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either side of the foramen magnum; on their inferior surfaces they bear the occipital condyles by means of which the skull articulates with the atlas. Of elongated oval form, the condyles are so disposed that their anterior extremities, in line with the anterior margin of the foramen magnum, lie closer together than their posterior ends, which extend as far back as the middle. of the lateral borders of the foramen. Convex from before backwards, they are skewed so that their surfaces, which are nearly plane from side to side, are directed slightly laterally. Each is supported on a boss of bone, pierced by the canalis hypoglossi (hypoglossal canal), which opens obliquely from within outwards and forwards on the floor of a fossa, situated just lateral to the anterior part of the condyle. The canal transmits the hypoglossal nerve, together with a meningeal branch of the ascending pharyngeal artery and its companion
External occipital protuberance
Highest nuchal line
veins. Behind the condyle is placed the fossa condyloidea, in the floor of which the canalis condyloideus (condyloid canal) frequently opens. Through this a vein passes which joins the transverse sinus. The fosse lodge the posterior margins of the superior articular processes of the atlas in extension of the head. The edge of the foramen magnum immediately posterior to the condyle is often grooved for the passage of the vertebral artery around it. Jutting laterally from the posterior half of the condyle is a stout bar of bone, serially homologous with the vertebral transverse processes; this is the processus jugularis; deeply notched in front (jugular notch) its anterior border is free and rounded, and forms the posterior boundary of the jugular foramen. Curving laterally from this margin, in line with the hypoglossal canal, there is often a small pointed projection, the processus intrajugulare, which serves to divide the jugular foramen into two compartments. Laterally, the jugular process articulates by means of a synchondrosis with the jugular surface of the petrous part of the temporal bone. Its posterior border is confluent with the inferior and lateral portion of the occipital squama, and its under surface is rough and tubercular for the attachment of the
rectus capitis lateralis muscle. The superior aspect of the lateral part displays on either side of the foramen magnum an elevated surface of oval form, the tuberculum jugulare; this corresponds to the part of the bone which bridges over the canal for the hypoglossal nerve. Its upper surface in many instances displays an oblique groove running across it; in this are lodged the glosso-pharyngeal, vagus, and accessory nerves. The jugular process is deeply grooved superiorly for the lower part of the transverse blood sinus, or sigmoid sinus, which here turns round the anterior free edge of the process into the jugular foramen. Joining this, close to its medial edge, is the opening of the canalis condyloideus, when it exists.
The basilar part of the occipital bone extends forwards and upwards from the foramen magnum. Its anterior extremity is usually sawn across, as,
FIG. 134.-OCCIPITAL BONE (Cerebral Surface).
Groove for transverse sinus and tentorium cerebelli
after adult life, it is necessary to sever it in this way from the sphenoid, the cartilage uniting the two bones having by that time become completely ossified. Broad and thin posteriorly, it narrows at the sides and thickens vertically in front where on section it displays a quadrilateral form. Projecting from its inferior surface some little distance anterior to the foramen magnum is the pharyngeal tubercle to which the fibrous raphe of the pharynx is attached; on each side of this the longus capitis and rectus capitis anterior muscles are inserted. superior surface forms a broad and shallow groove which slopes upwards and forwards from the thin anterior margin of the foramen magnum; in this rests the medulla oblongata. On each side its lateral edges are faintly grooved for the inferior petrosal venous sinuses, below which the lateral aspect of the bone is rough for the cartilage which unites it to the sides and apex of the petrous part of the temporal bone.
The foramen magnum, of oval shape, so disposed that its long axis lies in the
sagittal plane, is of variable size and form. The plane of its outlet differs somewhat in individual skulls; in most instances it is directed inferiorly and slightly forwards. Anteriorly the condyles encroach upon it, and narrow to some extent its transverse diameter. To its margins are attached the ligaments which unite it with the atlas and epistropheus. Through it pass the lower part of the medulla oblongata where it becomes continuous with the spinal medulla, the two vertebral arteries, the accessory nerves, and the blood-vessels of the meninges of the superior part of the spinal medulla.
Connexions. The occipital bone articulates with the two parietals in front and above, with the sphenoid in front and below, with the two temporals on either side, and with the atlas by means of its condyles.
