Dental Anatomy: A Graphic Representation of Crown Forms, with Technic for Their Reproduction*

D EN TA L ANATOM Y: A G R A PH IC REPRESENTATION OF CROW N FORMS, W IT H T E C H N IC FOR T H E IR REPRODUCTION* By M. DIAMOND, D.D.S., New York City N A T O M I C differences are most marked between crowns of the incisors, cuspids, bicuspids and molars, because these teeth represent the groups possessing different specialized functions. M arked as these differences are, there are a series of landmarks th at are common to the crowns of all teeth, some of these to only the upper teeth, others to those of the lower, and some that are identified w ith the particular tooth. These landmarks, to be presently enumerated, are intended as definite guideposts for the easier assimilation of the descriptive anatomic detail, as well as for the development of the technic for the reproduction of crown forms. W herever possible, the landm arks are to be identified w ith a definite purposeful­ ness on the part of N ature to contrive the most efficient means for their particu­ lar usefulness. W ith few exceptions, all teeth are de­ veloped from four prim ary parts, known as lobes. In considering the anatomic form of the crowns, it is im portant to consider ( 1 ) the form of each lobe, and (2 ) their placing or arrangem ent in rela­ tion to each other.

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*R evised m anuscript received for p u b lic a ­ tion, Sept. 14, 1929. *R ead before the Section on P a rtia l D en ­ ture P rosthesis a t the Seventieth A nnual Session of the A m erican D ental Association, M inneapolis, M inn., A ug. 21, 1928. Jo u r. A . D , A ., M a rch , 1930

I t is in an understanding of both of these factors th at an explanation of the gross anatomic variations between the crown of one tooth and th at of another is to be found, as is the explanation for the detailed anatomic variations between the crown of a tooth in the mouth of one person and the crown of a similar tooth in the mouth of another. T h e crown of the upper right central incisor in the mouth of one person basic­ ally resembles the crown of the upper right central incisor in the m outh of any other person, yet a careful examination discloses infinite variations, which in their extremes are very marked. So it is w ith the crowns of all of the teeth. Stripped of these variations, the close kinship between the teeth of all human beings is readily established in a remain­ ing common denominator or basic outline form. T his basic form I choose to call the symmetrical tooth form. T h e varia­ tions themselves I have also attempted to classify, and they w ill be separately discussed.

LANDMARKS 1. T h e crowns of all of the upper teeth are centered directly over the roots, so th at if in the anterior teeth the incisal angle were bisected, and the bisector continued through the apex of the root, the labiolingual diameter of the crown would also be bisected.

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D iam ond— D entai Anatom y 2. Crowns of the lower teeth bear a somewhat different relation to their roots, in th a t they are all bent over or inclined toward the lingual aspect. T h is inclina­ tion is due to the leaning over or con­ vergence of the labial surface of the anterior and the buccal surface of the posterior teeth, tow ard the lingual. T his convergence is greater in the posterior teeth, and greatest in the lower bicuspids, where if the angle of the buccal cusp were bisected at its summit and continued dow nw ard in the direction of the apex of the root, the bisector would pass through its apex. 3. In the crowns of the anterior teeth, the widest mesiodistal diameter is to be found at the junction of the incisal and middle thirds, and in the posterior teeth, the widest mesiodistal diameter is to be found at the junction of the occlusal and middle thirds. In this region, th at is, at the junction of the incisal and middle thirds of the anterior and the occlusal and middle thirds of the posterior teeth, a contact relation is established w ith the adjacent tooth. T h e contact point is exceedingly im­ portant in that it serves as a definite means for preventing foods from being impacted in the interproximal spaces, against the delicate tissues. 4. T h e mesial and distal boundaries of the lingual surfaces of the upper anterior teeth, and the mesial and distal bound­ aries of the occlusal surfaces of all the posterior teeth, present w ell rounded bands of enamel, which are called the mesial and distal m arginal ridges. These marginal ridges are rolled over tow ard the lingual surfaces of the upper anterior teeth, and tow ard the occlusal surfaces of the posterior teeth, and serve very much as the rim of a saucer to keep the masticated foods w ithin the lingual and occlusal surfaces, preventing them from

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being jammed against the contact point w ithin the interproximal spaces. These m arginal ridges are therefore an impor­ tant accessory to the contact points, in serving to protect the vulnerable tissues. I t is interesting to note the total absence of marginal ridges on the lingual sur­ faces of the lower anterior teeth, against which there is no masticatory stress. 5. T h e widest labiolingual diameter of the anterior and the widest buccolingual diameter of the posterior teeth are found near the junction of the cervical and middle thirds. Placed at this point, the widest labiolingual and buccolingual diameters serve to protect the soft tissues surrounding the labial surfaces of the anterior and the buccal surfaces of the posterior teeth by slanting the foods, dur­ ing mastication, off at a tangent, away from the soft tissues. 6. T h e mesial and distal surfaces of all teeth converge tow ard the cervical line from the widest mesiodistal diameter. 7. T h e mesial and distal surfaces of all teeth also converge tow ard the lin­ gual aspect, the convergence of the mesial surface being somewhat the greater. 8. A ll mesial surfaces are rather straight in their cervico-incisal or cervicoocclusal directions, and all distal surfaces are convex in the same direction. Lower bicuspids, and particularly the first, are exceptions to this rule. 9. T h e cusp eminences may be divided into three types: (1 ) the well-rounded elevation; (2 ) those w ith a shallow de­ pression or cavity, and (3 ) two irregular quadilateral planes at an angular rela­ tionship of approximately 120 degrees. 10. T h e cusp depth in the posterior teeth is usually one-fourth the height of their crowns. T h e greatest exception is w ith the upper first bicuspids, in which the depth of the cusp is between one-third

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and one-half the length of its crown. 11. T h e marginal ridges of the poste­ rior teeth have their respective positions in the cervico-occlusal diameter of the crown, and lie on the plane in between the summit of the cusps and the depth of the cusps, but close to the depth. 12. T h e depth of the cusp lies on the plane between the m arginal ridges and the point of contact, which is one-third the length of the crown. 13. A t the cervical line, a band of enamel is to be found surrounding the entire circumference of the crown, which rolls over tow ard the cervical line as it joins the root. 14. T h e labial surfaces of all the ante­ rior and the buccal surfaces of all the posterior teeth slope away tow ard the distal aspect, in their mesiodistal con­ vexities. T h e greatest slope is in the cervical third and gradually diminishes as the incisal or occlusal surfaces are approached. 15. T h e peripheral outline of the lingual surfaces of all the anterior teeth is smaller than the peripheral outline of the labial surfaces.

