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The normal adult cranial vault
The Normal Adult Cranial Vault lsadore Meschan, M.D.
T
HE NORMAL cranial vault is an interesting composite of intramembranous ossification, serving the important function of buttressing traumatic impacts, thereby shielding the brain. The calvarium, or cranium, is composed of eight bones: the paired parietal and temporal bones, and the frontal, occipital, sphenoid, and ethmoid bones, which are single. These are largely flat bones composed of a dense outer layer, known as the outer table; a less dense middle zone, called the diplo& and a dense inner zone known as the inner table. The calvarial thickness varies widely among different adults, averaging about 5 mm. The thickest part is at the external occipital protuberance where the bone is often 1.8 cm.’ The normal cranial vault should not exceed 1 cm; if it measures more, some degree of cerebral underdevelopment or a systemic disease, active or healed, should be suspected.3 Usually, the outer table is somewhat thicker than the inner, and is quite smooth and uniform. The inner table presents numerous depressions for vessels, arachonoidal granulations, dura mater, and the brain. The diploe is spongy and yet strong. The outer table tends to be thicker anteriorly when the frontal sinuses are not well developed. Often, it is thin along the parietal eminences. It may be perforated by many venous structures. The thinnest part of the skull vault is the squamous portion of the temporal bone. Other weak spots are the medial and superior walls of the orbit, the cerebellar fossae, the cribrose plate of the ethmoid, the tegmen tympani, and that part of the middle fossa underlying the mandibular fossa. Temporal enostoses are small bony ridges on the inner table of the squamous portion of the temporal bone, frequently seen along the inferior half. Usually, they are triangular in shape. They tend to project over the sellar region, and must not be misinterpreted as parasellar or suprasellar calcification. The bone of the cranial vault is supplied chiefly by the meningeal vessels (Fig. 1). In this respect, Isadore Meschan, M.D.: Director,
Department of RadiThe Bowman Gray School of Medicine of Wake Forest University, Winston-Salem, N. C. 60637. 0 1974 by Grune & Stratton, Inc. ology,
Seminarsin
Roenfgenology,
Vol.
IX, No. 2 (April),
1974
the pericranium differs from the periosteum elsewhere in the body in that it gives very little arterial supply to the bone. The pericranium adheres chiefly to the sutures of the cranial bones, so that a subpericranial effusion is limited by the sutures. GROWTH
AND
DEVELOPMENT
Most of the growth and differentiation of the skull after birth occurs during the first 2 yr of life. Growth is slowest from the seventh year until puberty. There is then a spurt of growth for about 2 yr. Not until about the twentieth year does the skull attain its final size and shape. The major grooves on the bones of the calvarium make their appearance during the first 2 yr of life. The major sutures of the cranial vault usually persist throughout infancy and childhood and do not begin to close until about the thirtieth year. A persistent metopic suture occurs in about 10% of the population. In these individuals, the frontal sinus is often underdeveloped unilaterally or bilaterally. LINES,
IMPRESSIONS, CHANNELS, AND SUTURES’
Various lines and impressions in the adult cranial vault are of great radiographic significance. These will be individually considered, irrespective of the bones in which they usually occur; thereafter the individual bones of the cranial vault will be considered in some detail. Granular Pits or Arachonidal (Pacchionian) Granulation Impressions (Fig. 2)6 These are small irregular parasagittal impressions that lodge the arachnoidal granulations. They are most frequently found adjoining the superior sagittal sinus. Occasionally, they are located more laterally along the meningeal or diploic veins. Not infrequently, they give even the experienced observer considerable difficulty in distinguishing them from abnormal areas of bone absorption They are occasionally found in the occipital bone. Usually, only the inner table of the bone shows indentations and small vascular channels can be identified leading into them.
125
126
ISADORE
MESCHAN
Fig. 1. Diagrammatic and radiographic appearance of arterial and venous impressions on the bones of the calvarium. (A) Diagram of meningeal artery circulation. The middle meningeal artery arises from the external carotid (EC), passes through the foramen spinosum of the sphenoid bone and enters the cranium. It runs forward in a groove on the great wing of the sphenoid bone and divides into two branches, the anterior and posterior. The anterior branch crosses the great wing of the sphenoid and then divides into branches that spread out between the dura mater and internal surface of the cranium. Some of its branches pass upward as far as the vertex of the skull, and others backward to the occipital region. The posterior branch curves backward on the squamous portion of the temporal bone and reaches the parietal, where its branches supply the posterior part of the dura mater and cranium. The various anastomoses of the branches of the middle meningeal are numbered. 1 indicates anastomoses with branches of the pericallosal artery; 2 represents anastomoses with branches of the middle cerebral artery that reach the calvarium; 3 is the site of anastomoses with the meningeal branches of the ophthalmic artery (On), which arises from the internal carotid artery (IC) at the carotid siphon; 4 represents anastomoses be tween the posterior and anterior branches of the middle meningeal; 5 indicates anastomoses between the meningeal branches of the ascending pharyngeal artery and the posterior branch of the middle meningeal; 5 is the site of anastomoses between the posterior branch of the middle meningeal and the occipital artery and its meningeal branches (0). The vertebral artery (V) sends out meningeal branches that anastomose with those of the ascending pharyngeal and posterior branch of the middle meningeal.
Arterial Grooves; Superior and Infetior Cerebral Veins A diagram of the arteries supplying the cranium with special reference to their communications is shown in Fig. 1. There are three separate and intercommunicating vascular systems: the cerebral and cerebellar, the meningeal-osseous and the vesselsto the superficial soft parts and bone (Fig. 1). The entire venous system is shown in Fig. 3.’ The most important vessels of the meningeal-osseous system are the middle meningeal arteries and their accompanying middle meningeal veins, and the diploic veins. The subcutaneous, muscular, and periosteal arteries and veins, comprising the third system, become especially important at muscular
attachments and at the emissary foramina, to be considered later. There are numerous intercommunications between the three systems. The main trunks of the cerebral arteries at the base of the skull supply small meningeal and dural communications. There are also parasagittal branches to the dura in the tentorial region. There are similar communications with the ophthalmic arteries and with the arteries of the auditory apparatus. On the venous side, the intercommunications occur at the base, parasagittally, at the tentorium, and at the falx. These communications become particularly important under pathologic conditions. There are no grooves in the calvarial vault cor-
THE NORMAL
ADULT
CRANIAL
VAULT
127
Fig. 1. (6) Diagram showing the main middle meningeal veins. Generally, the middle meningeal veins accompany the middle meningeal arteries and thus there is an anterior and a posterior branch complex, as in Fig. 1A. The dotted lines indicate some of the main diploic veins that can be identified on many skull films (D). The “frontal star” (FS) is a cluster of frontal diploic veins that ultimately anastomose with the middle meningaal. The “parietal star” (PS) is a similar collection in the parietal bone that ultimately draws into the middle meningeal, posterior or anterior. The lacunae (L) are venous lakes that communicate with the middle meningeal veins. Arachnoid granulations generally protrude into the lacunae. PP, pharyngeal plexus.