Ossification. The major part of the bone ossifies in cartilage, the upper part of the squamous part (interparietal) alone developing in membrane. The basilar part begins to ossify about the sixth week of foetal life by the appearance of two centres, one in front of the other; the anterior, according to Albrecht, constitutes the basiotic, the posterior the basi-occipital. These two centres-which there is some reason to believe may themselves be formed by the fusion of pairs placed laterally-rapidly unite, so that the occurrence of one centre alone is frequently described. From this the anterior part of the margin of the foramen magnum is formed, together with a portion of the anterior end of the occipital condyle on either side. It helps also to close up the front of the hypoglossal canal. Union with the condylic parts is complete about the fourth or fifth year. Ankylosis between the basi-occipital and the sphenoid takes place about the twenty-fifth year.
The lateral, condylic, or exoccipital parts begin to ossify from a single centre about the end of the second month of foetal life. The notch for the hypoglossal canal appears about the third month. From this centre is formed the posterior three-fourths of the occipital condyle. The exoccipital is usually completely fused with the squamous part by the third year or earlier.
As already noted, the squamous part consists of two parts-the one above the occipital crest, the other below it; the former develops in membrane, the latter in cartilage. In a three-months foetus this difference is very characteristic. The cartilaginous part (supra-occipital) begins to ossify from two centres (four according to Mall) about the sixth or seventh week, which rapidly join to form an elongated strip placed transversely in the region of the occipital protuberance. The centres for the superior part (interparietal) appear later. According to Maggi (Arch. Ital. Biol. tome 26, fas. 2, p. 301), they are four in number, of which two placed on either side of the median plane appear about the second month. The other pair, placed laterally, are seen about the third month; fusion between these takes place early, but their disposition and arrangement explain the anomalies to which this part of the bone is subject. The medial pair may persist as separate ossicles, or fuse to form the pre-interparietals, whilst the lateral pair may remain independent of the supra-occipital as a single or double interparietal bone, the former, owing to the frequency of its occurrence in Peruvian skulls, being sometimes called the "os Inca." Union between the supraFIG. 135.-OSSIFICATION OF THE OCCIPITAL occipital and the interparietal elements occurs about the third or fourth month; but evidence of their a, Basilar centre; b, Exoccipital; c, Ossicle of Kerkring; d, Supra-occipital (from car- separation is frequently met with even in the adult tilage); e, Fissure between supra-occipital by the persistence of a transverse suture running and interparietal; f, Interparietal (from inwards from each lateral angle of the squamous part, membrane); g, Fissure between inter- or, as above mentioned, there may be an os Incæ. parietals. The supra-occipital forms a small part of the median part of the posterior border of the foramen magnum, though here a small independent centre, known as the ossicle of Kerkring, is occasionally met with. independent centres are sometimes seen between the supra-occipital and the exoccipitals.
At birth the occipital consists of four parts-the interparietal and supra-occipital combined, the basi-occipital, and the exoccipitals-one on either side.
The temporal bone lies about the centre of the inferior half of either side of the skull, and enters largely into the formation of the cranial base. It is placed between the occipital behind, the parietal above, the sphenoid in front, and the occipital and sphenoid medially and below. At birth it consists of three parts -a superior and lateral part, the squama temporalis or squamous portion; a medial and posterior portion, the petro-mastoid, which contains the parts specially associated with the sense of hearing, together with the organ associated with equilibration; and an inferior or tympanic part, from which the floor and anterior wall of the external acoustic meatus is formed.