SYMMETRICAL TOOTH FORMS O f the four lobes that make up the upper central incisor, three are labial and the fourth lingual. T h e labial lobes all develop to approximately the same le n g th ; the central lobe is slightly shorter than the mesial, and the distal slightly shorter than the central. Mesiodistally, the cen­ tral lobe occupies about one-fourth of the mesiodistal diameter, the remaining area being approximately equally divided be­ tween the mesial and distal lobes, w ith the mesial slightly the wider. Evidence of union between the three labial lobes may be noted in the incisal and middle thirds, from the presence of developmental grooves which converge

slightly in the direction of the cervical third, where they lose themselves, the three labial lobes merging into one smooth whole. Each labial lobe has its individual con­ vexity in the incisal and middle thirds, w ith approximately the same degree of labial prominence, the central slightly subdued tow ard the mesial, and the distal slightly subdued tow ard the central lobe. T h e mesiodistal convexity of the cervical third is even, sloping away tow ard the distal margin. T his sloping away tow ard the distal margin of the mesiodistal convexity is to be noted on the labial and buccal surfaces of all teeth, and may be established as a universal landmark. T h e distal por­ tion of the cervical third is more sub­ dued than the distal portions of the middle, incisal or occlusal thirds. T he purpose of this is to produce symmetry in the arrangem ent of the teeth for the arch form. T h e cingulum is markedly convex in the mesiodistal direction, and very slightly convex in the cervical and incisal direc­ tions. T h e lingual aspects of the labial lobes are usually concave, forming to­ gether an even saucerlike depression, hemmed in by the marginal ridges, the incisal margin and the cingulum, all of which may be regarded as the rim of a saucer. Frequently, the centrolabial lobe w ill develop more fully in the direction of the lingual, and form a transverse ridge, running from the incisal margin and merging into the cingulum, which divides the even saucerlike depression into two depressions, the fossae, each being triangular in outline. Usually accom­ panying this condition, a tubercle, or a rounded elevation of enamel, w ill be found upon the cingulum. Developmental grooves are merely evi­ dences of union between the primary

D iam ond— D ental A natomy parts and may be described as similar to the sutures of the cranial bones. T hey are to be found at the places of union between the primary parts and are best observed immediately after eruption of the crown, before any changes in mastica­ tory stress have taken place. T hey d-o not play any role in the function of mastica­ tion. As a m atter of fact, when the union is quite complete, the developmental grooves are hardly discernible or are com­ pletely worn away w ith slight m astica­ tory stress. In the upper central incisor, the developmental grooves are therefore to be found on the labial surface, as al­ ready mentioned, and near the mesiolingual and distolingual line angles, where they separate the lingual lobe from the three labial lobes. T h e symmetrical outline of the upper lateral incisor is identical w ith th a t of the central. T h e difference is chiefly in size, the proportions remaining more or less the same. T h e labial surface of the lateral is usually slightly more convex in the mesiodistal direction than the labial of the central incisor. T h e character of the variations of the lateral incisor are usually identical w ith the character of the variations in the central incisors. T h e Upper Cuspid Teeth.— T ra n si­ tion in symmetrical form of the upper cuspid from the incisor is due entirely to a change in the development of the centrolabial lobe. T h is lobe develops much w ider mesiodistally, occupying about half of the mesiodistal diameter. I t develops more prominently labially, creating a much greater convexity mesiodistally, and develops longer than either the mesial or distal lobes, so that the eminence of the centrolabial lobe is approximately onethird longer than the eminences of the mesial and distal lobes. A t this point, it is interesting to note that the formation of the incisal margin, as generally noted

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consisting of two straight arms at an angular relationship to each other, is not to be seen prior to the eruption of the crown, or immediately after the eruption. Instead, the incisal m argin consists of three rounded eminences, which are the term inal ends of the labial lobes, and are formed on completion of the crown. These rounded eminences, called mamelons, are subsequently w orn away by the stress of mastication, and the two straight arms that we are more fam iliar w ith are produced. In certain isolated cases, these rounded eminences may also be seen in adults, when the crown is either in lingual occlusion or in extreme labial occlusion, out of contact with the line of occlusion during mastication. T h e centrolabial lobe also develops more fully in the direction of the lingual, and eliminates most of the concavity de­ scribed in the lingual aspect of the incis­ ors. T h e incisal and middle thirds of the lingual aspects are rather full, and only two very small triangular depressions are to be rioted mesially and distally, hemmed in by the marginal ridges, the centro­ labial lobe and the incisal arms. T h e w id­ est mesiodistal diameter of the cuspid is usually slightly less than the widest mesiodistal diameter of the central in­ cisor, and its crown length from the cervical line to the summit of the central lobe is rarely longer than the crown length of the central incisor. T h e fact that it appears so is due to the relative positions of the cuspids and central in­ cisors in the maxilla. T h e Upper Bicuspids.— In the transi­ tion between the cuspid and bicuspid, the lingual lobe plays the im portant role, in that the lingual lobe continues its growth in the direction of the occlusal aspect, almost reaching in its length to the sum­ m it of the buccal cusp. A t the same time, it carries all of the landmarks found on

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the lingual aspects of the anterior teeth to the occlusal surfaces of the bicuspids. T h e buccal half of the upper bicuspid is identical w ith the labial half of the cuspid. T h e proportions of the lobes mesiodistally are the same. T h e length of the crown is approximately one-third shorter than the length of the cuspid. O n eruption, the three rounded emi­ nences or mamelons may be noted on the occlusal border of the buccal surface. T hese are subsequently w orn away to form two arms, mesially and distally, similar to the arms of the incisal m argin of the cuspid. T h e buccal p art of the upper bicuspids occupies slightly more than half of the buccolingual diameter and is separated from the lingual half by the central groove, which extends mesi­ ally and distally across the m arginal ridges and can be seen, on occasion, ru n ­ ning across the entire length of the mesial and distal surfaces, close to the mesiolingual and distolingual line angles. These developmental grooves have no distinct purpose, although D r. Prim e ascribes a spillway function to them. I believe D r. Prim e is in error for two reasons : 1. T h e developmental grooves as they traverse the m arginal ridges are not con­ stant in their position and location, and are most persistently to be noted only across the mesial m arginal ridge of the upper first bicuspid. In most other in­ stances, they very soon disappear from wear. 2. N ature never could have intended to place spillways in the direction of the interproxim al spaces for the clearance of masticated foods, when she has attem pted to be so careful to protect the tissues w ithin the interproxim al spaces by means of contact points and m arginal ridges. Logical spillways are, and should be, in the direction of the point angles, where