responding to the cerebral arteries,5 but the cerebral veins, especially the superior cerebral veins, may lie in distinct grooves in the calvarium. They extend 1 cm or more from the border of the superior sagittal sinus, and are highly variable (Fig. 4). Most often, these grooves lie posterior to the bregma. However, in the presence of hyperostosis frontalis interna, similar grooves may appear on the inner aspect of the frontal bone. The inferior cerebral veins in the occipital region may, on occasion, have the same relationship to the transverse sinus as the superior veins have to the superior sagittal sinus. Distinct bone grooves for these veins may occasionally be identified adjacent to a transverse sinus. The middle meningeal artery can be followed from its entry into the base of the skull through the foramen spinosum more or less completely out to its third and fourth order branches (Fig. 1A). The grooves for the meningeal arteries and veins fan out from the foramen. They become relatively deeper and more visible with increasing age and accompanying deposition of bone on the internal table.5 The arteries’ share of the space in this sulcus, relative to that of the veins, varies considerably from patient to patient. In the posterior
groove, the artery takes up a relatively larger portion; the reverse is often true anteriorly. The grooves have irregular outlines, branch dichotomously, and diminish progressively in size as they extend distally. After it leaves the foramen spinosum, the main middle meningeal groove traverses the squamous temporal bone, and divides into an anterior and posterior branch, both of which are clearly visible on the parietal bones. The margins of these grooves usually have a slightly greater density than the surrounding bone. The course of the arteries is relatively smooth and undulating-never sharply angled, as in a fracture. Toward the midline, the groove may become slightly wider again, taking a less regular course, and here it becomes transformed chiefly into a venous groove. Tortuosity of the middle meningeal grooves may be found at any age in the region where the posterior branch crosses the temporoparietal suture. Lindblom has measured these grooves as follows: a distinct arterial groove at the pterion has an average breadth of 1.5 mm with a maximum of 2 mm. The groove for the posterior main trunk measures 1 mm with a maximum of 2 mm. He stated, “Any marked difference in the size of the
ISADORE
128
MESCHAN
Fig. 1. (Cl Lateral view demonstrating vascular impressions: 1, parieta1 star; 2, middle meningeal artery and vein, anterior division; 3, parietal star; 4, middle meningeal vein, posterior branch; 5, lateral sinus; 6, meningeal communications with the ophthalmic artery and vein. (D) PA demonstrating some of the venous impressions. 1, venous lacuna, showing the branching veins communicating with it; 2, arachnoid granulation impression; 3, diploic venous impressions.
grooves on the two sides is abnormal.“’ Arterial grooves may be differentiated from venous grooves in the calvarium in that they tend to taper progressively as they ascend toward the superior aspect of the skull. Venous grooves change very little in caliber throughout their course. There are two vascular grooves along the outer table of the cranial vault that have a straight linear appearance and may be confused with fracture. One is the supraorbital groove, said to be caused by the supraorbital artery; the other is in the outer table of the squamous portion of the temporal bone just above the external auditory meatus and anterior to it and is said to be produced by the
middle temporal branch of the superficial temporal artery (Fig. 5). These grooves tend to taper and branch as do other arterial grooves.3712 Venous Sinuses The impressions of the venous sinuses on the inner table of the skull appear as radiolucent channels bounded by curved bony ridges. The lateral or transverse sinus is the largest of these, and has its origin near the internal occipital protuberance, passing forward around the occipital bone with a slight upward convexity, to the pneumatized portion of the mastoid bone. Here it curves downward to become the sigmoid sinus.
THE NORMAL
Arathnold
ADULT
bob&a.
CRANIAL
VAULT Dwa m&r.
Arochnotd v~llus.
olx cerebri.
CO
Fig. 2. Arachnoid granulations. (A) Schematic diagram of a coronal section of the meninges and cerebral cortex. (El and Cl Close-up in Towne view and in lateral projection, respectively. (From Meschan 1;2,6 reproduced with permission of the WB Saunders Co., Philadelphia, Pa.)
A groove for the superior sagittal sinus may be found in the occipital, frontal, or parietal region. The frontal and parietal locations are most frequent beyond 10 yr of age, whereas the occipital is more common in children under 10 yr. The width of the superior sagittal sinus grooves, as measured roentgenographically, is usually 2-3 mm frontally; 6-g mm parietally; and 7-10 mm in the occipital region.5 The right transverse sinus groove is usually larger and more distinct than the left. The point of junction of the lateral sinuses and the superior sagittal sinus is the point of confluence of virtually all of the major dural sinuses. Hence, it is called the sinus confluens. It has a variable appearance, resembling a crossroad with or without a bony island within it (Fig. 6). The groove for the sigrnoid sinus is practically always identifiable, and its actual width is usually l-2 mm greater than the adjoining transverse sinus. The sphenoparietal sinus is another ppinent venous sinus. It begins in connection with the anterior parietal diploic vein just posterior to the coronal suture and courses along the inferior sur-
face of the lesser wing of the sphenoid to become a tributary of the cavernous sinus. It should be noted that the size of the groove roentgenographically is not the true size of the lumen of the venous sinus, because the shape of the cross section of the sinus is not constant. Meningeal Veins The arteries in bone canals are generally accompanied by veins (Fig. 1). Each canal usually contains two meningeal veins which are disproportionately small when compared with the accompanying artery, except in those instances noted previously. In the parasagittal region, the veins often increase in size and open into the superior sagittal sinus. The anterior branch of the middle meningeal artery extending toward the bregma has an accompanying vein that is usually larger than the artery. Especially in older individuals it lies in a prominent deep groove. In the parasagittal region, the meningeal veins gradually form lateral lacunae, and the pacchionian granulation impressions in the bone indicate this portion of their course. Normally, there may be a difference of
ISADORE
Fig. 3. Diagram of the veins and dural duced with permission of Acfa Radiologica.)
sinuses
of the cranium
1 mm in diameter between the veins on the two sides, When larger differences are found, pathology should be suspected. The Lateral Lacunae These should be considered widenings of the parasagittal portions of the meningeal veins.” They are usually lacking at birth, as are the pacchionian granulations, and develop later to form large irregular spaces tilled with these granulations. The lacunae are widest where the meningeal veins empty into them. They communicate with neighboring cerebral veins and with the vein of Trolard. The bone depressions for the lacunae are most clearly seen radiographically in an axial projection of the vertex. Somewhat similar lengthened depressions may be produced by sinusoidal cerebral veins and at times they are difficult to differentiate, particularly on lateral view alone. Lliploic Veins The impressions of the venous plexuses in the dip105 (Fig. 7) are visible in the frontal, parietal, and occipital bones, giving the skull a mosaic ap-
and their
communications.