The squamous part consists of a thin shell-like plate of bone placed vertically, having a medial (cerebral) and a lateral (temporal) surface and a semicircular upper border. Inferiorly, behind, and medially it is fused in early life with the petro-mastoid portion by means of the squamoso-mastoid and the petro-squamosal sutures, traces of which are often met with in the adult bone; whilst below and in front it is separated from the tympanic and petrous parts by the petro-tympanic fissure. Its temporal surface, smooth and slightly convex, enters into the formation of the floor of the temporal fossa, and affords attachment to the temporal muscle. Near its posterior part it is crossed by one or more ascending grooves for the branches of the middle temporal artery. In front and below there springs from it the processus zygomaticus. This arises by a broad attachment, the surfaces of which are inferior and superior; curving laterally and forwards, it then becomes twisted and narrow, so that its sides are turned medially and laterally and its edges directed upwards and downwards. Anteriorly it ends in an oblique serrated extremity which articulates with the temporal process of the zygomatic bone. Posteriorly the edges of the zygomatic process separate and are termed its roots. The superior edge, which becomes the posterior root, sweeps back above the external acoustic meatus, and is continuous with the supra-mastoid crest, which curves backwards and slightly upwards, and serves to define the limit of the temporal fossa posteriorly. Internally this ridge corresponds to the level of the floor of the middle cerebral fossa. The inferior edge turns medially and constitutes the anterior root; the inferior surface of this forms a transversely disposed rounded ridge, the tuberculum articulare (O.T. articular eminence), behind which there is a deep hollow, the fossa mandibularis, limited posteriorly by the tympanic plate, and crossed at its deepest part by an oblique fissure, the petrotympanic fissure. This cleft, which is closed laterally, transmits about its middle. the tympanic branches of the internal maxillary artery, and lodges the anterior process of the malleus. At its medial end the lips of this fissure are frequently separated by a thin scale of bone, a downgrowth from the tegmen tympani of the petrous part, which here separates the tympanic from the squamous elements, forming in its descent the major part of the lateral wall of the osseous auditory tube, which lies just medial to it. Between this scale of bone and the posterior edge of the fissure there is a canaliculus, which transmits the chorda tympani nerve. The part of the mandibular fossa in front of the petro-tympanic fissure, as well as the articular tubercle, articulates with the condyle of the mandible, through the medium of the interposed articular disc. The part of the fossa behind the fissure is non-articular and lodges a portion of the parotid gland. the angle formed by the divergence of the two roots of the zygoma, in correspondence with the lateral part of the articular tubercle, there is a rounded tubercle; to this are attached the fibres of the temporo-mandibular ligament of the mandibular joint. In front of the medial end of the articular tubercle there is a small triangular surface, limited anteriorly by the edge of the anterior root, and medially by a thick serrated margin which articulates with the temporal aspect of the great wing of the sphenoid; this area forms part of the roof of the infra-temporal (O.T. zygomatic) fossa. Just anterior to the external acoustic meatus and projecting
downwards from the inferior surface of the posterior root there is a conical process, called the post-glenoid tubercle, which forms a prominent anterior lip to the lateral extremity of the petro-tympanic fissure; it is the representative in man of a process which is developed in some mammals and prevents the backward displacement of the mandible. By some anatomists it is referred to as the middle root of the zygomatic process.
The zygomatic process by its inferior margin and medial surface gives origin to the masseter muscle, whilst attached to its superior edge are the layers of the temporal fascia. Behind the external acoustic meatus, and below the supramastoid crest, the squamous element extends downwards as a pointed process, which assists in forming the roof and posterior wall of the external acoustic meatus, where it unites inferiorly with the tympanic part and forms the lateral wall of a hollow within called the tympanic antrum. In the adult this process is occasionally
FIG. 136. THE RIGHT TEMPORAL BONE SEEN FROM THE PARIETAL SIDE.
sharply defined posteriorly by an oblique irregular fissure, the remains of the mastosquamosal suture. Immediately above and behind the external acoustic meatus there is often a little projecting spur of bone, the spina suprameatum (suprameatal spine).
The angular recess between this process and the supramastoid crest is of interest surgically, and is known as Macewen's triangle. The same authority has pointed out that the masto-squamosal suture frequently remains open till puberty and occasionally after, and may be of importance as a channel along which infective processes may extend.
The cerebral surface of the squamous part, less extensive than the parietal aspect owing to the bevelling of the parietal border, is marked by the impression of the gyri of the temporal lobe of the cerebrum, and is limited below by the petrosquamosal suture, the remains of which can frequently be seen. It is crossed in front by an ascending groove for the posterior branch of the middle meningeal artery and its accompanying vein, branches from which course backwards over the bone in grooves more or less parallel to its parietal border.
The parietal border of the squamous part is curved, sharp, and scale-like, being