masticated foods find outlet buccally into the vestibule of the mouth, and lingually into the mouth proper. T h e more evident the break across the m arginal ridge, the clearer the evidence of irregularity in the development of the parts. D r. Prim e also contends, in m at­ ters of restoration of the proximal sur­ faces (which include marginal ridges), that if these spillways are not placed across the m arginal ridge, evidence of irritation of the soft tissues will soon be noted. I cannot agree w ith D r. Prime. G in­ givitis is due to another condition, usually to the position of the marginal ridge itself. T h e tendency in restoration is to place the marginal ridge too high in the direc­ tion of the summit of the cusps. Location of the marginal ridge is definite and can readily be determined by its relative posi­ tion as it lies on the plane between the summit of the cusp and its depth. Its plane, while between the summit and the depth of the cusp, lies very close to the depth. T h e m arginal ridges begin at the term inal ends of the arms of the buccal cusp, and slightly decline in their course to meet the term inal ends of the arms of the lingual cusp at the point angles. T h e buccal cusp of the upper first bicuspid is very prominent, wider mesio­ distally than the lingual cusp and some­ w hat longer. T h e planes of the buccal cusps are irregularly quadrilateral in outline, and in angular relationship of approximately 120 degrees. T h e arm in common to both planes, I choose to call the cusp ridge. Black speaks of it as the “triangular ridge.” T h e bases of the planes together form the central groove, and the buccal margins of the planes form the mesial and distal arms of the cusp. T ogether, they form the occlusal border of the buccal surface. T hey merge to form the summit of the cusp and join

D iam ond— D entai Anatomy the m arginal ridges mesially and distally at the point angles. T h e mesial and distal margins of the cusp planes are the mesiobuccal and distobuccal grooves, which run from the term inal ends of the central groove mesially and distally in the direc­ tion of the point angles. These grooves are the logical spillways. T h e lingual cusp has an entirely dif­ ferent character. Its mesial and distal arms are rather convex, and merge at the summit to form a continuous convex line. Its planes are slightly concave and merge to form a continuous slight de­ pression. Rarely is there evidence of a cusp ridge, to form, along w ith the buccal cusp ridge, a transverse ridge, but when occasionally noted, it is an evidence of a slight overdevelopment, a tendency simi­ lar to the production of a tubercle upon a cingulum on the lingual lobe of the anterior teeth. T h e space between the cusp ridges, which Black speaks of as the sulcus, I suggest another term f o r : the “ inter-cusp space.” “ Sulcus” is in tru th a m isnom er; which may account for the general con­ fusion in its application. T h e base of the planes of the lingual cusps is the central groove, and their mesial and distal borders are the mesiolingual and distolingual grooves, which run in the direction of the lingual point angles. These grooves are the logical spillways. W ith in the boundaries of the mesial m arginal ridge, the mesiobuccal and mesiolingual grooves, and within the boundaries of the distal m arginal ridge, the distobuccal and distolingual grooves, small triangular depressions are found which are the mesial and distal triangular fossae. T h is completes the configuration of the occlusal surface. T h e upper second bicuspids differ from the first only in m atters of detail. T h e

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number of lobes and their arrangem ent are the same. T he crown, on the whole, is generally smaller in all directions than that of the first bicuspid. T h e crown is shorter and the cusp depth assumes the proportions of- one-fourth the length of the crown. T h e w idth of the central groove is shortened, and the marginal ridges are strangely heavier. O n the whole, the general outline appears much less defined. W ith the second bicuspid, there is a receding of the mesial surface tow ard the occlusal and lingual aspects. W ith the first, a receding of the distal surface tow ard the occlusal, which, w ith the mesial surface of the second, creates a rather wide lingual embrasure between the bicuspids. A general tendency is to regard the mesial surface of the upper first bicuspid as receding tow ard the lingual and occlusal, because of its ap­ proximation to the cuspid. T h e Upper M olars.— In the upper molars, we are faced w ith apparently considerably more complexity. Analysis of the molar crowns shows them to be made up of the same number of segments as the bicuspids and the anterior teeth ; but their arrangement and placing is radically different. Instead of three lobes tow ard the buccal aspect, we find only two, and two tow ard the lingual. O ne of the three buccal lobes skirts around to the lingual aspect and becomes the disto­ lingual lobe of the upper first molar. T h e mesiolingual cusp of the upper first molar, on careful observation, proves it to be identical w ith the lingual cusp of the bicuspid. T h e oblique ridge connecting the mesiolingual cusp w ith the distobuc­ cal might really be regarded as similar to the distal marginal ridge of the biscuspids. C areful observation w ill disclose the fact th at the distolingual cusp appears to

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be added to the crown form, which in itself is already complete in its bound­ aries. Indeed, we may note the dim inu­ tion of this distolingual cusp, out of proportion to the remaining parts, w ith the second and again w ith the third up­ per m olars; and in about 50 per cent of the upper third molars, the distolingual lobe is completely dropped, only the three cusped crown remaining, w ith the oblique ridge, now becoming the distal m arginal ridge. In the crown of the upper first molar, we find all of the three types of cusps on the one occlusal surface. Each of the two buccal cusps consists of two irregular quadrilateral planes at an angular rela­ tionship of approximately 120 degrees. T hey are approximately the same size, each occupying about half of the mesiodistal diameter, and about half of the buccolingual diameter. T h e distobuccal cusp is slightly shorter than the mesiobuccal. T h e separation between the arms and planes of the buccal cusps, which the w riter calls the “buccal notch,” is about 100 degrees. O f the two lingual cusps, the mesiolingual occupies about tw o-thirds of the mesiodistal diameter, and the distolin­ gual, the remaining third. T h e space separating the lingual cusps, which I choose to call the “lingual notch,” is formed entirely at the uexpense of the distal side of the mesiolingual cusp. I found this last observation a very impor­ tant detail in eliminating much of the confusion incident to establishing the proper contour of this region. F or the developmental groove separat­ ing the two buccal lobes I suggest the term “bucco-occlusal” groove, since half of its course is upon the occlusal surface, and half upon the buccal. Similarly, the developmental groove separating the two

