Skull
SKULL, the skeleton of the head, composed of 22 bones, 8 of which form the skeleton of the cranium, 14 that of the face. Except the lower jaw, which is movable, the bones are all firmly united by immovable joints. In the following article it is considered more profitable to treat the skull as a whole than to detail the bones separately, and for this purpose a normal European skull will be studied from in front (norma facialis), from above (norma verticalis), from the side (norma later alls), from behind (norma occipitalis) and from below (norma basalis). Afterwards the interior of it will be considered by means of sections.
THE SKULL FROM IN FRONT (norma facialis) (see fig. i). The forehead region is formed by the frontal bone, the two halves of which usually unite in the second year; sometimes, however, they fail to do so and then a suture (metopic) may remain to an advanced age. The lower limit of the forehead is formed by the upper margin of the orbit on each side, and by the articulation between the frontal and nasal bones near the mid line. At the junction of the inner and middle third of each supraorbital margin is the supraorbital notch for the nerve of that name. Above each supra-orbital margin is an elevation, better marked in adult males, called the supraciliary ridge, while between these ridges in the middle line is a slight prominence, the glabella. Below the forehead the two nasal bones form the skeleton of the upper part of the nose ; they articulate with one another in the mid line, but laterally they are joined by a suture to the nasal processes of the maxillae which run up to articulate with the frontal at the internal orbital process, thus forming the inner margin of the orbit.
Externally the malar bones (fig. I, g) articulate with the frontal at the external orbital process and form the lower and outer quadrant of the orbital margin.
The maxillae or upper jaws (fig. I, M) form the greater part of the skeleton of the face ; they complete the lower and inner quadrant of the orbit, and below the nasal bones leave the anterior nasal aperture (apertura pyriformis) between them, and project slightly at the middle of the lower border of this aperture to form the anterior nasal spine. About a quarter of an inch below the infra-orbital margin and just below the articulation with the malar the infraorbital foramen, for the infra-orbital branch of the fifth nerve, is seen on each side. The lower parts of the maxillae form the alveolar margin in which all the upper teeth are set. Laterally each maxilla is prolonged out into a buttress, the zygomatic process, which supports the malar bone.
Below the maxillae the mandible or lower jaw is seen in perspective (fig. I, m). The horizontal part or body is in two halves up to the second year, but after that complete bony union takes place, forming the symphysis. Above the body of the mandible is an alveolar margin containing the sockets of the lower teeth, while below, near the mid line, the bone projects forward to a variable extent and so forms the mental prominence (fig. I, o), one of the special characteristics of a human skull. Below the second bicuspid tooth on each side is the mental foramen for the exit of the mental branch of the fifth nerve.
The Orbit. Each orbit is a pyramidal cavity, the base of the pyramid being in front, at the orbital margin, and the apex behind, at the optic foramen, where the optic nerve and ophthalmic artery pass through. The four sides of the pyramid form the roof, floor, inner and outer walls of the orbit. The roof is arched from side to side and is made up of the frontal bone anteriorly, and the lesser wing of the sphenoid posteriorly. The floor is chiefly formed by the maxilla, though the malar forms a little of it in front. There is a groove for the infra-orbital nerve running forward in it, but before the margin of the orbit is reached the groove becomes a tunnel. The inner wall is antero-posterior and parallel with its fellow of the FIG. i.
opposite orbit; in front it is formed by the nasal process of the maxilla, behind which the lachrymal bone articulates ; together they enclose a vertical groove, for the lachrymal sac, which leads down into the nose, through the naso-lachrymal canal, transmitting the nasal duct (see EYE). Behind the lachrymal bone is the orbital plate of the ethmoid and in the suture between this and the frontal the anterior and posterior ethmoidal foramina are seen. Posteriorly the ethmoid articulates with the sphenoid, while at its lower and hinder part a small piece of the palate bone comes into the orbit. The outer wall of the orbit slopes backward and inward, the two opposite sides therefore converge as they run back. The malar bone, in front, and the great wing of the sphenoid, behind, form this wall. Between the roof and the outer wall there is a slit in the posterior part of the orbit called the sphenoidal fissure because it lies between the great and small wings of the sphenoid ; it transmits the third, fourth, first division of the fifth and sixth cranial nerves, as well as the ophthalmic vein.
Another slit called the spheno-maxillary fissure lies in the line of junction of the outer wall and floor, it leads into the spheno- maxillary and zygomatic fossae and transmits the second division of the fifth nerve and some veins.