(From
Lindblom
MESCHAN
K;s repro-
pearance on the radiograph. At times, these veins appear unusually accentuated. In the parietal bones particularly, they have been referred to as the “star” or “spider” (Fig. 1C). The diploic veins should be considered the nutrient veins of the calvarium. Because they contain thin bony lamellae and have very little resistance, the veins may widen rather quickly. It is hazardous to interpret abnormality on the appearance of the diploic veins alone. The number and position of the diploic veins are not constant. Often, one or more of them is lacking on one or both sides, and on occasion they appear to be doubled. They tend to collect into trunks and end in small emissary veins lying at sites of muscle insertion. They communicate with one another, independent of sutures, even traversing the midline. They also tend to communicate with the meningeal veins, lateral lacunae, venous sinuses, and cerebral veins. Diploic veins usually begin to be manifest at about 5 yr of age.’ Between 5 and 30 yr of age, one or more diploic veins may reach widths as great as 3 mm. After age 30, this is not at all in-
THE NORMAL
ADULT
CRANIAL
131
VAULT
@
83
Fig. 4. Line diagram showing variations of the normal impressions duced by superior cerebral veins adjoining the sagittal sinus. (After blom K;5 reproduced with permission of Acta Radiologica.)
frequent in the frontal or parietotemporal regions. A difference in size between corresponding veins on the two sides is found in about 5% of individuals, but usually this does not exceed 1 mm.’ Besides diploic veins, there are diploic lakes, which appear as irregular, well-demarcated, oval or round areas of radiolucency rarely exceeding 2 cm in greatest diameter. They are most common in the parietal bones.
proLind-
Emissary Veins In the cranial vault, emissary veins are most frequent in the region of the occipital protuberance and in the parasagittal and posterior aspect of the parietal bones. These represent anastomotic channels between intracranial and extracranial vascular systems. Some of the emissary veins achieve considerable size.
Fig. 5. superficial
The middle temporal branch temporal artery (arrows).
of
the
132
ISADORE
MESCHAN
Fig. 6. Schematic illustration of variations in the sinus confluens. The superior sagittal sinus (a), the occipital sinus (bl and right and left transverse sinus (c and d) are shown. The internal occipital protuberance lies at the crossroad position. The arrows indicate the probable direction of venous flow in the various configurations. (From Pendergrass EP, et al;” reproduced with permission of Charles C Thomas, Publisher, Chicago, Ill.)
At times a canal up to 4 cm in length runs in the diploe from the superior sagittal sinus laterally and downward to the region of the incisura supraorbitalis. This contains an emissary vein coming directly from the sinus, usually resembling a diploic vein. This canal may vary in width up to 2 mm, and is usually found on only one side. The foramen parietale is situated just above the lambda, about l-2 cm from the midline. It usually does not exceed 1 mm in diameter, but in exceptional cases it may increase to as much as 1 cm. Also at the internal occipital protuberance, one or more fine openings are found for canals communicating with the diploe. Normally, these canals and foramina have a width not exceeding 2 mm. The mastoid foramen contains an emissary vein connecting the sigmoid sinus and the veins of the adjacent superficial soft parts. This foramen may also contain a small branch of the occipital artery. At times, the mastoid foramen is actually a canal several cm in length. It may be from l-10 mm wide. It is important that it be identified prior to surgery, since this may help avoid unnecessary bleeding. The parietal bone may also contain en-
larged parietal foramina (Fig. 8). These foramina are ordinarily extremely small, no larger than 1 mm, and transmit an emissary vein. Abnormally large foramina occasionally occur, apparently as an inherited trait. The foramina are ordinarily situated close to the sagittal suture, about 2.5-3.5 cm above the lambdoid suture, just medial to the parietal tuberosity. Arteries and Veins of the Superficial Soft Parts These vascular structures are all external to the galea and therefore do not produce grooves in the calvarium. The only exceptions are the groove in the incisura supraorbitalis, which contains a vessel, and the deep groove for the occipital artery just medial to the mastoid process (Figs. 1 and 3). The Sutures A number of important sutures occur in the superior portion of the skull: The sagittal suture marks the junction of the two parietal bones superiorly, and extends from the bregma, its junction with the coronal suture, to the lambda, its junction with the lambdoid suture.
THE NORMAL
ADULT
CRANIAL
VAULT
Fig. 8.
Enlarged
The coronal suture is situated between the parieta1 bones and the frontal bone. It ends laterally by joining the sphenoid bone; the point of union is known as the pterion. The lambdoidal suture is situated between the parietal bones and the occipital bone. Its point of junction with the squamosal suture is known as the asterion. When the interparietal portion of the occipital bone exists as an independent element (the Inca bone), a transverse occipital suture is formed between it and the occipital bone proper (Fig. 9C, page 109). The two halves of the frontal bone normally fuse together by the fifth or sixth year of childhood. The line of fusion normally disappears. However, in about 10% of individuals it persists as the metopic suture (Fig. 3, page 117). This extends from the frontonasal suture to the coronal. Occasionally, incomplete fusion of the metopic suture occurs
Fig. 9. sphenoid, permission
par&al
foramina.
and leads to the erroneous interpretation of skull fracture. On the lateral aspect of the skull, the sphenofrontal, sphenoparietal, and squamosal sutures form a continuous irregular line, outlining the upper outer margin of the greater wing of the sphenoid and the squamous temporal bone. These extend from the posterior margin of the frontal bone to the occipital bone. There are several important variations in the interrelationship of the frontal, temporal, sphenoid, and parietal bones. Occasionally, the sphenoparietal suture is lacking, and the coronal suture is directly continuous with the suture between the greater wing of the sphenoid and the squamous temporal bone. At other times, there is an extra bone at this crossing known as the epipteric bone, which is a sutural or wormian bone. Other variations are shown in Fig. 9. The sphenozygomatic suture is situated between
Variations in the interrelationship of the frontal, temporal and parietal bones. (From Pendergrass EP;ll reproduced with of Charles C Thomas, Publisher, Chicago, Ill.)
