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The Ossicula Auditûs of the Sheep (Ovis Aries)
GENERAL ARTICLES.
THE OSSICULA AUDITUS OF THE SHEEP (OVIS ARIES).
By H. C. WILKIE, F.R.C.V.S., F.R.M.S., F.Z.S. THE auditory ossicles of the domestic sheep are of special anatomical interest on account of their morphologicaI variations, not only from the newly born lamb to the adult, but also between individual animals of the same breed and of similar weight and age. These notes relate only to adult sheep, and, in endeavouring to determine the exact anatomical features which may be taken as characteristic of the species, I have noted those characters which may, I believe, be accepted as truly typical, apart from the many and interesting variations which occur. The lIIalleus.- The most prominent general features of the malleus (Fig. I) are the flattened form of the head, the strongly curved and blunt processus muscularis, the almost rectangular processus brevis. and the two angles which the manubrium forms with the neck, the directions taken by this process being forward and inward. A straight line drawn from the summit of the head to the processus brevis measures nearly 6 mm., and another drawn from the processus brevis to the terminal point of the manubrium measures fuHy 5'50 mm. The head is not globlllaT on its anterior surface, but is slightly flattened, and its round edges have the appearance of being somewhat pressed out against the edges of the articulatory surface. This surface has a postero-internal aspect, and its facets are two in number, the upper of which is broad and nearly semi-circular, and the lower one from a broad base descends to a somewhat pointed and curved extremity. The upper facet shows a slight median vertical ridge, and the lower, a ridge which inclines from its base to a point on the outer margin, where the articular surface begins to narrow. The neck of the malleus is lightly curved, and the lamina, which originates superiorly, weIl up on the base of the head on the externat side, extends over a distance of 1'80 mm. to meet the point of the processus gracilis. The free margin of the lamina is markedly convex. The processus gracilis originates at the base of the neck and takes a long sweep upward and forward to terminate in the point above mentioned, which buries itself in the opening of the Glaserian fissure. The anterior ligament of the malleus (Iaxator tympani major of Sömmerring) is attached to this point, and for some little distance along the anterior edge of the process. The processus muscularis appears to take its root immediately beneath the lower margin of the inferior articulatory facet, and from that point describes nearly a semicircle, to terminate on ~he anterior part of the inner surface of the neck in a blunt, well-rounded, and prominent extremity to which the tendon of the Tensor tympani muscle is attached.
299
GENERAL ARTICLES.
The processus brevis is represented by a strangely formed angular projection on the external surface of the neck near the base of the manubrium. It gives attachment to the posterior ligament of the malleus. The drawing of the malleus is made from the inner side, as this shows most of the more important characteristics of the ossicle, but that aspect does not show the fuH character of the manubrium, because it is thus seen on its edge. This edge, broad at the base, ta pers down in its centre to terminate in a very slightly spatulate extremity, which is curved a Httle outwards. The whole manubrium is, however, flattened from side to side soon after leaving its somewhat cylindrical base, and from an anterior or posterior aspect exhibits the double curvature and pointed extremity shown in Fig. I (lower figure).
P.S
FIG.
J.
Upper figure. Right malleus-internal aspect. U.F., Upperfacet; L.F., Lowerfacet; L., Lamina; P.G., Processusgracilis; P.M., Processus muscularis; P. B., Processus brevis ; M., Manubrium. Lower figure.
Lateral aspect of the manubrium from its base immediately below the processus brevis. P. B., Processus brevis; I. E., In ternal edge; E. E., External edge.
The Incus.-The most prominent characteristics of this bone (Fig. z) are its strong and bulky body, its long and deep articulatory surface, the wide divergence of its crura, and the anomalous condition that the processus brevis is considerably longer than the processus longus or stapedial crus. The long diameter of the body is from the highest point of the articular ridge to midway between the crura, and this has a measurement of z'z5 mm. The articulatory surface consists of two facets, separated by a high ridge. The upper facet is wide and semi-circular in form, and the
3 00
GENERAL ARTICLES.