THE SKULL FROM ABOVE (norma verticalis). When looked at from above the frontal bone is seen forming the anterior part of the vertex and articulating with the two parietals posteriorly by a nearly transverse serrated suture (coronal suture). Running back from the middle of this is the median sagittal suture extending as far as the lambda on the norma occipitalis. The point where the sagittal and coronal sutures join is the bregma, the site of the lozenge-shaped anterior fontanelte in the infant's skull, but this closes during the second year of life. Small ossicles called Wormian bones are often found in the cranial sutures, and one of these (the interfrontal or os anti-epilepticum) is sometimes found at the bregma. About twothirds of the way back the sagittal suture becomes less serrated and on each side of it the small parietal foramen may be seen. This only transmits a small emissary vein (see VEINS) in the adult, but, as will be seen later, is of considerable morphological interest. As middle life is reached the cranial sutures tend to become obliterated and the bones can no longer be separated ; this fusion begins at the places where the sutures are least deeply serrated, and as a rule the sagittal suture disappears between the two parietal foramina between thirty and forty years of age.
THE SKULL FROM THE SIDE (norma lateralis). -On looking at the accompanying figure (fig. 2) it will be seen that the Calvaria or brain FIG. 2. Profile of the Skull.
Fr, Frontal bone.
Pa, Parietal.
SO, Supra-occipital.
Sq, Squamous-temporal.
MT.Mastoid-temporal.
Ty, Tympanic.
St, Styloid-temporal.
As, Ali-sphenoid.
E, Os planum of ethmoid.
L, Lachrymal.
N, Nasal.
MX, Superior maxilla. Ma, Malar. Mn, Mandible. bh, Basi-hyal. th, Thyro-hyal. ch, Cerato-hyal. em. External meatus. cs, Coronal suture. Is, Lambdoidal suture. ss, Squamous suture.
case forms all the upper part, while the face is below the anterior half. Taking the calvaria first the side view of the frontal bone (fig. 2, Fr) is seen extending back as far as the coronal suture (cs). Just above Fr is an elevation on each side, the frontal eminence, better seen in female than in male skulls. The junction between the frontal and malar (Ma) at the outer margin of the orbit has already been referred to as the external angular process and is an important landmark for measurements, and from it a curved line (the temporal crest) runs back crossing the coronal suture to reach the parietal bone (Pa, fig. 2) ; as it runs back this line divides into two. Below the crossing of the temporal crest the coronal suture is less serrated than above, and here it becomes obliterated first. The quadrilateral outline of the parietal bone is seen as well as its articulations; above it touches its fellow of the opposite side; in front, the frontal (Fr) ; below the great wing of the sphenoid or alisphenoid (As), the squamous part of the temporal or squamosal (Sq) and the mastoid part of the temporal (MT), while behind it articulates with the supra-occipital (SO), through the lambdoid suture (Is). All four angles of the parietal are points of special interest; the anterosuperior angle or bregma has been already noticed, and it will be seen to lie nearly above the ear opening or external auditory meatus in the temporal bone (em). The antero-inferior angle where the frontal, parietal and alisphenoid meet is the pterion and is the site of an occasional Wormian bone (epipteric). The posterior superior angle is the lambda and will be better seen on the nprma occipitalis, while the posterior inferior angle, where the parietal, supra-occipital, and mastoid temporal bones meet, is known as the asterion and marks the lateral sinus within the cranium. A little above and behind the middle of the parietal bone, and just above the superior temporal crest, is the parietal eminence where ossification starts. The squamous part of the temporalbone overlaps the parietal at the squamous suture, while from its lower part the zygomatic process projects forward to articulate with the malar. At the root of this process is the glenoid cavity where the condyle of the lower jaw articulates, and just behind this the external auditory meatus is seen (em). Behind this again the mastoid temporal is prolonged down into a nipple-shaped swelling, the mastoid process (MT), containing air cells and only found in the adult human skull, while just in front of the external auditory meatus is the styloid process (St), connected with the hyoid bone by the stylo-hyoid ligament (dotted). In the side view of the face the nasal and maxillary bones are seen, and from this point of view it will be noticed that just below the nasal aperture the maxillae, where they join, are produced forward into a little spur, the anterior nasal spine, which is a purely human characteristic. At the side of the maxilla the malar or jugal (Ma) bone is placed, and its lozenge-shaped outline is apparent; it forms the anterior part of the zygomatic arch. When the mandible is disarticulated and removed the posterior part of the maxilla is seen, and behind it the external pterygoid plate of the sphenoid. Between these two bones there is a vertical slit-like opening into a cave, the spheno-maxillary fossa, which communicates with the orbit through the spheno-maxillary fissure, with the nasal cavity through the spheno-palatine foramen, with the cranial cavity through the foramen rotundum, and with the mouth through the posterior palatine canal, as well as having other smaller openings.