134
ISADORE
the zygomatic bone and the greater wing of the sphenoid. The zygomaticofrontal suture is situated between the frontal process of the zygoma and the inferolateral portion of the frontal bones. The zygomaticotemporal suture joins the zygomatic processes of the zygomatic and temporal bones. It is usually situated at the junction of the anterior and middle thirds of the zygomatic arch. Extending horizontally from the squamosal suture to the lambdoid suture is a small irregular suture that remains rather prominent throughout life. It is known as the parietomastoid suture. It is situated between the mastoid process of the temporal bone and the parietal bone. On the posterior aspect of the mastoid process, between it and the occipital bone, is the occipitomastoid suture. Sutural Sclerosis There is often a dense band of sclerosis along sutures due to a continuous bridging process across the suture that is taking place. These vary in width up to 15 mm. This is a normal physiologic process that becomes more apparent with advancing age. DENSITY ALTERATIONS THE CALVARIUM
IN
MESCHAN
striking and readily visible. Beyond this age, or when unduly accentuated in younger children, digitate markings may indicate increased intracranial pressure. Parietal Thinning The parietal bones, particularly superiorly and laterally, may be unusually thin, owing to a lack of development of the diploe, the inner table being less affected than the outer. There may even be bony dehiscence. This thinning is usually bilateral and symmetrical, but occasionally is unilateral. Usually it is a normal variant, but occasionally it has pathologic significance. 7hickened Areas The internal and external occipital protuberances, as they project from the occipital bone, thicken the bone in this location. Occasionally, the external protuberance forms a large bony process. Not infrequently there is thick bone immediately above the frontal sinuses and in the vicinity of sutural junctions. Occasionally a small exostosis may occur in the external acoustic meatus. It may appear as a minute nodule or fill the entire meatus. It is usually bilateral.
Digitate or Convolutional Markings Irregular areasof increased and decreased density are found throughout the skull due to thinning of the inner table, as the result of pressure produced by the convolutions or gyri of the brain (Fig. 10, page 123). In normal individuals under approximately 16 yr of age, these may be moderately
Fig. 10. Line diagram demonstrating nuchal ridges on the occipital bone.
the
superior
and
MISCELLANEOUS
BONE
RIDGES
Occipital Bone In the occipital bone, the superior and inferior nuchal lines give origin to prominent muscles. Detectable ridges of bone are seen on the roentgenogram (Fig. 10).
inferior
Occipital Bone
THE NORMAL
ADULT
CRANIAL
VAULT
Fig. 11. Line diagram showing the superior inferior temporal ridges of the parietal bones. dotted areas indicate the parietal fossa.
and The
Parietal Bone The superior and inferior temporal lines on parietal bone are the sites of attachment of temporal fascia and muscle. These lie below parietal eminence, the point where ossification the parietal bone begins (Fig. 11).
the the the of
Frontal Bone The superciliary arch just above the supraorbital foramen or notch and the supraorbital margin, is a readily apparent prominence. The arches of the two sides are separated by the glabella (Fig. 12). The lateral extremity of the supraorbital margin ends in the zygomatic process, which articulates with the zygomatic bone. The temporal line (Fig. 12) converges upon the zygomatic process. It rep-
resents the anterior end of the superior and inferior temporal lines of the parietal bone previously described. Sphenoid Bone The sphenoid bone is, for the most part, situated in the base of the skull and thus is outside the scope of this presentation. Its temporal surface, however, is divided by a prominent infratemporal crest into a superior part, forming part of the wall of the temporal fossa and affording attachment to the temporal muscle, and an inferior part forming part of the wall of the infratemporal fossa and giving attachment to the lateral pterygoid muscle. The infratemporal crest is usually readily identified in lateral radiographs of the skull.
Fig. 12. Line diagram demonstrating the superciliary of the frontal bone and the temporal line.
arch
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136
Temporal Bone
MESCHAN
SEX DIFFERENCES
The temporal bone is situated at the side and base of the cranium. Its squamous portion is thin and expanded and forms the lateral aspect of the middle fossa. The temporal zygomatic process forms the base of the zygomatic arch. Its petrous and tympanic portions are not part of the cranial vault. Superior to the external acoustic meatus there is a vertical groove for the middle temporal artery (Fig. 5). The internal or cerebral surface of the squamous portion is marked by furrows for the convolutions of the brain, and grooves for the middle meningeal vessels. These have been previously described. The normal radiolucency of the squamous portion of the temporal bone may at times give the erroneous impression of complete absence of bone or perhaps increased intracranial pressure.
Until the age of puberty, there is little difference between the male and female skull. However, a woman’s skull is, as a rule, lighter and smaller and its cranial capacity about 10% less than a man’s.4 The walls of the female skull are thinner and its muscular ridge is less strongly marked; the glabella, superciliary arches, and mastoid processes are less prominent, and the air sinuses are smaller. The upper margin of the orbit tends to be sharper, the forehead more vertical, the frontal and parietal eminences more prominent, and the vault somewhat flatter than in men. At times, however, the characteristics are relatively indistinct, and sex determination by skull appearance is extremely difficult or impossible.
