THE SKULL. 



The head consists of the cranium and the face. The former is the brain-case ; 

 the latter is chiefly concerned in forming the jaws. The head also contains the 

 terminal organs of four special senses. That of hearing is entirely inside one of the 

 cranial bones, while the organs of sight and of smell lie in cavities formed partly by 

 cranial and partly by facial bones. The special organ of taste, a part of the surface 

 of the tongue, is in the mouth, bounded wholly by facial bones. Thus, while the 

 cranial bones have a share in forming the face, no facial bone has any part in forming 

 the brain-case. The latter is an egg-shaped cavity which communicates by a large 

 opening — the foramen magnum — with the spinal canal, through which the spinal cord 

 passes down from the brain. The brain-case has many smaller openings in the base, 

 through which nerves escape both to the face and to a large part of the body and 

 blood-vessels pass for the nutrition of the brain and its membranes and the walls of 

 the skull. 



As the bones of the head can be separated in a young subject, it is customary 

 to describe every bone by itself. It is too often forgotten that this knowledge is 

 merely a means to an end, — namely, the understanding of the skull as a whole. In 

 the following account this end is kept constantly in view. 



THE CRANIUM. 



The cranial cavity is formed by eight bones : the occipital, the sphenoid, the 

 two temporals, the ethmoid, \.\\e frontal, and the two parietals. The cranium consists 

 of the vault and the base. The vault is formed by Xhe parietals, the greater part oi 

 X.\\^ frontal , and a part of the sphenoid, of the temp07'als, and of the occipital. 



The base of the cranium is divided into three fossae extending across the skull. 

 The posterior fossa is the lowest ; it opens by the foramen magnum into the spinal"* 

 canal, and contains the cerebellum, the medulla, and the pons. The middle one is 

 narrow at the centre and expands laterally into the temporal regions. The anterior 

 is the highest, lying above the orbits and the nose. The anterior fossa transmits the 

 olfactory nerves, the middle the optic, the posterior the auditory and the glosso- 

 pharyngeal, the nerve of taste. 



THE OCCIPITAL BONE. 



The occipital bone' is divided for description into an anterior part, the basilar ; 

 two lateral ones, the condylar ; and a posterior one, the tabular or sqtiamous portion. 

 These correspond to the basi-occipital, the exoccipital, and the supra-occipital of 

 comparative anatomy. They all develop from separate centres and bound the 

 fora^nen viagman,^ a nearly circular opening, transmitting the spinal cord with its 

 enveloping membranes. The spinal accessory nerves and the vertebral arteries 

 ascend within the latter from the cavity of the spine to that of the cranium. 



The basilar portion ^ bounding the foramen magnum in front is originally rough 

 anteriorly, but shortly after puberty it coossifies with the body of the sphenoid. Its 

 superior surface is smooth and concave and supports the medulla oblongata. Just 

 internal to the edges is a very shallow groove for the inferior petrosal sinus. The 

 inferior surface is smooth for about one centimetre in front of the foramen magnum, 

 and rough in front of this for the rectus capitis anticus major and minor. In the mid- 

 dle line at the junction of the rough and smooth surfaces is \\\q pharyngeal tubercle,*" 

 Very rarely this aspect presents a depression, \}ci& pharyngeal fossa. Sometimes 

 there is a facet near the edge of the foramen for the anterior arch of the atlas. Also, 

 there may be a tubercle on the posterior part of the basilar portion against which the 

 odontoid process may rest, called the third condyle. Laterally, the basilar portion 



' Os occipitale. ' Foramen occipitale magnum. ' Parsbasilaris. ■* Tubcrculum pharyngeum. 

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