The side view of the mandible or lower jaw shows the body, already seen from in front, and the ramus projecting up from the back part of it at an angle of from 1 10 to 120 in the adult. Before the teeth come and after they are lost the angle is greater. The point just above ch (fig. 2) is known as the angle of the jaw. At the upper part of the ramus are two projections; the most anterior is the coronoid process for the attachment of the temporal muscle, while posteriorly is the condyle which articulates with the glenoid cavity of the temporal bone.
THE SKULL FROM BEHIND (norma occipitalis) (fig. 3). From this point of view the posterior ends of the parietal bones (PP), with the sagittal suture between them, are seen. Below these comes the supra-occipital bone (fig. 3, O) separated from them by the lambdoid suture which is deeply serrated and a frequent site of Wormian bones. Where the sagittal and lambdoid sutures meet is the lambda (L), and here a small Wormian bone is sometimes found, called the preinter parietal. In the mid line about a hand's breadth (2^-3 in.) below the lambda is a prominence, the external occipital protuberance or inion, for the attachment of the ligamentum nuchae, while running out on each side from this are the superior curved lines which attach muscles of the neck.
THE SKULL FROM BELOW (norma basalis) (fig. 4). Starting from in front, the superior alveolar arcade with the teeth sockets is seen. This in a European skull approaches a semicircle, but in lower races the sides become more parallel; this is known as a hypsiloid arcade. Within the arcade is the hard palate formed by the maxillae in front (fig. 4, m), and the palate bones (p) behind.
FIG. 3.
FIG. 4.
At the front of the median suture between the maxillae is the anterior palatine canal which, if it is looked into closely, will be seen to lead into four small foramina, two antero-posterior known as Scarpa's foramina, for the naso-palatine nerves, and two lateral called Stensen' s foramina for small arteries and the remains of the mouth opening of Jacobson's organ (see OLFACTORY SYSTEM). In young skulls a suture runs outward from the anterior palatine canal to between the lateral incisor and canine sockets, and sometimes another runs from the same place to between the central and lateral incisor teeth.
At each postero-lateral angle of the palate are the posterior palatine canals for the descending palatine nerves. The posterior margin of the hard palate is a free edge which forms the lower boundary of the posterior nasal apertures or choanae and attaches the soft palate (see PHARYNX). Behind the alveolar arcade on each side are the external and internal pterygoid plates pf the sphenoid; the external is a muscular process for the attachment of the pterygoid muscles, while the internal ends below in the hook-like hamular process which is directed backward and outward. Dividing the posterior nasal aperture into two is the vertical hind edge of the vomer (v), which articulates above with the body of the sphenoid (basi-sphenoid), and just behind this the sphenoid is united by bone with the basioccipital (b), though up to twenty years of age there is a synchrondrosis (see JOINTS) called the basilar suture) between them. It is therefore very easy to tell an adult's skull from that of a young person. Passing back in the mid line the foramen magnum (f) is seen, through which pass the spinal cord and its membranes, the vertebral arteries and the spinal accessory nerves. A little in front of this is a small tubercle, the pharyngeal spine, to which the constrictors of the pharynx are attached. On each side of the foramen magnum and in front of its mid transverse diameter are the condyles (c), which articulate with the atlas, while just above these are the anterior condylar foramina, one on each side, for the exit of the hypoglossal nerves.
External to the pterygoid plates the base of the skull is formed by the ali-sphenoid, which projects backward into a point, the spine of the sphenoid, and just in front of this is the small foramen spinosum for the passage of the middle meningeal artery. In front and a little internal to the foramen spinosum is a larger opening, the foramen male, through which the third division of the fifth nerve leaves the skull. Into the re-entering angle between the ali-sphenoid and basi-occipital is fitted the petrous part of the temporal, which, however, does not quite fill the gap but leaves a space on each side of the site of the basilar suture to be closed in by fibro-cartilage, and this is known as the middle lacerated foramen. On the lower surface of the petrous bone is the round opening of the carotid canal through which the internal carotid artery and its accompanying sympathetic nerves pass into the skull, while more externally the styloid process projects downward and forward and is more or less ensheatned at its root by the rampart-like ridge of the vaginal process. Between the styloid process and the occipital condyle lies the jugular or posterior lacerated foramen through which pass the lateral and inferior petrosal sinuses, and the glosso-pharyngeal, vagus and spinal accessory nerves. The bone which bounds this foramen behind, and which bears the posterior two-thirds of the occipital condyle, is the ex-occipital part of the occipital. A little behind and external to the styloid process is the tip of the mastoid process, just internal to which is the deep antero-posterior groove for the digastric muscle, and internal to that another slighter groove for the occipital artery. Behind the styloid process and between it and the mastoid is the stylo-mastoid foramen through which the facial nerve passes, while in front of the process the glenoid cavity can be seen in its entirety, bounded in front by the eminentia articularis and divided into an anterior articular part and a posterior tympanic plate by the Glaserian fissure. Just internal to the glenoid cavity is the opening of the bony Eustachian tube.