REFERENCES 1. Anson BJ: Morris’ Human Anatomy, a Comprehensive Systemic Treatise (ed 12). New York, McGraw-Hill, 1966 2. Bailey P: Intracranial Tumors (ed 2). Springfield, Ill, Thomas, 1948 3. Ethier R: Thickness and texture, in Newton TH, Potts DG (eds): Radiology of the Skull and Brain, vol 1. St. Louis, Mosby, 1971, pp 154-215 4. Lewis WH (ed): Gray’s Anatomy of the Human Body (ed 24). Philadelphia, Lea & Febiger, 1942 5. Lindblom K: A roentgenographic study of the vascular channels of the skull. Acta Radio1 (Stockh) Suppl. 30, 1936 6. Meschan I: An Atlas of Normal Radiographic Anatomy (ed 2). Philadelphia, Saunders, 1959, pp 229
7. Meschan I: Radiographic Positioning and Related Anatomy. Philadelphia, Saunders, 1968 8. Meschan 1: Analysis of Roentgen Signs in General Radiology. Philadelphia, Saunders, 1973, p 503 9. Meschan I: Anatomy Basic to Radiology. Philadelphia, Saunders (in press) 10. O’Connell JEA: Some observations on the cerebral veins. Brain 57:484-503,1934 11. Pendergrass EP, Schaeffer JP, Hodes PJ: The Head and Neck in Roentgen Diagnosis (ed 2). Springfield, III, Thomas, 1972 12. Schunk H, Maruyama Y: Two vascular grooves of the external table of the skull which simulate fractures. Acta Radio1 54:186-194,196O 13. Taveras JM, Wood JH: Diagnostic Neuroradiology. Baltimore, Williams &Wilkins, 1964
T
HE NORMAL cranial vault is an interesting composite of intramembranous ossification, serving the important function of buttressing traumatic impacts, thereby shielding the brain. The calvarium, or cranium, is composed of eight bones: the paired parietal and temporal bones, and the frontal, occipital, sphenoid, and ethmoid bones, which are single. These are largely flat bones composed of a dense outer layer, known as the outer table; a less dense middle zone, called the diplo& and a dense inner zone known as the inner table. The calvarial thickness varies widely among different adults, averaging about 5 mm. The thickest part is at the external occipital protuberance where the bone is often 1.8 cm.’ The normal cranial vault should not exceed 1 cm; if it measures more, some degree of cerebral underdevelopment or a systemic disease, active or healed, should be suspected.3 Usually, the outer table is somewhat thicker than the inner, and is quite smooth and uniform. The inner table presents numerous depressions for vessels, arachonoidal granulations, dura mater, and the brain. The diploe is spongy and yet strong. The outer table tends to be thicker anteriorly when the frontal sinuses are not well developed. Often, it is thin along the parietal eminences. It may be perforated by many venous structures. The thinnest part of the skull vault is the squamous portion of the temporal bone. Other weak spots are the medial and superior walls of the orbit, the cerebellar fossae, the cribrose plate of the ethmoid, the tegmen tympani, and that part of the middle fossa underlying the mandibular fossa. Temporal enostoses are small bony ridges on the inner table of the squamous portion of the temporal bone, frequently seen along the inferior half. Usually, they are triangular in shape. They tend to project over the sellar region, and must not be misinterpreted as parasellar or suprasellar calcification. The bone of the cranial vault is supplied chiefly by the meningeal vessels (Fig. 1). In this respect, Isadore Meschan, M.D.: Director,
Department of RadiThe Bowman Gray School of Medicine of Wake Forest University, Winston-Salem, N. C. 60637. 0 1974 by Grune & Stratton, Inc. ology,
Seminarsin
Roenfgenology,
Vol.
IX, No. 2 (April),
1974
the pericranium differs from the periosteum elsewhere in the body in that it gives very little arterial supply to the bone. The pericranium adheres chiefly to the sutures of the cranial bones, so that a subpericranial effusion is limited by the sutures. GROWTH
AND
DEVELOPMENT
Most of the growth and differentiation of the skull after birth occurs during the first 2 yr of life. Growth is slowest from the seventh year until puberty. There is then a spurt of growth for about 2 yr. Not until about the twentieth year does the skull attain its final size and shape. The major grooves on the bones of the calvarium make their appearance during the first 2 yr of life. The major sutures of the cranial vault usually persist throughout infancy and childhood and do not begin to close until about the thirtieth year. A persistent metopic suture occurs in about 10% of the population. In these individuals, the frontal sinus is often underdeveloped unilaterally or bilaterally. LINES,
IMPRESSIONS, CHANNELS, AND SUTURES’
Various lines and impressions in the adult cranial vault are of great radiographic significance. These will be individually considered, irrespective of the bones in which they usually occur; thereafter the individual bones of the cranial vault will be considered in some detail. Granular Pits or Arachonidal (Pacchionian) Granulation Impressions (Fig. 2)6 These are small irregular parasagittal impressions that lodge the arachnoidal granulations. They are most frequently found adjoining the superior sagittal sinus. Occasionally, they are located more laterally along the meningeal or diploic veins. Not infrequently, they give even the experienced observer considerable difficulty in distinguishing them from abnormal areas of bone absorption They are occasionally found in the occipital bone. Usually, only the inner table of the bone shows indentations and small vascular channels can be identified leading into them.
125
126
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MESCHAN
Fig. 1. Diagrammatic and radiographic appearance of arterial and venous impressions on the bones of the calvarium. (A) Diagram of meningeal artery circulation. The middle meningeal artery arises from the external carotid (EC), passes through the foramen spinosum of the sphenoid bone and enters the cranium. It runs forward in a groove on the great wing of the sphenoid bone and divides into two branches, the anterior and posterior. The anterior branch crosses the great wing of the sphenoid and then divides into branches that spread out between the dura mater and internal surface of the cranium. Some of its branches pass upward as far as the vertex of the skull, and others backward to the occipital region. The posterior branch curves backward on the squamous portion of the temporal bone and reaches the parietal, where its branches supply the posterior part of the dura mater and cranium. The various anastomoses of the branches of the middle meningeal are numbered. 1 indicates anastomoses with branches of the pericallosal artery; 2 represents anastomoses with branches of the middle cerebral artery that reach the calvarium; 3 is the site of anastomoses with the meningeal branches of the ophthalmic artery (On), which arises from the internal carotid artery (IC) at the carotid siphon; 4 represents anastomoses be tween the posterior and anterior branches of the middle meningeal; 5 indicates anastomoses between the meningeal branches of the ascending pharyngeal artery and the posterior branch of the middle meningeal; 5 is the site of anastomoses between the posterior branch of the middle meningeal and the occipital artery and its meningeal branches (0). The vertebral artery (V) sends out meningeal branches that anastomose with those of the ascending pharyngeal and posterior branch of the middle meningeal.
Arterial Grooves; Superior and Infetior Cerebral Veins A diagram of the arteries supplying the cranium with special reference to their communications is shown in Fig. 1. There are three separate and intercommunicating vascular systems: the cerebral and cerebellar, the meningeal-osseous and the vesselsto the superficial soft parts and bone (Fig. 1). The entire venous system is shown in Fig. 3.’ The most important vessels of the meningeal-osseous system are the middle meningeal arteries and their accompanying middle meningeal veins, and the diploic veins. The subcutaneous, muscular, and periosteal arteries and veins, comprising the third system, become especially important at muscular
attachments and at the emissary foramina, to be considered later. There are numerous intercommunications between the three systems. The main trunks of the cerebral arteries at the base of the skull supply small meningeal and dural communications. There are also parasagittal branches to the dura in the tentorial region. There are similar communications with the ophthalmic arteries and with the arteries of the auditory apparatus. On the venous side, the intercommunications occur at the base, parasagittally, at the tentorium, and at the falx. These communications become particularly important under pathologic conditions. There are no grooves in the calvarial vault cor-
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127
Fig. 1. (6) Diagram showing the main middle meningeal veins. Generally, the middle meningeal veins accompany the middle meningeal arteries and thus there is an anterior and a posterior branch complex, as in Fig. 1A. The dotted lines indicate some of the main diploic veins that can be identified on many skull films (D). The “frontal star” (FS) is a cluster of frontal diploic veins that ultimately anastomose with the middle meningaal. The “parietal star” (PS) is a similar collection in the parietal bone that ultimately draws into the middle meningeal, posterior or anterior. The lacunae (L) are venous lakes that communicate with the middle meningeal veins. Arachnoid granulations generally protrude into the lacunae. PP, pharyngeal plexus.