lingual cusps, I suggest calling the “linguo-occlusal” groove. Both of the lingual cusps are slightly shorter than the buccal. T h e distolingual is slightly shorter than the mesiolingual, or, to put it another way, the lingual half of the crown is slightly shorter than the buccal, and the distal half slightly shorter than the mesial half. T h ere remains little more to say about the upper first molar, except to call atten­ tion to the similarity of the pattern to that of the bicuspids. T h e marginal ridges are noted in a similar location, as are the triangular fossae. T h e mesiobuccal and mesiolingual, distobuccal and distolingual grooves run in the direction of the point angles, which bound the triangular fossae and are the logical spillways. T h e linguoocclusal groove merges into the disto­ buccal groove at the central groove, and the bucco-occlusal groove runs into the central groove at the deepest portion of the occlusal surface. T h e depression of the mesiolingual cusp here has the name of central fossa. T his depression also encroaches slightly on the distal plane of the mesiobuccal cusp, and the mesial plane of the distobuccal cusp. T h e central groove traverses the oblique ridge as it merges w ith the disto­ buccal and linguo-occlusal grooves. T h e general outline of the upper sec­ ond m olar is very similar to th at of the first molar. Its crown is shorter, and it is wider buccolingually than mesiodistally, the outstanding difference being the diminution of the distolingual cusp. T he third molar has identically the same pat­ tern, but tends to become smaller, some­ times remaining a four cusped crown w ith a diminutive distolingual cusp, and sometimes three cusped w ith the disto­ lingual entirely absent. T h e crowns of the lower teeth, while generally having a similar design to those

D iam ond— D entai Anatomy of the upper, are differently adapted to perform their particular function in the process of occlusion during mastication. T h e lower teeth are a part of the m andi­ ble, and so of the active driving force in the process of mastication. T h e upper teeth are the recipient of the force. T h e whole picture of the upright position of the crowns, the relation of the crowns to the roots, the arrangem ent and direction of the roots, show them to be squatly and firmly planted to receive the force, even as the human being would assume a posture w ith the legs spread apart and feet firmly planted on the ground, his body erect, w aiting to receive an oppo­ nent charging against him. O n the other hand, when the football player makes his charge, the body is bent forw ard, the legs are rath er close together; suggesting the position and contour of the lower teeth. T h e ir crowns generally are inclined tow ard the lingual aspect in relation to their roots, and the roots are formed and arranged accordingly. T h e crowns of the low er anterior teeth do not appear, in their position in the mouth, to have a lingual inclination in relation to their roots. T h a t is merely an illusion due to the position of the whole tooth in rela­ tion to the mandible, in th at its central axis is inclined outw ardly tow ard the labial aspect. T h e lower incisors are very delicate and very graceful in their appear­ ance. T h ey conform to the general pat­ tern of four lobes, of which three are labial, and one lingual. T h e same gen­ eral landmarks are to be n o ted : converg­ ence of the approximal walls tow ard each other in the direction of the cervical line and convergence of the approximal walls tow ard the lingual aspect. As has already been noted, the land­ marks on the lingual aspect are lacking, because they do not need the defensive mechanism of marginal ridges and fossae,

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noted on the lingual surfaces of the up­ per anterior te e th ; for the lingual sur­ faces of the lowers play no part in the active function of mastication. T h e lat­ eral incisor is somewhat larger than the central in all measurements. Its distal surface is convex in the cervico-incisal direction, while the distal of the central incisor is practically flat. T he L ow er Cuspids.— T h e lower cus­ pid presents a number of difficulties, and seems to be one of the most difficult teeth to reproduce. These difficulties are dis­ solved when a careful analysis is made of the arrangement of the primary parts and their individual contours. O f the three labial lobes, the centrolabial, as in the upper cuspid, occupies about half of the mesiodistal diameter, and is some­ w hat longer than the mesial or distal lobes. I t is not nearly so prominent labially in relation to the other two, showing a lesser convexity of the labial surface in the mesiodistal direction. T h e distal lobe, while shorter than the mesial, is w ider mesiodistally than the mesial, and bellies out distally toward the first bicuspid. T h e mesial surface is straight in its cervico-incisal direction, and may be noted to be rather continuous w ith the mesial aspect of its root. O f the two incisal arms, the mesial is rather short, partly because the summit of the centrolabial lobe is only slightly longer than the summit of the mesial, and partly because the mesial lobe is rather narrow , making the distance between the two summits rather small. T h e distal arm therefore, is considerably longer. T h e bellying out of the distal surface is in the incisal half of the crown le n g th ; in its cervical half, it suddenly becomes concave as it terminates at the cervical line. Its lingual aspect is rather free of markings, except those indicating the coalescence of parts. W hen the lower anterior tooth is

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seen out of the mouth, a most graceful curve can be noted, describing the whole labial aspect of the tooth, from apex of root to incisal summit of crown. T h e L ow er Bicuspids.— T h e general peripheral outline of the lower bicuspid crowns is very different from that of the upper bicuspids. T h e general outline of the upper bicuspids may be described as quadrilateral, the approximal sides con­ verging slightly tow ard the lingual aspect. T h e general contour of the lower bicus­ pid may be described as circular. T h e segments and landmarks fit into a circu­ lar pattern, instead of a quadrilateral. T h e lingual inclination of the crowns is most marked w ith the bicuspids owing to the “ bending over” of the buccal sur­ faces. T h e inclination of the buccal su r­ face begins at the junction of the cervical and middle thirds, and ends in the sum­ m it of the buccal cusp, and the summit of the buccal cusp almost approximates the central axis of the tooth. T h e landm ark already referred to as regards the passing of the bisector of the buccal cusp angle through the apex of the root proves to be very helpful in determining the relation of the crown and the root. In the first bicuspid, the development of the lingual lobe is usu­ ally not very marked, the buccal lobe re­ maining very bold and appreciably longer. T h e pattern of the occlusal surface can be identified w ith the general pattern already familiar. Usually, the intercusp space is elimi­ nated by an overcalcification of the cusp ridges, bridging the buccal and lingual cusps w ith a band of enamel, called the transverse ridge. T h e central groove may be traced traversing the transverse ridge, and descending mesially and distally to the usual cusp depth. From these points arise the mesiobuccal and mesiolingual, distobuccal and distolingual

grooves. T h e m arginal ridges and the triangular fossae are again noted in their usual anatomic positions. T h e planes of the buccal cusps are usually fuller and somewhat convex, while the lingual emi­ nence appears as little more than a large well-rounded tubercle. T h e centrobuccal lobe is considerably more prominent, occupying about half of the mesiodistal diameter, the mesial and the distal lobes being subdued, and divid­ ing the remaining mesiodistal diameter between them. T h e crown of the lower second bicuspid is somewhat larger than the crown of the first. Its chief interest is a duplication of lobes lingually, pro­ ducing a three-cusped tooth, two lingual and one buccal. T h e lingual cusps usu­ ally develop to the length of the buccal, and begin to assume the character of the buccal cusps of the upper molars. In most other respects, the general pattern and arrangem ent of parts is similar to that of the first, w ith the exception that the transverse ridge is rarely in evidence. T h e variations in form of the lower bicuspids are so numerous, so sharp and extreme, th at they may be specifically grouped into several typal forms. I t is difficult w ithin the limits of this paper to describe them all, but a number of typical variations of the lower second bicuspid may be pointed out. These variations are produced for the most part by a change in the course of the central groove, and in the position of the linguo-occlusal groove. In one type, the central groove is rather straight in its mesiodistal direction. From it radiates the linguo-occlusal grooves, dividing the lingual lobes, so that the mesiolingual is wider mesiodistally than the distolingual. In another type, the central groove consists of two parts, at an angular rela­ tionship of approximately 100 degrees, with the point of the angle tow ard the