The posterior part of the norma basalis behind the foramen magnum is formed by the supra-occipital part of the occipital bone, so that all the four parts of the bone, which are separate up to the "third year, help in the formation of that large opening. Between the foramen magnum and the external occipital protuberance and superior curved line already noticed, the bone attaches the deep muscles of the neck.
THE INTERIOR OF THE CRANIUM. If the roof of the skull be sawn off the interior or cerebral surface of both the vault and the base may be examined. The vault shows the cerebral aspects of parts of the frontal, parietal and occipital bones, and of the sutures between them. In the mid line is a shallow antero-posterior groove for the superior longitudinal blood sinus, and on each side of this irregular depressions are often seen for the Pacchionian bodies (see BRAIN). The base (fig. 5) is divided into three fossae, anterior, middle and posterior, each being behind and on a lower level than the one in front of it.
The anterior cranial fossa is formed by the cribriform plate of the ethmoid, near the mid line, freely perforated for the passage of the olfactory nerves. In the mid line, near the front, is a triangular plate rising up which attaches the falx cerebri (see BRAIN) and is called the crisis, galli. On each side of p IG - this is the nasal slit for the nasal branch of the first division of the fifth nerve. On each side 'of the cribriform plate is the orbital plate of the frontal, while the back part of the fossa has for its floor the body of the sphenoid (pre-sphenoid) near the mid line and the lesser wing (orbito-sphenoid) on each side. Each lesser wing is prolonged back into a tongue-like process, the anterior clinoid process, just internal to which is the optic foramen (fig. 5, n), and the two foramina are joined by the optic groove for the optic commissure. Behind this groove is a transverse elevation, the olivary eminence (22), which marks the junction of the pre- and basi- sphenoid parts of the body of the sphenoid bone.
The middle cranial fossa is like an hour-glass placed transversely, as there is a central constricted, and two lateral expanded, parts. The central part forms the pituitary fossa (fig. 5, 3) for the pituitary body (see BRAIN) and is bounded behind by the wall-like dorsum sellae, at the sides of which are the posterior clinoid processes (5, 4). The olivary eminence, pituitary fossa and dorsum sellae together resemble a Turkish saddle and are often called the sella turcica. The lateral expanded part of the middle cranial fossa is bounded in front by the great wing of the sphenoid (alisphenoid), behind by the front of the petrous part of the temporal (periotic) and laterally by the squamous part of the temporal (squampsal). Between the alisphenoid and orbitosphenoid is the sphenoidal fissure ; already noticed in the orbit, and a little behind this, piercing the alisphenoid, is the posterior opening of the foramen rotundum, through which the second division of the fifth nerve passes into the spheno-maxillary fossa. Further back the alisphenoid is pierced by the foramen ovale (o) and foramen spinosum (s), both of which have been already noticed on the norma basalis. From the latter a groove for the middle meningeal artery runs forward and outward, and soon divides into anterior and posterior branches, the former of which deepens into a tunnel near the pterion. At the apex of the petrous bone and at the side of the dorsum sellae is the middle lacerated foramen (c), already noticed, and running inward to this from an aperture in the petrous bone is a groove for the great superficial petrosal nerve which is overlaid by the Casserian ganglion of the fifth nerve.
The posterior cranial fossa is pentagonal in outline, having an anterior border formed by the dorsum sellae, two antero-lateral borders, by the upper borders of the petrous bones, and two posterolateral curved borders, by the grooves for the lateral sinuses (fig. 5, ll). In the middle of this fossa is the foramen magnum, bounded by the four parts of the occipital bone, which unite during childhood. In front of the foramen magnum the floor of the fossa is formed by the basi-occipital and basi-sphenoid bones, which unite soon after twenty and form a steep slope, downward and backward, known as the clivus (b). This is slightly grooved from side to side, and lodges the pons and medulla (see BRAIN) and the basilar artery. On each side of the basi-occipital the posterior surface of the petrous bone bounds the fossa, and lying over the suture between them is the groove for the inferior petrosal venous sinus which leads backward and outward to the jugular foramen already noticed on the norma basalis. About the middle of the posterior surface of the petrous bone is the internal auditory meatus, through which pass the facial and auditory nerves, the pars intermedia (see NERVES, CRANIAL) and the auditory artery. Close to the antero-lateral part ' of the foramen magnum is the inner opening of the anterior condylar foramen which is sometimes double for the two bundles of the hypoglossal nerve, and a little in front of and outside this is a heaping up of bone called the tuberculum jugulare, which marks the union of the basi- and ex-occipital bones. The hindmost limit of the posterior fossa in the mid line is marked by an elevation called the internal occipital protuberance, and at this point the grooves for the superior longitudinal (s), and two lateral sinuses (ll) join to form the torcular Herophili (see VEINS). Running from the internal occipital protuberance toward the foramen magnum in the mid line is the internal occipital crest, which attaches the falx cerebelli (see BRAIN) and on each side of this is the cerebellar fossa.