responding to the cerebral arteries,5 but the cerebral veins, especially the superior cerebral veins, may lie in distinct grooves in the calvarium. They extend 1 cm or more from the border of the superior sagittal sinus, and are highly variable (Fig. 4). Most often, these grooves lie posterior to the bregma. However, in the presence of hyperostosis frontalis interna, similar grooves may appear on the inner aspect of the frontal bone. The inferior cerebral veins in the occipital region may, on occasion, have the same relationship to the transverse sinus as the superior veins have to the superior sagittal sinus. Distinct bone grooves for these veins may occasionally be identified adjacent to a transverse sinus. The middle meningeal artery can be followed from its entry into the base of the skull through the foramen spinosum more or less completely out to its third and fourth order branches (Fig. 1A). The grooves for the meningeal arteries and veins fan out from the foramen. They become relatively deeper and more visible with increasing age and accompanying deposition of bone on the internal table.5 The arteries’ share of the space in this sulcus, relative to that of the veins, varies considerably from patient to patient. In the posterior
groove, the artery takes up a relatively larger portion; the reverse is often true anteriorly. The grooves have irregular outlines, branch dichotomously, and diminish progressively in size as they extend distally. After it leaves the foramen spinosum, the main middle meningeal groove traverses the squamous temporal bone, and divides into an anterior and posterior branch, both of which are clearly visible on the parietal bones. The margins of these grooves usually have a slightly greater density than the surrounding bone. The course of the arteries is relatively smooth and undulating-never sharply angled, as in a fracture. Toward the midline, the groove may become slightly wider again, taking a less regular course, and here it becomes transformed chiefly into a venous groove. Tortuosity of the middle meningeal grooves may be found at any age in the region where the posterior branch crosses the temporoparietal suture. Lindblom has measured these grooves as follows: a distinct arterial groove at the pterion has an average breadth of 1.5 mm with a maximum of 2 mm. The groove for the posterior main trunk measures 1 mm with a maximum of 2 mm. He stated, “Any marked difference in the size of the
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128
MESCHAN
Fig. 1. (Cl Lateral view demonstrating vascular impressions: 1, parieta1 star; 2, middle meningeal artery and vein, anterior division; 3, parietal star; 4, middle meningeal vein, posterior branch; 5, lateral sinus; 6, meningeal communications with the ophthalmic artery and vein. (D) PA demonstrating some of the venous impressions. 1, venous lacuna, showing the branching veins communicating with it; 2, arachnoid granulation impression; 3, diploic venous impressions.
grooves on the two sides is abnormal.“’ Arterial grooves may be differentiated from venous grooves in the calvarium in that they tend to taper progressively as they ascend toward the superior aspect of the skull. Venous grooves change very little in caliber throughout their course. There are two vascular grooves along the outer table of the cranial vault that have a straight linear appearance and may be confused with fracture. One is the supraorbital groove, said to be caused by the supraorbital artery; the other is in the outer table of the squamous portion of the temporal bone just above the external auditory meatus and anterior to it and is said to be produced by the
middle temporal branch of the superficial temporal artery (Fig. 5). These grooves tend to taper and branch as do other arterial grooves.3712 Venous Sinuses The impressions of the venous sinuses on the inner table of the skull appear as radiolucent channels bounded by curved bony ridges. The lateral or transverse sinus is the largest of these, and has its origin near the internal occipital protuberance, passing forward around the occipital bone with a slight upward convexity, to the pneumatized portion of the mastoid bone. Here it curves downward to become the sigmoid sinus.
THE NORMAL
Arathnold
ADULT
bob&a.
CRANIAL
VAULT Dwa m&r.
Arochnotd v~llus.
olx cerebri.
CO
Fig. 2. Arachnoid granulations. (A) Schematic diagram of a coronal section of the meninges and cerebral cortex. (El and Cl Close-up in Towne view and in lateral projection, respectively. (From Meschan 1;2,6 reproduced with permission of the WB Saunders Co., Philadelphia, Pa.)
A groove for the superior sagittal sinus may be found in the occipital, frontal, or parietal region. The frontal and parietal locations are most frequent beyond 10 yr of age, whereas the occipital is more common in children under 10 yr. The width of the superior sagittal sinus grooves, as measured roentgenographically, is usually 2-3 mm frontally; 6-g mm parietally; and 7-10 mm in the occipital region.5 The right transverse sinus groove is usually larger and more distinct than the left. The point of junction of the lateral sinuses and the superior sagittal sinus is the point of confluence of virtually all of the major dural sinuses. Hence, it is called the sinus confluens. It has a variable appearance, resembling a crossroad with or without a bony island within it (Fig. 6). The groove for the sigrnoid sinus is practically always identifiable, and its actual width is usually l-2 mm greater than the adjoining transverse sinus. The sphenoparietal sinus is another ppinent venous sinus. It begins in connection with the anterior parietal diploic vein just posterior to the coronal suture and courses along the inferior sur-
face of the lesser wing of the sphenoid to become a tributary of the cavernous sinus. It should be noted that the size of the groove roentgenographically is not the true size of the lumen of the venous sinus, because the shape of the cross section of the sinus is not constant. Meningeal Veins The arteries in bone canals are generally accompanied by veins (Fig. 1). Each canal usually contains two meningeal veins which are disproportionately small when compared with the accompanying artery, except in those instances noted previously. In the parasagittal region, the veins often increase in size and open into the superior sagittal sinus. The anterior branch of the middle meningeal artery extending toward the bregma has an accompanying vein that is usually larger than the artery. Especially in older individuals it lies in a prominent deep groove. In the parasagittal region, the meningeal veins gradually form lateral lacunae, and the pacchionian granulation impressions in the bone indicate this portion of their course. Normally, there may be a difference of
ISADORE
Fig. 3. Diagram of the veins and dural duced with permission of Acfa Radiologica.)
sinuses
of the cranium
1 mm in diameter between the veins on the two sides, When larger differences are found, pathology should be suspected. The Lateral Lacunae These should be considered widenings of the parasagittal portions of the meningeal veins.” They are usually lacking at birth, as are the pacchionian granulations, and develop later to form large irregular spaces tilled with these granulations. The lacunae are widest where the meningeal veins empty into them. They communicate with neighboring cerebral veins and with the vein of Trolard. The bone depressions for the lacunae are most clearly seen radiographically in an axial projection of the vertex. Somewhat similar lengthened depressions may be produced by sinusoidal cerebral veins and at times they are difficult to differentiate, particularly on lateral view alone. Lliploic Veins The impressions of the venous plexuses in the dip105 (Fig. 7) are visible in the frontal, parietal, and occipital bones, giving the skull a mosaic ap-
and their
communications.