Iower facet is broad at its base (the ridge ), and becomes somewhat pointed at its lower extremity, just above the base of the stapedial crus. The processus brevis is wide at its base, which is a littie angular on the external surface, and tapers down almost immediately, becoming cylindrical in form and taking a nearly straight upward and backward direction towards its termination in a rounded peg-like extremity to which is attached the posterior ligament of the incus. The stapedial crus is widely divergent from the processus brevis and takes a curved direction from its base forwards and inwards. It is at first nearly cylindrical in form, but towards its extr.e mity (which is only at a distance of I mm. from the base) it becomes flattened, then slightly concave internally, and turning inwards forms a shell-like hooked extremity.
""""""'-\1.8.
R.F.
LE.
FIG. 2 (on the left). The right incus. A. Internal aspect. B. External aspect. P. B., Processus brevis; S. C., Stapedial crus; S.A., Sylvian apophysis ; R. F. , Ridge between the articulating facets. FIG. 3 (on the right). The right stapes. Internal aspect. H., The head; S., Surface for attachment of the stapedies tendon; A.C., Anterior crus ; 1., Interclural canal; V.B., Vestibular surface ofthe base; E.E., External edge.
This slight concavity seems to be the trace of a much greater concavity which occurs in the same situation in some other mammals. In the badger (Meles taxus) it forms a deep and widely open channel, and in the mole (Talpa europcea) the stapedial crus, which occupies three-fifths of the bulk of the entire bone, is excavated by a very large cavity, wh ich, with the sm all body at one end and a rounded epiphysis at the other, gives the incus of this animal the appearance of a life boat. Thc Sylvian apophysis of the incus (the os orbiculare or the os Ienticulare Sylvii of so me authors), is a beautifully formed elliptical
GENERAL ARTICLES.
disc of bone which is attached to the hooked extremity of the stapedial crus of the incus by a strong pedunde. The attachment is not great even in the adult, and in many cases in the course of preparation this gives way, be fore the ligament of the incudo-stapedial joint. The Stapes.- This osside (Fig. 3) shows the most strongly marked generic characters. It is relatively large in size, trapezoid al in form, with a large head, thick and hollow crura, and a typical oval intercrural canal of smalJ size. The total height of the bone, measured along its anterior crus from the base to the head, is nearly 3 mm. and that along the posterior crus 2' 50 mm. The head of the bone is large, with a ridge which slightly overhangs the crura, especiaJly the posterior crus. This ridge of the head though well-marked anteriorly becomes much thickened posteriorly, where on the inner aspect it forms a roughened surface for the attachment of the tendon of the stapedius musde. Tbe anterior crus, as seen by the measurements given above, is longer than the posterior, and from its attachment to the base it indines a little backwards. The posterior crus has a slightly concave posterior edge which is overhung by a sharp angular projection of the head of the bone. There is no large foramen perforating the anterior crus such as is seen in many mammals, notably the commOll badger, where it is of considerable size. The crura are deeply channelled towards the canal, and this hollowing is continued under the head and above the base. The aperture of the intercrural canal has a regular oval form and a long diameter of I mm. The base of the stapes is tbick and slightly concave towards the vestibule, and a striking feature of it is the very square-cut formation of its edges. These edges extend considerably beyond the limits of the insertion of the crura, and the form of its vestibular surface is an irregular ovoid, the characters of which are shown in Fig. 3. The morphological variations most commonly seen in the ossicula of adult sheep, apart, of course, from true abnormalities, point rather to the retention of embryo characters without any arrest of actual normal growth. Most, if not all , of these variations may be traced to the forms which are constant in the newly born lamb. In the lamb at birth the manubrium is much more curved anteriorly upon itself than is the case in the adult, the processus brevis is shorter and less rectangular, the body of the incus much less deep and its crura are far more equal in length, the stapes is much more triangular in form, its head smaller, and its intercrural canal considerably larger and more circular. These differences will, I think, be generaJly found to constitute the basis of most of the variations in the adult. The scale of the figures is -l- inch to I mm. X2
THE OSSICULA AUDITUS OF THE SHEEP (OVIS ARIES).
