86 THE SKELETON 



The POSTERIOR iRANiAL FOSSA is tlu' (Icepcst poi'tiou of the cavity. It is bounded 

 by the dovsuin ephippii and the superior borders of the petrosals, tlie mastoid 

 portion of the petrosals, the posterior inferior angle of the parietals, and the squanio- 

 oecipital below the level of the crest (supra-occipital). The upi)er limits are 

 indicated by the grooves for the lateral sinuses. It is marked by the following 

 sutures : — the petro-occipital, occipito-mastoid, parieto-mastoid, and, in young 

 skulls, the basilar suture. 



The ridges limiting this fossa give attachment to the tentorium cerebelli, and 

 the fossa lodges the cerebellum with the pons and medulla. It communicates with 

 the general cranial cavity by means of the foramen of Pacchionius when the ten- 

 torium is /*( dtu. 



It has the following objects of interest: — 



The clivus, extending from the dorsum ephii)pii to the anterior margin of the 

 foramen magnum. This is in relation with the basilar artery, the i)ons, the 

 medulla, the sixth nerves and the basilar sinus. 



The notch for the sixth nerve on each side of the dorsum epliii)pii. This is 

 sometimes a foramen, termed the petro-sphenoid. 



The foramen magnum, ])resenting on eacli side a tubercle for the check liga- 

 ments; and tlic anterior condyloid foramen (sometimes sul)divided by a spicu- 

 lum of bone) for the hypoglossal nerve. Behind the foramen there is a vertical 

 ridge of bone, the internal occipital crest, for the falx cerebelli. This sometimes 

 presents a depression known as the vermiform fossa. 



The anterior boundaries of the fossa present: — 



A notch for the passage of the fifth nerve. This is a foramen when the ten- 

 torium is present. 



The internal auditory meatus, for the facial and auditory nerves, and tlie 

 auditory branch of the liasilar artery. 



The jugular foramen, which transmits the glosso-pharyngeal, vagus, and spinal 

 accessory nerves, the internal jugular vein, the meningeal branch of the ascending 

 pharyngeal artery, aqueductus vestibuli and hiatus subarcuatus. 



Tlie termination of the groove for the lateral sinus, with the internal orifice of 

 the mastoid foramen. 



The cranium of an average European has a capacity of 1450 cc. The circumfer- 

 ence, taken in a plane passing through the ophryon anteriorly, the occipital point 

 posteriorly, and the pterion laterally, is 52 cm. The length from the ophryon to 

 the occipital point is 17 cm. ; the 'width between the parietals at the level of the 

 zygomata is 12-5 cm. ; and the height from the basion to the bregma is nearly the 

 same. The cranio-facial angle is about 96°. 



THE TEETH 



An adult individual with perfect dentition possesses thirty-two teeth, equally 



distril)uted to the maxilla and mandible. The upper set are called maxillary 



teeth and the lower set mandibular. The four central teeth in each dental arch are 



termed incisors; the tooth next these on each side is the canine; behind these are two 



premolars or bicuspids; and lastly, three molars. This relation of teeth is expressed 



by means of a formula: — 



.2 1 2 S 

 I — c - mm ~ in = 32. 

 2 1^ 2 ;? 



Each tooth has a portion coated with enamel exposed al)ove the gum — named 

 the crovvn ; and a portion coated with cementum embedded in bone — this is tlie 

 root. The line of union of the crown and root is termed the neck. 



The surface of the tooth directed towards the lips and cheek is called labial 

 and buccal respectively, and that towards the tongue lingual. It is also necessary 

 to apply definite terms to the opposed surfaces of teeth, hence the surface directed 

 towards tlie middle line of the mouth if the alveolar arch were straightened out 

 is median, and the opposite side is distal. Each tooth has distinguishing fea- 

 tures. 



