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the hard palate the pyramidal process of the palate bone appears wedged in between the two plates, and so enters into the formation of the floor of the pterygoid fossa. The medial pterygoid lamina separates the nasal cavity from the pterygoid fossa; to the posterior edge of the medial pterygoid lamina are attached the pharyngeal aponeurosis, the superior constrictor of the pharynx, and the pharyngopalatinus muscle. Above, the posterior border of this plate is channelled to form the small scaphoid fossa, which curves laterally over the summit of the pterygoid fossa, and furnishes a surface for the origin of the tensor veli palatini muscle. The sharp medial margin of this fossa, continuous below with the posterior border of the medial pterygoid lamina, extends upwards, and on either side of the body of the sphenoid forms a blunt pointed process, the pterygoid tubercle, which extends backwards towards the apex of the petrous part of the temporal bone. Just lateral to this, and concealed by it, is the posterior extremity of the pterygoid canal, through which pass the artery and nerve of the canal. The medial surface of the medial pterygoid lamina is directed towards the nasal cavity. Superiorly this surface curves medially to meet the inferior surface of the body of the sphenoid, forming on either side a lipped edge, the vaginal process, between which the alæ of the vomer, which here forms the nasal septum, are wedged. Between the two a small interval, however, is occasionally left, which forms on either side the basipharyngeal canal. A little lateral to the line of union of the vaginal process with the vomer is the opening of the pharyngeal canal. This lies between the inferior surface of the vaginal process and the sphenoidal process of the palate bone, which here articulates with the inferior surface of the body of the sphenoid. The pharyngeal branch of the spheno-palatine ganglion and the pharyngeal branch of the internal maxillary artery pass through this canal. Inferiorly the pterygoid processes project below the level of the hard palate. The medial plate ends in a slender recurved process, called the hamulus pterygoideus, which turns backwards and laterally (this is frequently broken off in skulls which have been roughly handled). It reaches as low as the level of the alveolar margin, and lies just within and behind the posterior extremity of the alveolar process. It can readily be felt in the living by placing the finger against the soft palate behind and just within the gum around the root of the dens serotinus (O.T. wisdom tooth). On the front of and below this process the tendon of the tensor veli palatini muscle glides in a groove.
The choanæ (O.T. posterior nares) lie between the two pterygoid processes. Of a shape much resembling two Gothic windows, their bases or inferior boundaries are formed by the horizontal part of the palate bone. Laterally they are bounded by the medial surfaces of the medial pterygoid laminæ, whilst above, the lateral side of the arch is formed by the vaginal processes of the same laminæ; medially they are separated by the thin vertical posterior border of the vomer, whilst, above, the everted alæ of the same bone form the medial sides of the arch. The plane of these apertures is not vertical but oblique, corresponding usually to a line drawn from the bregma above through the last molar tooth of the maxilla below. Their size varies considerably, but the height is usually equal to twice the width.
The region of the cranium which lies lateral to the maxilla and lateral pterygoid lamina corresponds to the infra-temporal fossa, which has been already described, as it is seen from the side (Lateral Aspect of the Skull, p. 168). Viewed from below, the infra-temporal fossa is bounded in front by the infra-temporal surface of the body of the maxilla and the medial surface of the zygomatic bone. The roof, which is traversed by the spheno-squamosal suture, is formed in front by the inferior surface of the great wing of the sphenoid, and behind by a small triangular surface of the under side of the squamous part of the temporal bone, immediately in front of the tuberculum articulare.
Circumscribed laterally and behind by the anterior root of the zygoma, which curves forward to become continuous in front with the infra-temporal crest crossing the lateral surface of the great wing of the sphenoid, the roof of the fossa is separated from its anterior wall by the inferior orbital fissure, which is so inclined that with its fellow of the opposite side it forms an angle of 90°. Superiorly the infra-temporal fossa communicates freely with the temporal fossa medial to the zygomatic arch, though the student must bear in mind the fact that when the mandible is in position the lateral limits of the space are very much reduced (p. 168).