By H. C. WILKIE, F.R.C.V.S., F.R.M.S., F.Z.S. THE auditory ossicles of the domestic sheep are of special anatomical interest on account of their morphologicaI variations, not only from the newly born lamb to the adult, but also between individual animals of the same breed and of similar weight and age. These notes relate only to adult sheep, and, in endeavouring to determine the exact anatomical features which may be taken as characteristic of the species, I have noted those characters which may, I believe, be accepted as truly typical, apart from the many and interesting variations which occur. The lIIalleus.- The most prominent general features of the malleus (Fig. I) are the flattened form of the head, the strongly curved and blunt processus muscularis, the almost rectangular processus brevis. and the two angles which the manubrium forms with the neck, the directions taken by this process being forward and inward. A straight line drawn from the summit of the head to the processus brevis measures nearly 6 mm., and another drawn from the processus brevis to the terminal point of the manubrium measures fuHy 5'50 mm. The head is not globlllaT on its anterior surface, but is slightly flattened, and its round edges have the appearance of being somewhat pressed out against the edges of the articulatory surface. This surface has a postero-internal aspect, and its facets are two in number, the upper of which is broad and nearly semi-circular, and the lower one from a broad base descends to a somewhat pointed and curved extremity. The upper facet shows a slight median vertical ridge, and the lower, a ridge which inclines from its base to a point on the outer margin, where the articular surface begins to narrow. The neck of the malleus is lightly curved, and the lamina, which originates superiorly, weIl up on the base of the head on the externat side, extends over a distance of 1'80 mm. to meet the point of the processus gracilis. The free margin of the lamina is markedly convex. The processus gracilis originates at the base of the neck and takes a long sweep upward and forward to terminate in the point above mentioned, which buries itself in the opening of the Glaserian fissure. The anterior ligament of the malleus (Iaxator tympani major of Sömmerring) is attached to this point, and for some little distance along the anterior edge of the process. The processus muscularis appears to take its root immediately beneath the lower margin of the inferior articulatory facet, and from that point describes nearly a semicircle, to terminate on ~he anterior part of the inner surface of the neck in a blunt, well-rounded, and prominent extremity to which the tendon of the Tensor tympani muscle is attached.
299
GENERAL ARTICLES.
The processus brevis is represented by a strangely formed angular projection on the external surface of the neck near the base of the manubrium. It gives attachment to the posterior ligament of the malleus. The drawing of the malleus is made from the inner side, as this shows most of the more important characteristics of the ossicle, but that aspect does not show the fuH character of the manubrium, because it is thus seen on its edge. This edge, broad at the base, ta pers down in its centre to terminate in a very slightly spatulate extremity, which is curved a Httle outwards. The whole manubrium is, however, flattened from side to side soon after leaving its somewhat cylindrical base, and from an anterior or posterior aspect exhibits the double curvature and pointed extremity shown in Fig. I (lower figure).
P.S
FIG.
J.
Upper figure. Right malleus-internal aspect. U.F., Upperfacet; L.F., Lowerfacet; L., Lamina; P.G., Processusgracilis; P.M., Processus muscularis; P. B., Processus brevis ; M., Manubrium. Lower figure.
Lateral aspect of the manubrium from its base immediately below the processus brevis. P. B., Processus brevis; I. E., In ternal edge; E. E., External edge.
The Incus.-The most prominent characteristics of this bone (Fig. z) are its strong and bulky body, its long and deep articulatory surface, the wide divergence of its crura, and the anomalous condition that the processus brevis is considerably longer than the processus longus or stapedial crus. The long diameter of the body is from the highest point of the articular ridge to midway between the crura, and this has a measurement of z'z5 mm. The articulatory surface consists of two facets, separated by a high ridge. The upper facet is wide and semi-circular in form, and the
3 00
GENERAL ARTICLES.