D iam ond— D entai Anatom y lingual aspect. From this point radiates the linguo-occlusal groove, dividing the lingual lobes into approximately equal parts. In still another type, the central group is convex, w ith its convexity point­ ing lingually. T h e linguo-occlusal groove radiates from it at a point distally from the center, which makes the mesiolingual lobe w ider mesiodistally than the distal lobe. T h e L ow er M olars.— T h e lower first molar, like the lower second bicuspid, is made up of five prim ary parts. T h e three buccal lobes spread out to form three individual elevations or cusps. T h e gen­ eral contour of the occlusal surface may be described as that of an irregular trapezoid, w ith its mesiodistal diameter, buccally, w ider than its mesiodistal diameter lingually. A n interesting observation is th at the lingual cusps very definitely re­ semble in type the buccal cusps of the upper molars, and the buccal cusps re­ semble the distolingual cusp of the upper first m olar as they are usually well rounded or convex. Again, I take the liberty to suggest some changes in terminology to retain the pattern presented, and to make for somewhat more clarity. T h e three buc­ cal cusps, I suggest calling, respectively, the mesiobuccal, the centrobuccal and the distobuccal, and the developmental grooves separating them, the mesiobuccoocclusal and distobucco-occlusal grooves. O f the three buccal lobes, the mesio­ buccal occupies about half of the mesio­ distal diam eter; the centrobuccal, about two-thirds of the remaining half, and the distobuccal, the remaining third. T h e central groove in its course describes a sort of shallow “W ,” w ith the base of the W facing buccally. T h e linguo-occlu­ sal groove radiating from the central point of the W divides the lingual cusps into approximately equal parts. T h e

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mesio-lingual cusp is usually slightly longer. T h e mesiobuccal and mesiolingual, distobuccal and distolingual grooves run in the direction of their respective point angles. M arginal ridges and the tri­ angular fossae may be noted in the usual locations. Buccolingually, the diameter is greater mesially than distally, owing to a rather marked sloping away of the buccal surface tow ard the distal aspect. In the lower second molar in its typical form, the distobuccal cusp is dropped, a four lobed or four cusped tooth resulting, irregularly quadrilateral in outline. T h e lingual cusps resemble the pattern of the buccal cusps of the upper molar, and the buccal cusps resemble the pattern of the distolingual cusps of the upper molar. T h e central groove runs rather straight across in a mesiodistal direction, dividing the lingual from the buccal into about equal halves, and the linguo-occlusal and bucco-occlusal grooves join as they inter­ sect the central groove, dividing the mesial from the distal portions into about equal halves, thereby form ing a cross. From the term inal ends, mesially and distally of the central groove, the spill­ ways radiate, and again the marginal ridges and the triangular fossae, in their usual position. T h e lower third m olar is either a typical first molar in pattern or a typical second molar in pattern. So much for the symmetrical outline of crown forms, sketchily described w ith the idea of establishing a pattern more or less applicable to all of the teeth.

VARIATIONS AND ANOMALIES Variations of crown forms are very numerous. M any attem pts have been made to classify the crown forms into types. These classifications, while never so expressed, were really attempts to

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classify the variations. Failure of the existing classifications, I believe, is due to the fact th at (1 ) all of the conceivable variations of crown forms were not al­ lowed for, and (2 ) all of the observations w ere made at random after certain changes had taken place from all of the forces at work during the masticatory function. O ne classification now obsolete is the so-called “ temperament grouping,” the nervous tooth, sanguineous, bilious and lymphatic. J . Leon W illiam s, who is re­ sponsible for relegating this classification into the discard, is at the same time responsible for another classification equally inadequate. T h e temperament grouping took into consideration only the anterior teeth. W illiam s bases his classi­ fication only upon the directions taken by the mesial and distal borders of the labial surface of the upper central incisors. In his first grouping, the approximal borders are parallel in their course. In the second grouping, they converge tow ard each other in the direction of the cervical line. In the third grouping, the distal borders describe a double curve in their course. D r. W illiam s offers this classification, not only for modern Europeans, but also as a constant for all races including primitive peoples and pri­ mates, thereby hoping to establish, through the courses of the approximal borders of the labial surfaces of the upper central incisors, definite evidence of the evolutionary theory. D r. W illiam s’ observations are not based upon crown forms prior to, or im­ mediately after, their eru p tio n ; nor is he taking into concideration the changes from incisal and approximal attrition. Obviously, if the incisal th ird of the cen­ tral incisors is w orn away, the outline and the course of the approximal borders w ill appear entirely different from when

observed upon the fully formed crown. Obviously, w ith lateral attrition of the approximal surfaces, the contours of the approximal borders will also become ap­ preciably altered. These observations, are, I believe, suffi­ ciently striking to raise an issue w ith D r. W illiam s’ point of view, but I am en­ gaged in answering him at somewhat greater length in a separate paper. I t is not my desire or intention to en­ ter the field of controversy in the studies of comparative dental anatomy. T h e observations and classifications recorded are entirely on the basis of physical evi­ dence in the human dentition. Contro­ versial aspects, more particularly with relation to the concrescence theory, do not, I believe, interfere w ith the present observations and classifications. W hether the human tooth, from the standpoint of paleontologic evidence, has developed through the ages from a primitive single cone, or whether, as Gregory maintains, the human tooth has been formed through the ages as the result of a series of spe­ cialized modifications due to environmen­ tal needs, does not interfere w ith the physical evidence in the observation of the human tooth, in that the human tooth at least appears to be a unit of individual segments w ith evidence of union between them. T h e histologic evidence in the development and growth of the human tooth appears to strengthen the idea that its development begins w ith a number of individual centers, which seem to form independently, merging tow ard each other as their formation is completed. T h is evidence, w ith this conclusion, does not necessarily contradict D r. G reg­ ory’s point of view. In his explanation of the evolution of the human tooth, his logic particularly appeals to me, because it fits in very w ell w ith the general evo­ lutionary doctrine. N or w ill the addi­