The inferior surface of the great wing of the sphenoid is here V-shaped. The angle corresponds to the spine, the lateral limb to the spheno-squamosal suture, whilst the medial limb corresponds to a narrow cleft, the fissura spheno-petrosa, which separates it from the petrous portion of the temporal bone, to which it is united in the recent condition by a synchondrosis. Along the line of this latter fissure the edges of the adjacent bones (sphenoid and petrous part of the temporal) are bevelled so as to form a groove, which extends from the root of the medial pterygoid lamina medially, to the medial side of the base of the angular spine laterally, where the groove ends by entering an osseous canal. In the groove (sulcus tubæ auditivæ) the cartilaginous part of the auditory tube is lodged, whilst the osseous canal includes the bony part of the same tube, together with the tensor tympani muscle, which is lodged in a separate compartment immediately above it. The anterior extremity of the cartilaginous part of the auditory tube is supported by the posterior edge of the medial pterygoid lamina, which is often notched for its reception. Between the root of the lateral pterygoid lamina and the angular spine there are two foramina which lie immediately in front of the sulcus tubæ auditivæ. Of these the larger and anterior is the foramen ovale, through which pass the motor root and mandibular division of the trigeminal nerve, together with the accessory meningeal artery. The smaller, which, from its position immediately in front of the angular spine, is called the foramen spinosum, transmits the middle meningeal artery and vein, and sympathetic plexus surrounding the artery. The lesser superficial petrosal nerve here passes through the base of the skull to join the otic ganglion either through a small foramen (canalis innominatus) placed between the foramen ovale and the foramen spinosum, or through the foramen ovale or through the spheno-petrosal fissure. The position of the suture between the basi-occipital and basi-sphenoid corresponds to a line connecting the tips of the pterygoid tubercles at the root of the medial pterygoid laminæ.
Occasionally in the centre of this line there is a small pit with a foramen leading from it. This probably represents the lower end of the cranio-pharyngeal canal.
The inferior surface of the basilar part of the occipital bone (basi-occipital) stretches between the body of the sphenoid in front and the anterior margin of the foramen magnum behind; projecting from its centre is a slight elevation, the pharyngeal tubercle, to which the pharyngeal raphe, together with the central part of the anterior atlanto-occipital membrane, is attached. It should be noted, that when the atlas is in position the pharyngeal tubercle lies in line with the tubercle on the anterior arch of that bone. Curving laterally and backwards from the pharyngeal tubercle, on either side, is an irregular ridge (crista muscularis), in front and behind which are attached the longus capitis and rectus capitis anterior muscles. On either side of the basi-occipital, in front, there is an irregular opening of variable size; this is placed between the root of the pterygoid process anteriorly, the apex of the petrous portion of the temporal bone laterally, and the lateral edge of the basi-occipital and basi-sphenoid medially. It is called the foramen lacerum. Opening into it in front, just lateral to the pterygoid tubercle, is the pterygoid canal, whilst, in correspondence with the apex of the petrous part of the temporal, the large orifice of the carotid canal may be seen entering it behind and from the lateral side. In the recent condition the lower part of the foramen lacerum is occupied by fibro-cartilage, over the upper surface of which the internal carotid artery and greater superficial petrosal nerve pass to reach their respective foramina, whilst a small meningeal branch of the ascending pharyngeal artery occasionally enters the cranium through it. Leading laterally from the foramen lacerum in the direction of the angular spine of the sphenoid is the spheno-petrosal fissure, which lies at the bottom of the sulcus tubæ auditivæ, and disappears from view within the bony part of the auditory tube. Passing backwards from the foramen lacerum there is a fissure between the lateral side of the basi-occipital and the posterior and medial border of the petrous part of the temporal bone. This, which is called the petro-occipital fissure, opens posteriorly into the jugular foramen. In the recent condition the fissure is filled up with cartilage. The inferior surface of the petrous bone included between these two fissures is rough and irregular, and affords attachments near its apex to two small muscles, the levator veli palatini and the tensor tympani. Immediately behind the angular spine the petrous part of the temporal is pierced by a circular hole, the inferior opening of the carotid canal. This passes upwards, and then turns medially and forwards towards the apex of the bone, where it may again be seen opening into the lateral and upper side of the foramen lacerum. Laterally the wall of the vertical part of this canal, which is usually very thin, separates it from the cavity of the tympanum, as may be seen by holding the skull up to the light and looking into the external acoustic meatus. The carotid canal transmits the internal carotid artery, together with the sympathetic plexus around it. It is noteworthy that the two carotid canals lie in line with the anterior edges of the two external acoustic meatuses.
The jugular foramen is an opening of irregular shape and variable size placed between the petrous part of the temporal in front and the jugular process of the occipital bone behind. The former is excavated into a hollow called the jugular fossa, which forms a roof to the upper and lateral part of the space, whilst the latter, by a curved edge, either rounded or sharp, constitutes its posterior border. There is often considerable difference in the size of the jugular foramina ; that on the right side (with the skull in its normal position) is usually the larger. The foramen is occasionally subdivided into two by spicules of bone which bridge across it. Lodged within the fossa is the bulb of the internal jugular vein, in front of which the inferior petrosal sinus passes down to join the internal jugular vein below the foramen. Effecting an exit between the two veins, in order from before backwards, are the glosso-pharyngeal, vagus, and accessory nerves. Small meningeal branches from the ascending pharyngeal and occipital arteries also enter the foramen.