Iower facet is broad at its base (the ridge ), and becomes somewhat pointed at its lower extremity, just above the base of the stapedial crus. The processus brevis is wide at its base, which is a littie angular on the external surface, and tapers down almost immediately, becoming cylindrical in form and taking a nearly straight upward and backward direction towards its termination in a rounded peg-like extremity to which is attached the posterior ligament of the incus. The stapedial crus is widely divergent from the processus brevis and takes a curved direction from its base forwards and inwards. It is at first nearly cylindrical in form, but towards its extr.e mity (which is only at a distance of I mm. from the base) it becomes flattened, then slightly concave internally, and turning inwards forms a shell-like hooked extremity.
""""""'-\1.8.
R.F.
LE.
FIG. 2 (on the left). The right incus. A. Internal aspect. B. External aspect. P. B., Processus brevis; S. C., Stapedial crus; S.A., Sylvian apophysis ; R. F. , Ridge between the articulating facets. FIG. 3 (on the right). The right stapes. Internal aspect. H., The head; S., Surface for attachment of the stapedies tendon; A.C., Anterior crus ; 1., Interclural canal; V.B., Vestibular surface ofthe base; E.E., External edge.
This slight concavity seems to be the trace of a much greater concavity which occurs in the same situation in some other mammals. In the badger (Meles taxus) it forms a deep and widely open channel, and in the mole (Talpa europcea) the stapedial crus, which occupies three-fifths of the bulk of the entire bone, is excavated by a very large cavity, wh ich, with the sm all body at one end and a rounded epiphysis at the other, gives the incus of this animal the appearance of a life boat. Thc Sylvian apophysis of the incus (the os orbiculare or the os Ienticulare Sylvii of so me authors), is a beautifully formed elliptical
GENERAL ARTICLES.
disc of bone which is attached to the hooked extremity of the stapedial crus of the incus by a strong pedunde. The attachment is not great even in the adult, and in many cases in the course of preparation this gives way, be fore the ligament of the incudo-stapedial joint. The Stapes.- This osside (Fig. 3) shows the most strongly marked generic characters. It is relatively large in size, trapezoid al in form, with a large head, thick and hollow crura, and a typical oval intercrural canal of smalJ size. The total height of the bone, measured along its anterior crus from the base to the head, is nearly 3 mm. and that along the posterior crus 2' 50 mm. The head of the bone is large, with a ridge which slightly overhangs the crura, especiaJly the posterior crus. This ridge of the head though well-marked anteriorly becomes much thickened posteriorly, where on the inner aspect it forms a roughened surface for the attachment of the tendon of the stapedius musde. Tbe anterior crus, as seen by the measurements given above, is longer than the posterior, and from its attachment to the base it indines a little backwards. The posterior crus has a slightly concave posterior edge which is overhung by a sharp angular projection of the head of the bone. There is no large foramen perforating the anterior crus such as is seen in many mammals, notably the commOll badger, where it is of considerable size. The crura are deeply channelled towards the canal, and this hollowing is continued under the head and above the base. The aperture of the intercrural canal has a regular oval form and a long diameter of I mm. The base of the stapes is tbick and slightly concave towards the vestibule, and a striking feature of it is the very square-cut formation of its edges. These edges extend considerably beyond the limits of the insertion of the crura, and the form of its vestibular surface is an irregular ovoid, the characters of which are shown in Fig. 3. The morphological variations most commonly seen in the ossicula of adult sheep, apart, of course, from true abnormalities, point rather to the retention of embryo characters without any arrest of actual normal growth. Most, if not all , of these variations may be traced to the forms which are constant in the newly born lamb. In the lamb at birth the manubrium is much more curved anteriorly upon itself than is the case in the adult, the processus brevis is shorter and less rectangular, the body of the incus much less deep and its crura are far more equal in length, the stapes is much more triangular in form, its head smaller, and its intercrural canal considerably larger and more circular. These differences will, I think, be generaJly found to constitute the basis of most of the variations in the adult. The scale of the figures is -l- inch to I mm. X2