From the internal occipital protuberance the two wide grooves for the lateral venous sinuses (n) run nearly horizontally outward till they reach the posterior inferior angles of the parietal bones; here they turn downward with an S-shaped curve, grooving the mastoid portion of the temporal and later on the exoccipital bones, until they reach the jugular foramina. To the edges of the horizontal parts of these grooves, and to the upper edge of the petrous bones the tentorium cerebelli is attached.
THE SKULL IN SAGITTAL SECTION. If the skull be sawn down just to the right of the mid line and the left half be looked at, the appearance will be that reproduced in fig. 6. The section of the cranial bones shows that they are formed of an outer and inner table of hard bone, while between the two is a layer of cancellous tissue called the diploe. In certain places the diploe is invaded by ingrowths from the air passage which separate the two tables and form the air sinuses of the skull, though it is important not to confuse these with the intracranial blood or venous sinuses. In the section under consideration two of these spaces, the frontal (fs) and the sphenoidal (PS) air sinuses are seen. Behind the frontal sinus is the crista r> FIG. 6. Section through the Skull immediately to the right of the Mesial Plane (see also lettering in fig. 2) :
BO, Basi-occipital. SC, Septal cartilage of nose.
EO, Ex-occipital. V, Vomer.
PT, Petrous-temporal. PI, Palate.
BS, Basi-sphenoid. Pt, Pterygoid of sphenoid.
PS, Pre-sphenoid (the letters are fs, Frontal sinus.
placed in the sphenoidal Pf, Pituitary fossa.
sinus). fm, Foramen magnum.
OS, Orbito-sphenoid. a, Angle.
ME, Mes-ethmoid. s, Symphysis of lower jaw.
galli already mentioned, while below is the bony septum of the nose formed, by the mes-ethmoid plate (ME), the vomer (V), and the line of junction of the palatine processes of the two maxillae and two palate bones. The re-entering angle between the mes-elhmoid and vomer is filled in the recent state by the septal cartilage (SC).
Below the face is the inner surface of the body and ramus of the mandible, and half-way down the latter is the inferior dental foramen where the inferior dental branch of the fifth nerve accompanied by its artery passes into the inferior dental canal in the substance of the bone to supply the lower teeth. Just in front of this foramen is a little tongue of bone called the lingula attaching the spheno- mandibular (long internal lateral) ligament, while running downward and forward from this is the mylo-hyoid ridge with the groove of the same name just below it.
If the cut surface of the right half of the skull be looked at, the outer wall of the nasal cavity will be seen with the three turbinated bones each overhanging its own meatus, but the anatomy of this part has already been dealt with in the article on the olfactory system For further details see any standard anatomical textbook uain, Gray, Cunningham, etc. For charm of style, The Human keleton by G. M. Humphry (London, 1858), although somewhat out of date, is unsurpassed.
Embryology.
The notochord (see SKELETON: Axial) extends forward to the ventral surface of the middle cerebral vesicle (see BRAIN) or as far as the place where the dorsum sellae will be. It is partly surrounded by the mesenchyme just as it is completely in the rest of the axial skeleton, and this mesenchyme extends dorsally on each side to wrap round the nerve cord, which is here the brain. In this way the brain becomes enclosed in a primitive membranous cranium, the inner part of which persists in its primitive condition as the dura mater, while the outer part may chondrity, chondrify and ossify, or ossify without a cartilage stage. That part of the cranium which is in front of the notochord is called prechordal, while the posterior part into which the notochord extends is chordal. On each side of the notochord chondrification takes place and a basicranial plate of cartilage is formed which soon meets its fellow of the opposite side, and forms the floor of the skull as far forward as the dorsum sellae, and as far back as the external occipital protuberance. Laterally it comes in contact with the mesenchyme surrounding the jnternal ear, which is also chondrifying to form the cartilaginous periotic capsule, and the two structures fuse together to form a continuous floor for the back of the skull. A. Froriep has shown that in the hinder occipital region of the calf there are evidences of four vertebrae having been incorporated with the basicranial plate, that is to say that the plate and its coalesced vertebrae represent five mesodermic somites (" Zur Entwickelungsgeschichte der Wirbelsaule, insbesondere des Atlas und Epistropheus und der Occipitalregion," Archiv fur Anal. u. Phys., Anat. Abth., 1886). It has more recently been shown by Levi that the same thing is true for man. K. Gegenbauer has pointed out that the primitive membranous skull shows, in the chordal region, signs of metameric segmentation in the way in which the cranial nerves pierce the dura mater one behind the other. These segments, however, had