The two jugular foramina lie in line with a line drawn through the centres of the two external acoustic meatuses. Following the direction of a line connecting the angular spine of the sphenoid and the mastoid process of the temporal, and placed immediately lateral to the apertures of the carotid canal and jugular foramen, is the vaginal process of the tympanic plate of the temporal bone, the edge of which is sharp and thin, and serves to separate the inferior surface of the petrous part of the temporal from the non-articular part of the mandibular fossa. Springing from this crest immediately lateral to the jugular fossa, and in line with the middle of the external acoustic meatus, is the styloid process of the temporal bone. Its relation to the jugular foramen is of great importance as the internal jugular vein lies close to its medial side.
Immediately behind the root of the styloid process, medial to and in line with the front of the mastoid process, is the stylo-mastoid foramen, which is the inferior aperture of the canalis facialis. Through it the facial nerve passes out and the stylo-mastoid branch of the posterior auricular artery passes in. The medial surface of the mastoid process is deeply grooved at its base for the origin of the posterior belly of the digastric muscle. Medial to this, and running along, just wide of the occipito-mastoid suture, is a shallow groove in which the occipital artery is lodged. Just medial to the stylo-mastoid foramen is the synchondrosis between the extremity of the jugular process of the occipital bone and the petrous part of the temporal. The jugular process is a bar of bone which limits the jugular fossa posteriorly and abuts on the occipital condyles medially; its inferior surface is convex from before backwards and affords attachment to the rectus capitis lateralis muscle. The occipital condyles are placed between the jugular processes and the foramen magnum. Limited in front by a rounded thickening which becomes confluent with the anterior border of the foramen magnum, they form by their medial sides the lateral boundaries of that aperture on its anterior half. Laterally they are continuous with the jugular processes, in front of which they overhang a fossa which is pierced behind by the canalis hypoglossi, through which passes the hypoglossal nerve, together with a small vein and occasionally a small meningeal branch derived from the ascending pharyngeal artery.
The posterior condylic fossæ are situated just behind the posterior extremities of the condyles. Not infrequently the floor of each is pierced by the condyloid canal, through which the posterior condylic vein emerges. The base of the skull behind the jugular processes and condyles of the occipital bone is formed by the nuchal surface of the squamous part of that bone. Posteriorly this surface is bounded by the superior nuchal or curved line, in the centre of which is placed the projecting external occipital protuberance. Laterally the squamous part of occipital bone is separated from the mastoid portion of the temporal bone by the occipito-mastoid Juture, which curves backwards and laterally, from the extremity of the jugular process in front, around the base of the mastoid process behind. In front and in the median plane this plate of bone is pierced by the foramen occipitale magnum, the anterior half of which has been already seen to lie between the occipital condyles. Usually of oval form, though in some cases it tends to approach the circular, the plane of this opening is inclined downwards and slightly forwards. The extreme anterior edge of the foramen is sometimes called the basion, whilst the extreme posterior margin is termed the opisthion. The lower border of the medulla oblongata, where it becomes continuous with the spinal medulla, is lodged within the foramen, together with the meninges which cover it, whilst the vertebral arteries and the spinal portions of the accessory nerves pass upwards through it. The anterior and posterior spinal arteries, some small veins, and the roots of the first cervical nerves, also traverse it from above downwards.
The student will, no doubt, experience considerable difficulty in bearing in mind the relative positions of the various foramina and processes which he has studied on the inferior surface of the base of the skull.
If a line be drawn on either side from the incisive foramen in front, through the stylomastoid foramina posteriorly, it will be found to cut or pass near to the following objects :-On the hard palate it will lie close to the greater and lesser palatine foramina. It will then pass between the hamulus and the lateral pterygoid lamina, overlying the foramen ovale, the foramen spinosum, the opening of the osseous part of the auditory tube and the angular spine of the sphenoid ; behind this it will cut through the root of the styloid process and define laterally the limits of the jugular fossa. After passing through the stylo-mastoid foramen, if the line be prolonged backwards it will usually be found to pass over the mastoid foramen in the occipitomastoid suture. Another line of much value is one drawn across the base of the skull from the centre of one external acoustic meatus to the other. This will be found to pass through the root of the styloid process, the jugular foramen, the hypoglossal canal; it then crosses the front of the occipital condyles, and corresponds with the anterior edge of the foramen magnum.
A line which may be found useful is one drawn from the stylo-mastoid foramen of one side to the greater palatine foramen of the opposite side. This will be seen to overlie, from behind forwards, the lateral part of the jugular foramen and the inferior opening of the carotid canal. The line indicates the direction of the carotid canal, and cuts the foramen lacerum anteriorly ; in front of this it usually corresponds to the position of the posterior aperture of the pharyngeal canal.