(From
Lindblom
MESCHAN
K;s repro-
pearance on the radiograph. At times, these veins appear unusually accentuated. In the parietal bones particularly, they have been referred to as the “star” or “spider” (Fig. 1C). The diploic veins should be considered the nutrient veins of the calvarium. Because they contain thin bony lamellae and have very little resistance, the veins may widen rather quickly. It is hazardous to interpret abnormality on the appearance of the diploic veins alone. The number and position of the diploic veins are not constant. Often, one or more of them is lacking on one or both sides, and on occasion they appear to be doubled. They tend to collect into trunks and end in small emissary veins lying at sites of muscle insertion. They communicate with one another, independent of sutures, even traversing the midline. They also tend to communicate with the meningeal veins, lateral lacunae, venous sinuses, and cerebral veins. Diploic veins usually begin to be manifest at about 5 yr of age.’ Between 5 and 30 yr of age, one or more diploic veins may reach widths as great as 3 mm. After age 30, this is not at all in-
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CRANIAL
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VAULT
@
83
Fig. 4. Line diagram showing variations of the normal impressions duced by superior cerebral veins adjoining the sagittal sinus. (After blom K;5 reproduced with permission of Acta Radiologica.)
frequent in the frontal or parietotemporal regions. A difference in size between corresponding veins on the two sides is found in about 5% of individuals, but usually this does not exceed 1 mm.’ Besides diploic veins, there are diploic lakes, which appear as irregular, well-demarcated, oval or round areas of radiolucency rarely exceeding 2 cm in greatest diameter. They are most common in the parietal bones.
proLind-
Emissary Veins In the cranial vault, emissary veins are most frequent in the region of the occipital protuberance and in the parasagittal and posterior aspect of the parietal bones. These represent anastomotic channels between intracranial and extracranial vascular systems. Some of the emissary veins achieve considerable size.
Fig. 5. superficial
The middle temporal branch temporal artery (arrows).
of
the
132
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MESCHAN
Fig. 6. Schematic illustration of variations in the sinus confluens. The superior sagittal sinus (a), the occipital sinus (bl and right and left transverse sinus (c and d) are shown. The internal occipital protuberance lies at the crossroad position. The arrows indicate the probable direction of venous flow in the various configurations. (From Pendergrass EP, et al;” reproduced with permission of Charles C Thomas, Publisher, Chicago, Ill.)
At times a canal up to 4 cm in length runs in the diploe from the superior sagittal sinus laterally and downward to the region of the incisura supraorbitalis. This contains an emissary vein coming directly from the sinus, usually resembling a diploic vein. This canal may vary in width up to 2 mm, and is usually found on only one side. The foramen parietale is situated just above the lambda, about l-2 cm from the midline. It usually does not exceed 1 mm in diameter, but in exceptional cases it may increase to as much as 1 cm. Also at the internal occipital protuberance, one or more fine openings are found for canals communicating with the diploe. Normally, these canals and foramina have a width not exceeding 2 mm. The mastoid foramen contains an emissary vein connecting the sigmoid sinus and the veins of the adjacent superficial soft parts. This foramen may also contain a small branch of the occipital artery. At times, the mastoid foramen is actually a canal several cm in length. It may be from l-10 mm wide. It is important that it be identified prior to surgery, since this may help avoid unnecessary bleeding. The parietal bone may also contain en-
larged parietal foramina (Fig. 8). These foramina are ordinarily extremely small, no larger than 1 mm, and transmit an emissary vein. Abnormally large foramina occasionally occur, apparently as an inherited trait. The foramina are ordinarily situated close to the sagittal suture, about 2.5-3.5 cm above the lambdoid suture, just medial to the parietal tuberosity. Arteries and Veins of the Superficial Soft Parts These vascular structures are all external to the galea and therefore do not produce grooves in the calvarium. The only exceptions are the groove in the incisura supraorbitalis, which contains a vessel, and the deep groove for the occipital artery just medial to the mastoid process (Figs. 1 and 3). The Sutures A number of important sutures occur in the superior portion of the skull: The sagittal suture marks the junction of the two parietal bones superiorly, and extends from the bregma, its junction with the coronal suture, to the lambda, its junction with the lambdoid suture.
THE NORMAL
ADULT
CRANIAL
VAULT
Fig. 8.
Enlarged
The coronal suture is situated between the parieta1 bones and the frontal bone. It ends laterally by joining the sphenoid bone; the point of union is known as the pterion. The lambdoidal suture is situated between the parietal bones and the occipital bone. Its point of junction with the squamosal suture is known as the asterion. When the interparietal portion of the occipital bone exists as an independent element (the Inca bone), a transverse occipital suture is formed between it and the occipital bone proper (Fig. 9C, page 109). The two halves of the frontal bone normally fuse together by the fifth or sixth year of childhood. The line of fusion normally disappears. However, in about 10% of individuals it persists as the metopic suture (Fig. 3, page 117). This extends from the frontonasal suture to the coronal. Occasionally, incomplete fusion of the metopic suture occurs
Fig. 9. sphenoid, permission
par&al
foramina.
and leads to the erroneous interpretation of skull fracture. On the lateral aspect of the skull, the sphenofrontal, sphenoparietal, and squamosal sutures form a continuous irregular line, outlining the upper outer margin of the greater wing of the sphenoid and the squamous temporal bone. These extend from the posterior margin of the frontal bone to the occipital bone. There are several important variations in the interrelationship of the frontal, temporal, sphenoid, and parietal bones. Occasionally, the sphenoparietal suture is lacking, and the coronal suture is directly continuous with the suture between the greater wing of the sphenoid and the squamous temporal bone. At other times, there is an extra bone at this crossing known as the epipteric bone, which is a sutural or wormian bone. Other variations are shown in Fig. 9. The sphenozygomatic suture is situated between
Variations in the interrelationship of the frontal, temporal and parietal bones. (From Pendergrass EP;ll reproduced with of Charles C Thomas, Publisher, Chicago, Ill.)
