The Skeleton of tpie Head. 75 



The four middle segments are similar in appearance, and form the 

 body (corpus sterni). The sixth segment, described as the xiphoid 

 process (processus xiphoideus), is an elongated strip of bone, to the 

 posterior end of which is attached a broad, thin plate of cartilage. 



The first costal articulation is situated at about the middle of the 

 manubrium, the remaining six at the points of junction of the segments. 

 Five of them occur singly, while the sixth and seventh costal cartilages 

 are attached together at the point of junction of the last segment of the 

 corpus sterni with the xiphoid process. 



THE SKELETON OF THE HEAD. 



The head-skeleton comprises: (1) the series of elements constituting 

 the skull ; and (2) the hyoid bone, with its connections. The skull, or 

 cranium — using that term in a general sense — includes the cranium 

 proper, that portion enclosing the brain and containing in its wall the 

 auditory capsules, and the bones of the face (ossa faciei), the latter in- 

 cluding the series of elements related for the most part to the jaws and 

 palate. The primary relations of the constituents of the head-skeleton 

 have already been indicated above (p. 31). 



A. THE SKULL AS A WHOLE. 



The skull is a composite structure, consisting of a large number of 

 elements, which, with the exception of the mandible, are united by 

 synarthroses, so that they produce the effect of a continuous mass. The 

 mandible is a more or less independent structvire, articulated with the 

 main body of the skull by a tvpical joint. 



The skull is roughly divisible into two portions, namely, an anterior, 

 facial portion, and a posterior, cranial portion. The cranial portion has 

 a somewhat conical shape, its apex being directed forward. It is sepa- 

 rated from the facial portion by a depression on either side of the skull, 

 the orbital cavity (orbita), which serves for the accommodation of the 

 optic bulb. Unlike the remaining special sense-organs, the eye is not 

 included within the skull-wall. The two portions are united both 

 medially and laterally, the lateral connection being established by the 

 zygomatic arch (arcus zygomaticus), which bridges the lateral portion of 

 tlic orbit. The facial portion has also a somewhat conical shape, its 

 apex being formed by the anterior extremity of the upper jaw and the 

 incisor teeth. Its base is formed in part by the connection with the 

 cranial portion, as already described, and also by the anterior walls of the 

 orbits. 



The cranial portion exhibits an extensive posterior, nuchal surface 

 (planum nuchale), situated in general at right angles to the cervical 

 portion of the vertebral column and also to the dorsal, lateral, and ventral 

 walls of the skull. This surface includes the external surface of the 

 occipital bone, with the exception, chiefly, of the basilar portion of the 

 latter. Its dorsal portion forms an area of attachment for the spinal and 

 special occipital musculature. Its ventral portion is perforated by a 