Diam ond— D entai Anatom y tional evidence listed under anomalies particularly contradict D r. G regory’s point of view, for he seems to anticipate them on the basis of endocrine disturb­ ances w ithin a particular individual, as against the reversion to the primitive single-cone theory. Variations m ust be classified as natural and acquired. A n im portant distinction must be made between these two groups. T h e acquired variations are the result of external irritants, w hether bacterial, chemical or mechanical, and have nothing to do w ith the inherited development of the crown form. These variations, which in themselves are very numerous, and are not to be discussed in this paper, m ust be carefully distinguished from the natural variations. N atu ral variations are predetermined and congenital, and are best observed after the crown is completely formed, either prior to eruption or immediately after eruption. W h a t the active factors in the formation of natural variations may be is still very much a m atter of speculation. T h e laws of heredity, of course, play an im portant part in the explanation of a large group. Endocrine disturbances may be responsible for som e; the mechanical resistance or absence of mechanical resistance during the calci­ fication of the crown may be a very likely explanation for a good many. M uch w ork is still to be done in this direction. Any attem pt to classify forms into types must prove futile, because of the infinite variations found, unless such a classifica­ tion prove helpful for practical purposes in designating artificial teeth from supply houses. T h e physical classification of square, oval or tria n g u la r; narrow or wide, short or long, may, for such purposes, prove adequate, but is on the other hand obvi­ ously quite inadequate in a study of varia­

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tions. T h e answer must be found in the classification of the types of variations to be found, and in order to accomplish this, some attem pt must first be made to observe the constants. These constants can be found in the basic form of the crown, the modifications of which are the variations produced. T his basic outline form has already been discussed under “ Symmetrical T ooth Form .” T o sum it up, the constants to be found a re : (1 ) the number of segments or lobes comprising the completed form of the crow n; (2 ) the form and contour of each segment; (3 ) the arrangem ent of the segments, and (4 ) the relation that one segment bears to another. T he natural variations associated with these constants may be classified as: 1. Any proportionately greater or lesser development of all of the segments, retaining the same number, same arrange­ ment and same contour, w ith the same relation between all of the segments, merely a difference in size resulting. 2. A disproportionate development of the segments, containing the same num­ ber, the same arrangem ent and the same relation between the segments, but hav­ ing variations in the form of each seg­ ment, which results in variations in dimensional relationships of the surfaces of the crown. 3. Accentuation or overcalcification of parts of individual lobes. 4. Imperfections or deficiencies in or undercalcification of parts of individual lobes. W ith this grouping, all of the con­ ceivable variations may be accounted for. An attem pt to list all of the variations th at I have observed and fitted w ithin their respective groups is material for another paper, and is included in my textbook. I t w ill therefore suffice to give individual illustrations.

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T h e first two groupings are more or less obvious. In the first grouping, we find teeth more or less identical in gen­ eral character, but different in size, retain­ ing the same proportions in all directions. In the second grouping, we find teeth which vary in their dimensional relation­ ship. T h e w idth of the lobes may remain the same, but there is underdevelopment in length. T h is changes the proportion between the mesiodistal diameter and the cervico-incisal. O r they remain the same in length and show underdevelop­ ment in the w idth of the individual seg­ ments, a longer and narrow er tooth re­ sulting. Tubercles found on the cingulum of the upper anterior or on the marginal ridges of the posterior teeth and on the point of union between lobes are varia­ tions to be included w ithin the third grouping. T ransverse ridges or overcalcification of the base of cusps at the point of coal­ escence between buccal and lingual lobes also fit into this grouping; and similarly, transverse ridges noted upon the lingual surfaces of the upper anterior teeth, the result of an overdevelopment of the centrolabial lobe. T h is type of overcalcifi­ cation is also responsible for many varia­ tions in the crown forms of the lower bicuspids. Pits and fissures belong to the fourth grouping, and are evidence of the lack of improper union of two or more segments. Pits and fissures, which have a very im portant bearing on our operative pro­ cedure from the standpoint of preventive dentistry, are now receiving a great deal of attention through the efforts of the H y a tt Study Club. Pits and fissures are conceded to be a definite invitation to bacteria, w ith the subsequent production of caries. In an effort to indicate some­ thing of their danger, in th at they are

prone to cause caries, I supplied the term “precarious” fissures. Smooth depres­ sions usually found upon the labial sur­ face of anterior teeth, particularly the upper lateral incisors, are also evidences of undercalcification and apply to the last grouping. Anomalies are unusual variations of comparative rare occurrence. T hey too are either congenital or predetermined, or are influenced by metabolic disturb­ ance during the process of calcification. Some of the anomalies noted are a com­ plete absence of one or more of the lobes, as, for example, in the peg-shaped lateral incisors. T his type presents a congenital absence of both the mesiolabial and distolabial lobes. O n occasion, the upper lateral incisor is entirely absent, congen­ itally, either on one side or on both. A nother example of undercalcification, resulting in anomaly, is the so-called “ H utchinson” tooth, in which the devel­ opment of the parts is completely ar­ rested, as the result of hereditary syphilis. T h is type of anomaly is characterized by an atrophy of all the segments of all of the anterior teeth, including the first molars. T his is to be differentiated from a type of atrophic anomaly which devel­ ops as the result of some contagious or infectious disease, or general cachetic disturbance at a very early age. D uration of the influence of these metabolic dis­ turbances is usually much shorter than the influence of hereditary syphilis, and, for that reason, not so many of the teeth are interfered w ith during their calcifi­ cation. T h e atrophy will, therefore, be noted only in the case of the anterior teeth and, on occasion, only the incisors. T h is is an im portant point in differential diagnosis. Extrem e overdevelopment of one or more of the lobes also results in a type of anomaly included w ithin the fourth

D iam ond— D entai Anatom y grouping. 1 have in my possession indi­ vidual specimens showing overdevelop­ ment, in some cases of only one of the lobes, to practically the size of a small crown. In another instance, a specimen in which all of the three lobes of the lower central incisor are overly developed, each to the size of a full crown, gives the appearance of three crowns all on one root. A dditional lobes, which I have particularly observed on the aspect of the mesiobuccal lobe of the upper second molar, also belong to the last grouping.