134
ISADORE
the zygomatic bone and the greater wing of the sphenoid. The zygomaticofrontal suture is situated between the frontal process of the zygoma and the inferolateral portion of the frontal bones. The zygomaticotemporal suture joins the zygomatic processes of the zygomatic and temporal bones. It is usually situated at the junction of the anterior and middle thirds of the zygomatic arch. Extending horizontally from the squamosal suture to the lambdoid suture is a small irregular suture that remains rather prominent throughout life. It is known as the parietomastoid suture. It is situated between the mastoid process of the temporal bone and the parietal bone. On the posterior aspect of the mastoid process, between it and the occipital bone, is the occipitomastoid suture. Sutural Sclerosis There is often a dense band of sclerosis along sutures due to a continuous bridging process across the suture that is taking place. These vary in width up to 15 mm. This is a normal physiologic process that becomes more apparent with advancing age. DENSITY ALTERATIONS THE CALVARIUM
IN
MESCHAN
striking and readily visible. Beyond this age, or when unduly accentuated in younger children, digitate markings may indicate increased intracranial pressure. Parietal Thinning The parietal bones, particularly superiorly and laterally, may be unusually thin, owing to a lack of development of the diploe, the inner table being less affected than the outer. There may even be bony dehiscence. This thinning is usually bilateral and symmetrical, but occasionally is unilateral. Usually it is a normal variant, but occasionally it has pathologic significance. 7hickened Areas The internal and external occipital protuberances, as they project from the occipital bone, thicken the bone in this location. Occasionally, the external protuberance forms a large bony process. Not infrequently there is thick bone immediately above the frontal sinuses and in the vicinity of sutural junctions. Occasionally a small exostosis may occur in the external acoustic meatus. It may appear as a minute nodule or fill the entire meatus. It is usually bilateral.
Digitate or Convolutional Markings Irregular areasof increased and decreased density are found throughout the skull due to thinning of the inner table, as the result of pressure produced by the convolutions or gyri of the brain (Fig. 10, page 123). In normal individuals under approximately 16 yr of age, these may be moderately
Fig. 10. Line diagram demonstrating nuchal ridges on the occipital bone.
the
superior
and
MISCELLANEOUS
BONE
RIDGES
Occipital Bone In the occipital bone, the superior and inferior nuchal lines give origin to prominent muscles. Detectable ridges of bone are seen on the roentgenogram (Fig. 10).
inferior
Occipital Bone
THE NORMAL
ADULT
CRANIAL
VAULT
Fig. 11. Line diagram showing the superior inferior temporal ridges of the parietal bones. dotted areas indicate the parietal fossa.
and The
Parietal Bone The superior and inferior temporal lines on parietal bone are the sites of attachment of temporal fascia and muscle. These lie below parietal eminence, the point where ossification the parietal bone begins (Fig. 11).
the the the of
Frontal Bone The superciliary arch just above the supraorbital foramen or notch and the supraorbital margin, is a readily apparent prominence. The arches of the two sides are separated by the glabella (Fig. 12). The lateral extremity of the supraorbital margin ends in the zygomatic process, which articulates with the zygomatic bone. The temporal line (Fig. 12) converges upon the zygomatic process. It rep-
resents the anterior end of the superior and inferior temporal lines of the parietal bone previously described. Sphenoid Bone The sphenoid bone is, for the most part, situated in the base of the skull and thus is outside the scope of this presentation. Its temporal surface, however, is divided by a prominent infratemporal crest into a superior part, forming part of the wall of the temporal fossa and affording attachment to the temporal muscle, and an inferior part forming part of the wall of the infratemporal fossa and giving attachment to the lateral pterygoid muscle. The infratemporal crest is usually readily identified in lateral radiographs of the skull.
Fig. 12. Line diagram demonstrating the superciliary of the frontal bone and the temporal line.
arch
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136
Temporal Bone
MESCHAN
SEX DIFFERENCES
The temporal bone is situated at the side and base of the cranium. Its squamous portion is thin and expanded and forms the lateral aspect of the middle fossa. The temporal zygomatic process forms the base of the zygomatic arch. Its petrous and tympanic portions are not part of the cranial vault. Superior to the external acoustic meatus there is a vertical groove for the middle temporal artery (Fig. 5). The internal or cerebral surface of the squamous portion is marked by furrows for the convolutions of the brain, and grooves for the middle meningeal vessels. These have been previously described. The normal radiolucency of the squamous portion of the temporal bone may at times give the erroneous impression of complete absence of bone or perhaps increased intracranial pressure.
Until the age of puberty, there is little difference between the male and female skull. However, a woman’s skull is, as a rule, lighter and smaller and its cranial capacity about 10% less than a man’s.4 The walls of the female skull are thinner and its muscular ridge is less strongly marked; the glabella, superciliary arches, and mastoid processes are less prominent, and the air sinuses are smaller. The upper margin of the orbit tends to be sharper, the forehead more vertical, the frontal and parietal eminences more prominent, and the vault somewhat flatter than in men. At times, however, the characteristics are relatively indistinct, and sex determination by skull appearance is extremely difficult or impossible.
REFERENCES 1. Anson BJ: Morris’ Human Anatomy, a Comprehensive Systemic Treatise (ed 12). New York, McGraw-Hill, 1966 2. Bailey P: Intracranial Tumors (ed 2). Springfield, Ill, Thomas, 1948 3. Ethier R: Thickness and texture, in Newton TH, Potts DG (eds): Radiology of the Skull and Brain, vol 1. St. Louis, Mosby, 1971, pp 154-215 4. Lewis WH (ed): Gray’s Anatomy of the Human Body (ed 24). Philadelphia, Lea & Febiger, 1942 5. Lindblom K: A roentgenographic study of the vascular channels of the skull. Acta Radio1 (Stockh) Suppl. 30, 1936 6. Meschan I: An Atlas of Normal Radiographic Anatomy (ed 2). Philadelphia, Saunders, 1959, pp 229
7. Meschan I: Radiographic Positioning and Related Anatomy. Philadelphia, Saunders, 1968 8. Meschan 1: Analysis of Roentgen Signs in General Radiology. Philadelphia, Saunders, 1973, p 503 9. Meschan I: Anatomy Basic to Radiology. Philadelphia, Saunders (in press) 10. O’Connell JEA: Some observations on the cerebral veins. Brain 57:484-503,1934 11. Pendergrass EP, Schaeffer JP, Hodes PJ: The Head and Neck in Roentgen Diagnosis (ed 2). Springfield, III, Thomas, 1972 12. Schunk H, Maruyama Y: Two vascular grooves of the external table of the skull which simulate fractures. Acta Radio1 54:186-194,196O 13. Taveras JM, Wood JH: Diagnostic Neuroradiology. Baltimore, Williams &Wilkins, 1964