lost their distinctness even before the cartilaginous cranium had become developed, so that there is no real segmental value in the elements of this, still less in those of the bony skull. The only place in which segmental elements can be distinguished is in the occipital region, which is in structure transitional between the head and vertebral column. The notochord, it has been shown, ends just behind the place where the stomodaeum pouches up through the cranial base to form the anterior part of the pituitary body (see BRAIN). Where it ends two curved bars of cartilage are formed, which run forward till they meet the olfactory capsules, which are also now chondrifying. These bars are the prechordal cartilages or trabeculae cranii and enclose between them the cranio-pharyngeal canal by which the pituitary body ascends, but later on, as they grow, they join together and cut off the pituitary body from the pharynx. By their growth outward they form the floor of the prechordal part of the chondro-cranium, so that from them is developed that part of the cartilaginous skull which will later on be part of the basisphenoid, the presphenoid, orbitosphenoid and alisphenoid regions. It has hitherto been assumed that this process held good for man, but recent research shows that the anterior part of the base of the skull chondrifies in the same way that ice appears on a pond and that the trabeculae are at no time definite structures. Chondrification of the nasal capsules is later than that of the parts of the skull behind, so that there is a steady progress in the process from the occipital to the ethmoidal region. There is a median centre of chondrification, the mesethmoid cartilage, which projects down into the fronto-nasal process (see OLFACTORY SYSTEM), and two lateral ectethmoid cartilages which eventually join with the mesethmoid to form the cartilaginous ethmoid.
The cartilaginous base of the cranium is now formed, but the vault is membranous. While -the base has been developing the two anterior visceral arches have been also forming and have gained an attachment to the cranium, but the formation and fate of these is recorded in the article SKELETON ( Visceral). About the sixth week of foetal life ossification begins at different points in the membranous vault of the skull. In this way the frontal, parietal, supra-occipital, c b a b c and a little later the squamous part of the temporal bones are formed. About the eighth week, too, the lachrymal, nasal and vomer appear in the membrane lying superficial to different parts of the olfactory capsule. All these are dermal bones, comparable to the deeper parts of the scales of fishes, and developed in the mesenchyme lying deep to and in contact with the ectoderm. It is therefore necessary to think of the primitive skull as a three-layered structure, the deepest layer persisting as the dura mater, the middle forming the chondro-cranium, which ossifies to form the base, and a superficial layer close to the skin or mucous membrane (ectoderm), from which the bones of the vault and superficial parts of the olfactory TeilBaok of d e d Arthur Thomson, in Cunningham'^ Anatomy.
FIG. 7. Ossification of Sphenoid. a, Presphenoid; 6, Orbito-sphenoids ; c, Alisphenoids; d, Internal pterygoid plates; e, Basi-sphenoid.
capsules are derived. At the four angles of the parietal, ossification is checked for some time to form fontanelles, of which the bregma is the most important, and at each of these points, as well as elsewhere in the sutures, accessory centres of ossification may occur to form Wormian bones.
Along the middle line of the base of the skull the same progress of ossification from behind forward ,. j^~ - - \*~&3t& ' ' -i f is seen that was noticed in * 7^ 3)5 / the process of chondrification. Bilateral centres for the basioccipital appear about the sixth week, for the basisphenoid in the eighth, and for the presphenoid in the tenth, while the lateral mass of the ethmoid does not ossify till the fifth month and the mesethmoid not until the first year of extrauterine life. In the lateral part of the base the exoccipitals and alisphenoids begin to ossify about the eighth week and the presphenoids about the tenth. In connexion with the alisphenoid there is a Arthur Thomson, in Cunningham's Text-Book o] small extra centre of Anatomy.
morphological interest FIG - S.-^-Ossification or Occipital Bone, only, which forms a little a > Basilar centre.
Exoccipital.
Ossicle of Kerkring.
Supra-occipital (from cartilage).
Fissure between supra-occipital and interparietal.
Interparietal (from membrane).
Fissure between interparietals.
The auditory or periotic capsule, like the olfactory, is late in ossifying; it has four centres (pro-otic, epiotic, opisthotic and pterotic) which do not come until the fifth month
Some parts of the chondro-cranium do not ossify at all; this is the case in the anterior part of the mesethmoid, which remains as the septal cartilage of the nose, while, as has been already pointed out, a buffer of cartilage persists between the basioccipital and basisphenoid until the twentieth year of life.