Mandible and Atlas in Position.—The examination of the base of the skull is incomplete unless the student examines it with the mandible and atlas in position. The relation of the ramus of the mandible to the infra-temporal fossa has been already sufficiently studied (p. 169); one or two points, however, may be emphasised. The angular spine of the sphenoid lies just medial to the condyle of the maudible when that structure is in position in the articular part of the mandibular fossa, and it is noteworthy that immediately to the medial side of the angular spine is the commencement of the osseous part of the auditory tube. The root of the styloid process occupies the centre of the interval between the mandibular ramus and the front of the mastoid process.
Anteriorly the arcade formed by the body of the mandible adds greatly to the depth of the hard palate. In this space are lodged the tongue and the structures which form the floor of the mouth. The medial surface of each side of the body of the mandible is traversed by the mylo-hyoid line, which commences posteriorly just behind the root of the last molar tooth and runs downwards and forwards towards the symphysis in front.
When the atlas is in articulation with the occipital bone it is well to recognise the relation of its transverse processes to the surrounding structures. The extremities of these processes lie in line with the ends of the jugular processes of the occipital bone, and thus come to be placed just medial to and immediately below and slightly in front of the tips of the mastoid processes. They can thus be easily felt in the living subject. Anteriorly they are separated by a short interval from the styloid processes, and the stylo-mastoid foramina lie immediately in front and
slightly to the lateral side of their extremities. The student will note that there is no hole in the jugular process of the occipital bone corresponding to the arterial foramen in the transverse process of the atlas through which the vertebral artery passes. The course of this vessel over the upper surface of the posterior arch behind the superior articular processes of the atlas will be seen to coincide with the posterior condylic fosse and the margins of the foramen magnum immediately medial thereto, where a slight grooving of the edge often indicates the course of the artery
In front the anterior tubercle of the atlas falls in line with the pharyngeal tubercle on the under surface of the basi-occipital, and the student must not overlook the fact that the anterior surface of the cervical column does not coincide with the anterior margin of the foramen magnum, but lies nearly half an inch in front of that, in a frontal plane passing immediately in front of the external acoustic meatuses. Behind, the upper surface of the posterior arch of the atlas overlaps the posterior margin of the foramen magnum, and it is by the apposition of these two surfaces that extension is checked at the occipito-atlantal articulation.
THE SKULL IN SECTION. By the removal of the skull-cap the cerebral aspect of the cranial cavity is exposed. The deep surface of the cranial vault is grooved in the median plane for the superior sagittal sinus, on either side of which are seen numerous depressions for the lodgment of arachnoideal granulations. On holding the bone up to the light, the floor of these little bollows is oftentimes seen to be very thin. A short distance in front of the lambda, and on either side of the sagittal suture, are the cerebral openings of the parietal foramina. The inner tables of the frontal and parietal bones are grooved for the meningeal arteries. The principal branch of the middle meningeal runs more or less parallel to and at a variable distance behind the line of the coronal suture. Along the bottom of these grooves small foramina may be seen for the passage of nutrient arteries to the bone, and the floor of the sagittal sinus is likewise pierced by small apertures for the transmission of veins.
Basis Cranii Interna. Cranial Fossæ.—The upper surface of the base of the skull is divided into three fossæ, of which the cerebrum occupies the anterior and middle, whilst in the posterior is lodged the cerebellum.
The anterior fossa is defined posteriorly by the sharp, thin edge of the small wings of the sphenoid, which curve laterally and slightly upwards, as well as backwards, to reach the region of the pterion laterally. The floor is formed from before backwards, in the median plane, by the superior surface of the ethmoid and the anterior part of the body of the sphenoid ; laterally it is constituted by the orbital parts of the frontal and the small wings of the sphenoid. On these the inferior surface of the frontal lobes of the cerebrum rests. In front the fossa is divided in the median plane by the frontal crest, to which the falx cerebri is attached. This is confluent below with the anterior part of the crista galli, from which, however, it is separated by the foramen cæcum, which usually transmits a small vein from the nose. On either side of the crista galli there are grooves which vary considerably in depth and width: therein are lodged the olfactory bulbs. The floor and sides of the groove are pierced by numerous foramina; of these the largest number transmit the olfactory nerves from the nasal cavity. In front an elongated slit, placed on either side of the crista, affords a passage to the nose for the anterior ethmoidal branch of the naso-ciliary nerve and a small branch of the anterior ethmoidal artery which accompanies it. To the lateral side of the olfactory groove and the cribriform plate, the anterior fossæ communicate on either side by means of the two ethmoidal foramina with the cavities of the orbits. The anterior foramen transmits the anterior ethmoidal nerve and the anterior ethmoidal artery; the posterior affords passage to the posterior ethmoidal artery and nerve (O.T. spheno-ethmoidal nerve of Luschka). Lateral to the olfactory groove, the floor of the fossa, which here corresponds to the roof of the orbit, is very thin, as may