DISCUSSION

James M ark Prime, Omaha, Nebr.:

D r. D iam ond gives us a som ew hat different ap ­ p roach to the m uch discussed subject of den­ ta l anatom y. Ju d g in g from the beautiful w ork th a t we h ave seen from his hands, w e a re com pelled to give consideration to his m ethod. H e studies the tooth crow n from a developm ental point of v ie w ; tak in g each lobe as a definite p a r t and v isu a liz in g the re ­ sults as these se p a ra te p a rts are united to com plete the crow n. H is m ethod h a s value in c la rify in g the presence of some tooth-form c h aracteristics th a t could not, perhaps, be accounted fo r so easily by o ther approaches. A ny m ethod is good only so f a r as it is able to assist us in recognizing a n d reproducing physiologic characteristics. I t is not w ithin o u r province o r w ith in possibility to form crow ns of teeth a fte r the m ethod em ployed by N a tu re. Fundam entally, therefo re, w e are not so m uch interested in how she does it as w hy she does it. T h e relativ e im portance of the tw o m ay be illu stra te d by com paring the teeth to other o rg an s of the body. T h e allim p o rta n t function of the eye is vision. W ould a com prehensive know ledge of N a tu re ’s m ethod of developing it m ate ria lly aid us in c orrecting the irre g u la ritie s of this org an ? W e m ay h ave v ery accurate ideas as to how N a tu re develops other o rg an s and be ill p re ­ p a red , w ith th a t know ledge alone, to restore th e ir functions w hen they become im p a ired or lost. Function, therefo re, forces its w ay to the fro n t as the m ajo r consideration w ith all o rg a n s of the body. Since form plays an im ­ p o rta n t role in dental functions, we find our­ selves dealing w ith a m ajo r subject in den­

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tistry. T h e functions of the dental organs of m an a re m any. T h e principal one, m astica­ tion, is effected th ro u g h occlusion. T h e occlusal surface is th e business end of the tooth. A ny form ch ara cte ristic th a t aids in this function is an asset a n d of first im por­ tance. A ny efforts a t restoring these form s w hich fa il to aid or in te rfere in th is function a re a m enace and a liability. D r. D iam ond w ell says, “A natom ic differences a re most m arked in crow ns of teeth,” because, as he states, of a v a ria tio n in specialized function. “M a rk ed as these differences are, there are a series of landm arks common to the crow ns of all teeth. Some to the upper, others to the low er and some to the p a rtic u la r tooth.” V ariatio n is the law of N ature. I t is d a n g e r­ ous to la y dow n rules or take the position th a t a c ertain c h ara cte ristic is constant. Rules a re ill applied to organs so v a ria b le in th eir form s. T h e best rule is th a t there is no rule. On the other h and, w e find characteristics o ccurring so re g u la rly as to unm istakably point to the fa c t th a t N a tu re intended them as functional attributes. Studying tooth form from a functional point of view offers a basis fo r easy com prehension. M any studies in the p ast have lost m uch of th eir value, because, as D r. D iam ond states, they w ere m ade on extracted teeth, m any of w hich w ere de­ form ed th ro u g h c aries a n d w ear. I t is p e r­ fectly obvious th at, if w e desire to a rriv e at a common, scientific u n d e rstan d in g of this sub­ ject, w e m ust g a th e r o u r’ facts from teeth on w hich th ere a re no m arks of m utilation from caries o r deform ity from w e ar. W e should m ake our observations from these teeth m ore th a n from lectures, c arv in g s, essays or text­ books. D r. D iam ond uses the term “p lan e” in reference to the occlusal surfaces of the bicuspids a n d m olars. T h ese surfaces as they come from the h a n d of N atu re a re ridged, tub ercu lated and c u rv ed . P lanes a re to be found on them only a fte r they h ave been deform ed through occlusal w e ar. T h ese su r­ faces, before they a re w orn, m eet each other, a t sm all points, w h en the ja w s are closed. T h is can easily be dem onstrated if w e place a piece of carbon p a p e r betw een the teeth of a young person a n d ask him to close. Small dots w ill a p p e a r on the occlusal surfaces w here the occlusal contacts w ere m ade. N a ­ tu re w a s jealous of th is form a n d m ade a heroic effort to m ain tain it by b uilding these surfaces of the h a rd e st tissue known, enam el. T o fu rth e r p e rp etu ate those form s, she so

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a rra n g e d the rods th a t w e a r occurs on th eir ends. F la t surfaces a re ra re ly if ev er observed on tooth crow ns before they are subjected to w e a r. T h e surfaces of the teeth a re b u ilt on curves. T h e y harm onize w ith the id ea s of the g re a t a rtist w ho said, “ T h e law of beauty is curve upon curve.” I t is tim e th a t the dental profession recognize “inclined p lan e s” as acquired deform ities, a n d w e should re fe r to them in o u r lite ra tu re a n d textbooks as such. T h e low er bicuspids and m olars a re inclined lingually. A t the cuspid, the line of inclina­ tion changes, and the incisors incline lab ia lly . T h e contact points betw een the low er bicus­ pids and la te ra l incisors a re located a t the m iddle of the incisal th ird of the tooth, w hile the contact points of the other a n te rio r teeth are a t the incisal edges, fo rm in g a continuous blade or cutting edge. T h is is in harm ony w ith the designed function of these teeth. D r. D iam ond sees no function in m astication through the developm ental grooves in central incisors. In this, I w ould concur, but w hen he applies this to the bicuspids and m olars, he does N a tu re and the tru th a gross injustice. D evelopm ental grooves aid ed by supplem en­ tal grooves play a m ost im p o rta n t role in function in all bicuspids and m olars. G a te ­ w ays, reliefw ay s a n d spillw ays a re provided by them . I f I u n d e rstan d D r. D iam ond cor­ rectly, he tac tfu lly sees one se rv in g him fo r w h a t he calls a “ logical spillw ay.” I m ight v en tu re the suggestion th a t spillw ays a re not developed, m aintained or m ade to function by the rules or law s of logic, but r a th e r by law s of biology a n d physiology. D r. D iam ond thinks th a t he senses a disagreem ent w ith me on this. H is dispute is not w ith m e b u t w ith N ature. T h e spillw ays a re th ere persistently. T h e y do not, as he w ould im ply, d iv e rt food into the interp ro x im al spaces. T h e y aid strongly in p re v en tin g th a t v e ry thing. T h ey a re not w o rn aw ay until the cusps a re lost through w e a r. W o u ld he contend th a t cusps h a v e no function because they a re m any tim es lost th ro u g h w e a r? T h e y a re not confined to the up p er bicuspids, but a re consistently presen t on other teeth. T h e ir presence, in some degree of developm ent on m ost teeth, and th e ir functions a re too w ell understood to ju stify going m ore deeply into the subject here and now. D r. D iam ond offers fo r our consideration some new dental term s. If they w ill aid us in a better u n d erstan d in g of this im p o rtan t subject, w e are fo r them . T h e essayist’s rem arks on the typal form s as postu­

lated by J. Leon W illia m s ag ree w ith those of m y ow n. H is logic is c le a r and square w ith the facts, as I see them . D r. W illia m s claim s to h ave discovered a definite relatio n in face form w ith u pper c en tral incisor form . M y o bservations extending over m any years h ave offered no proof fo r such theory.