From what has been said it is evident, and it will be still more evident if the article SKELETON (Visceral) be looked at, that some of the bones of the adult skull are compounded of various contributions from the different elements which make up the adult cranium. These, recapitulated, are (i) the dura mater or entocranium, which in man does not ossify except perhaps in the crista galli. (2) The chondro-cranium or mesocranium. (3) The superficial part of the mesenchyme ( ectocranium) from which dermal bones are formed. (4) The olfactory and auditory sense capsules. (5) The visceral arches. (6) Some fused vertebrae posteriorly.
tongue-shaped process , called the lingula, pro- c < jecting back into the "-, middle lacerated foramen e < and apparently corresponding to the sphenotic / bone of lower vertebrates.
Arthur Thomson, in Cunningham's Text- ampfe h C as Pi th a e t^pit^ ^J ^Z?** exoccipital and basal part of Right lemporal Bone at Birth.
the supra-occipital derived ?' from the chondro-cranium ' and fused vertebrae, while the C J d, Tympanift ring.
Inner wall of tympanum.
Fenestra rotunda.
Foramen ovale.
Mastoid.
Mastoid process.
with foramen for transmission of vessels. h, Squamo-zygomatic.
latter centres have fused with the interparietal, but an indication of their line of junction is seen on each side of g. The bone of Kerkring (c) is an abnormality, the meaning of which is not understood.
The temporal is also a very composite bone; in it the petromastoid portion represents the auditory sense capsule; the tabular vault part of the supra- . occipital has four dermal $' centres of ossification corre- '' , sponding to the interparietal - Masto-squamosal suture, and preinterparietal bones of external auditory meatus is formed by the outgrowth of the tympanic ring (fig. 9, a) which is probably part of the first visceral arch (see SKELETON, Visceral) ; the squamozygomatic part is a dermal bone, while the styloid process is a part of the second visceral arch.
The mastoid process is not present at birth, but appears about the second year and becomes pneumatic about puberty. From what has been seen of the skull bones in the above necessarily concentrated and abridged account, it is obvious that they do not correspond to the traces of segmentation as indicated by the cranial nerves, and for this and other reasons the " vertebrate theory of the skull " is no longer believed in.
For further details and references see Quain's Anatomy (London, 1908) ; Cunningham's Anatomy (Edinburgh, 1906) ; The Development of the Human Body, J. P. McMurrich (London, 1906).
Comparative Anatomy.
In this section only those parts of the skull which form the covering for the brain and the capsules for the olfactory and auditory apparatus are considered. Those parts of the face and jaws which are developed in connexion with the visceral arches are dealt with in the article SKELETON (Visceral). In the Acrania (Amphioxus) the enlarged anterior end of the nerve cord is merely surrounded by fibrous tissue continuous with the sheath of the rest of the nerve cord; there is therefore, in a sense, no true cranium.
In the Cyclostomata (hags and lampreys) a cartilaginous cranium is developed, the anterior part of which forms an unpaired olfactory capsule connected with the rest of the cranium by fibrous tissue only. In the floor, just in front of the anterior end of the notochord, an aperture, the basi-cranial fontanelle, remains unchondrified for the passage of the pituitary diverticulum into the skull.
In the Elasmobranchh (sharks and rays) and Holocephali (Chimaera) among the fishes the skull is still a complete cartilaginous box, though calcification of the cartilage often takes place. Taking the skull of the dogfish as a type, two large olfactory capsules are seen in front, and behind these the cranial brain-box is narrowed, being excavated at its sides for the great orbits. More posteriorly the auditory capsules widen the skull, and on the posterior (caudal) aspect the foramen magnum is seen with an occipital condyle on each side of it for the first vertebra to articulate with. On the upper (dorsal) surface of the skull are two apertures in the middle line; the more anterior of these is sometimes called the anterior fontanelle, though it has nothing to do with the bregma, described in man's skull, but forms a rudimentary median orbit for the pineal eye (see BRAIN). The posterior fontanelle is a depression which leads into two lateral tubes, each of which passes into the auditory capsule and is known as an aqiieductus vestibuli (see EAR).
In the cartilaginous ganoid fishes (sturgeon), which, like the elasmobranchs, are of great antiquity, the chondro-cranium is partly ossified so that ali- and orbito-sphenoids are found; in addition to this a large number of dermal bones have made their appearance, such as nasals, frontals, parietals, supra and post temporals, while in the roof of the mouth and pharynx a long membrane bone, the parasphenoid, is formed, and lies ventral to and strengthens the cartilaginous base of the skull. It will be noticed that these fish are important morphological landmarks, because in them the almost unchanged chondro-cranium coexists with a dermal ectocranium.
In the bony ganoids such as the " bow fin " (Amia) the dermal bones are still more numerous and, among others, squamosals, prootics and exoccipitals appear. These fish are also remarkable for a fusion of the anterior part of the vertebral column with the occipital region of the skull, an arrangement recalling Froriep's observations on the skull of the calf embryo mentioned in the section on embryology.
In the bony fishes (Teleostei) the membrane or dermal bones are still more numerous, and many of them are unrepresented in the mammalian skull, while others, which are there quite rudimentary, are very large. The chondro-cranium tends to disappear in the vault, but the base is fully ossified. Among other cartilage bones the five ossifications of the auditory capsule are seen, the pro-, epi-, opisth-, pter- and sphen-otics, all of which are found as centres of ossification in man. Jn the cod, for example, the sphenotic, which is represented in man by the little lingula sphenoidahs, is larger than the alisphenpid.
In the Dipnoi (mud-fish) the chondro-cranium is very slightly ossified, only exoccipitals being found, but there is the same coalescence with anterior vertebrae which was noticed in the ganoids. Dermal bones are plentiful.
In the Amphibia the chondro-cranium persists and is only ossified in front by the girdle bone or sphenethmoid, and behind by the prootics and exoccipitals, the latter of which bear the two cpndyles. The anterior fontanelle is well marked in the chondro-cranium, but is completely overlaid and concealed by the dermal fronto-parietals. The membrane bones though large are much less numerous than in the bony fishes.
In the Reptilia the skull varies immensely in the different orders, but speaking broadly, the chondro-cranium is less distinct than in the Amphibia, except in the ethmoidal region. In the base of the skull the basioccipital and basisphenoid are tending to replace the membranous parasphenoid, and instead of two exoccipital condyles only one in the mid line is present, though this in many forms (e.g. Chelonia) consists of three parts, a median borne on the basioccipital and two lateral on the exoccipitals. The parietal foramen is usually definitely marked in the dermal part of the skull and forms a median orbit for the pineal eye; this is especially the case in the Lacertilia (lizards). Except in the Ophidia (snakes) and Amphisbaenidae (worm-like lizards) there is a fibro-cartilaginous septum between the orbits so that the cranial cavity does not reach forward to the ethmoidal region. The pro-, epi- and opisth-otic bones are all developed, but the epiotic usually fuses with the supra-occipital and the opisthotic with the exoccipital.
In the Crocodilia the first attempt at pneumaticity is seen in the basisphenoid, which is traversed by a complicated system of Eustachian passages leading eventually to the tympanum. In the class Aves the general scheme of the reptilian skull is maintained, though the bones fuse together very early, thus obliterating the sutures between them. Almost all of them have air in their interior, and so are said to be pneumatic.
The single occipital condyle, if looked at in a young specimen, is seen to consist of a basioccipital and two exoccipital elements, though these are indistinguishable in the adult. The parasphenoid is represented by a broad plate which is called the basitemporal. The pro-, epi- and opisth-otic bones fuse together to form the auditory capsule.
In the Mammalia the calvaria varies considerably in the different orders, the characteristic features being best marked in adult males. Usually the different bones are interlocked by sutures, as in man, until adult life, but in some orders (e.g. Monotremata, Edentata and Carnivora) they fuse together quite early.
In the basicranium the cartilage bones presphenoid, basisphenoid, and basioccipital, are so well developed that the parasphenoid has disappeared. In the basisphenoid of the rabbit the cranio- pharyngeal canal (see section on embryology) persists as a foramen at the bottom of the pituitary fossa. In the lower orders the face lies well in front of the brain case, as it does in reptiles and amphibians, but as the Primates are reached the increasing size of the calvaria causes it to overlie the face. Many of the bones are pneumatic, the process reaching its maximum in the elephant and the adult male gorilla. The periotic capsule blends with the squamosal and tympanic to form the petrous bone, though it is practically only in man that the second visceral arch ossifies on to this as a styloid process. There are usually two occipital condyles which have basi- and exoccipital elements, though there are many mammals in which there is one large crescentic condyle surrounding the anterior half of the foramen magnum.
Ossification of the processes of the dura mater occurs in the tentorium cerebelli of the carnivora and in the falx cerebri of the ornithorhynchus and porpoise. The orbits are in most mammals continuous with the temporal fossae. Sometimes, as in many of the ungulates and in the lemurs, they are outlined by a bony ring, but it is not until the higher Primates are reached that the two cavities are shut off and even then a vestige of their original continuity remains in the spheno-maxillary fissure.
For further details see W. H. Flower, Osteology of the Mammalia (London, 1885) ; S. H. Reynolds, The Vertebrate Skeleton (Cambridge, 1897); R. Wiedersheim ; C. Gegenbaur, Vergleich. Anat. der Wirbelthiere (Leipzig, 1901). (F. G. P.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