Austin F. James, Chicago, 111.: D r. D iam ond’s rev iew of d e n ta l anatom y calling attention to types of tooth fo rm atio n is of in te rest to the periodontist, because he realizes, more th an any one else, w h a t N a tu re has done to protect the delicate supporting structures. If, in D r. D iam o n d ’s clinic, he is able to dem onstrate some definite m ethod to assist the g e n era l o p e ra to r in reproducing n a tu ra l tooth form s in all restorations, he w ill indeed h a v e add ed v a lu a b le inform ation to the profession. I h ave h a d the opportunity of observing num bers of cases com ing from the h ands of a few op erato rs w ho h av e the ability to reproduce tooth form in fixed bridgew ork, as w ell as in the construction of larg e and sm all inlays. I realize how much better the supporting stru c tu re s react w here this id ea has been c a rrie d out. T h e periodon­ tist is confronted w ith m any cases in w hich there is a slight m alocclusion. Or, as D r. T is h le r has said, “occlusal h a b it neuroses” h ave developed pathologic conditions in the supporting structures of otherw ise w ell form ed arches and perfectly form ed n a tu ra l teeth. M any of these patients a re past the age w hen orthodontic correction could be em­ ployed. R elief m ust be secured in some m an ­ ner, so the periodontist has attem pted to b a l­ ance the occlusion by g rin d in g the n a tu ra l teeth. In cases in w hich questionable ju d g ­ m ent has been used, m uch m utilation of the teeth has fo llo w e d ; w hich has caused some conscientious dentists to hesitate in attem pt­ ing this procedure. M y ow n respect fo r the n a tu ra l tooth has led me to spend m uch tim e in attem p tin g to solve this problem . M y w hole effort has been to reproduce the n a tu ra l tooth form in a ll w o rn opposing teeth, and the relief th a t can be obtained is re ally quite satisfactory. Dr. Diamond (closing): D r. P rim e seems to group all cuspal form s as of one type, the rounded or convex em inence, but carefu l -ex­ am ination discloses three definite types a ll of w hich can be seen on one occlusal surface, th a t of the upper first m olar. Surely,: the buccal cusps a re not the sam e as the mesiolingual cusp, nor has the m esiolingual Cusp

D iam ond— D entai Anatom y the sam e ch ara cte r as the d istolingual. T h e buccal cusp is m ade up of tw o irre g u la r q u a d ­ rila te ra l planes a t a relatio n sh ip of 120 de­ grees, a n d I p re fe r to speak of them as planes ra th e r th a n as inclined p lanes. T h e m esiolingual cusp definitely p resen ts a depression or concavity and only the d isto lin g u al is rounded or bulbous. T h a t these planes of the buccal cusps, a t the tim e of eruption, p re sen t ridges or elev atio n s w hich a re soon w orn dow n by the stress of m astication is no evidence th a t these elevations or ridges a re intended to rem ain. T h a t they have a function, there is no doubt, but this function, sim ilar to th a t of the te rm in a l ends of the labial lobes of the u p p e r c en tral incisors, is to aid the tooth in its e ruption. T h e incisors are soon w o rn dow n by the stress of m asti­ cation a fte r the c ro w n ’s eruption, leaving a sh a rp incisal edge w hich D r. P rim e, I am sure, w ould agree is m ore fitted fo r its specialized function of incision. D r. Prim e, in re sto rin g the incisal su rface of an a d u lt up p er central incisor, w ould not think of re sto rin g a trip le lo b u lar elevation because, I am sure he m ust agree, th eir function ceased w ith the eruption of the crow n. Sim ilarly, the elevations upon the planes of buccal cusps a re intended to aid the crow n in the process of its eruption. T h ey seem to d isa p p ea r all too soon in the norm al hum an m outh to m ake one believe th a t they have any other function in the process of m astication. R e g ard in g th e question of spillw ays, D r. P rim e a n d I do not disag ree about th e ir im p o rta n c e ; w e m erely disagree about th e ir location. A continuation of the cen tral developm ental groove across the m e­

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sial m arg in al ridge, som etim es ru n n in g the length of the entire m esial surface, is not, to m y w a y of thinking, a spillw ay, because: 1. W e do not find it tra v e rsin g the m arg in al ridges of a ll posterior teeth, though w e do find it constantly tra v e rsin g the m esial m a r­ g in al rid g e of the up p er first bicuspid. 2. T h e b reak across the m arg in al ridge m ust neces­ sa rily tend to d irec t the m asticated foods in the in terproxim al space, a n d I do not believe th a t N a tu re could have, a t the sam e tim e, de­ vised an elaborate defensive m echanism , such as the m a rg in a l rid g e a n d point of contact to p re v en t m asticated foods from en te rin g the interp ro x im al spaces, and then defeat its p u r­ pose by designing a spillw ay in the location th a t w ould perm it them to do th a t v e ry thing. T h a t w e find v a ria tio n s in crow n form s w hich a re conceded as defects, there is no doubt. T h ese defects a re chiefly found in those re ­ gions of coalescence betw een cusps, p a rtic u ­ la rly betw een the th ree buccal lobes and the single lingual lobe of bicuspid teeth, and betw een all of the fo u r lobes in the m o lar teeth. T h e re also seems to be no doubt th a t the m ore evident the b reak in the region of coalescence, the m ore evident is the lack of com pleted developm ent. On the o ther hand, the grooves usually spoken of as supplem en­ ta ry are, to m y m ind, the im p o rtan t grooves as f a r as spillw ays a re concerned, and fo r th at reason I speak of them as the logical spillw ays. T h e ir course leads the m asticated foods in the direction of the respective point angles, eith er in the vestibule of the m outh or in the m outh pro p er a n d a w ay fro m the delicate supporting tissues.