496 



MAMMALIAN ANATOMY 



FIG. 399. 



become less conspicuous and retreat almost to the bregma; the pro- 

 file of the face becomes slightly convex. If the height of the cranial 

 cavity be compared with its length by means of the cranial height 



. T /cranial height X 100\ T n -, 



index - . /: -), we observe that as the skull develops the 



\ cranial length / 



index decreases from 75 to 59 millimetres. 



The skull is developed partly from cartilage, partly from mem- 

 brane ; the cartilage occupies the base and lower parts of the sides of 

 the cranium, and gives rise to the occipital, the greater part of the 

 temporals, the sphenoid, the ethmoid, and the maxillo-turbinals. 



The skull shortly before birth (Fig. 399) is characterized by an 

 almost globular form, due to the relatively large cranium and to the 



small size and inferior position of the face. 

 The greater number of the constituent bones 

 are more or less ossified ; a few are still repre- 

 sented by membrane and cartilage. Because 

 of the extreme delicacy of the ossifications 

 and the firm attachments of muscles and 

 fibrous tissue to the bones and cartilages, 

 especially at the base of the cranium, the 

 student must exercise great care in preparing 

 the skull for study. 



The occipital bone is represented by four 

 distinct pieces separated by strips of cartilage. 

 Of ^ ese the supraoccipital is oval, convex, 

 and smooth ; the basioccipital is long, narrow, 

 and hexagonal ; and the exoccipitals are cres- 

 centic, and bear flattened condyles but no distinct paroccipital pro- 







cesses. The iuterparietal is well ossified ; it is larger than the supra- 

 occipital. The parietals are externally strongly convex ; internally 

 they bear narrow curved plates representing the tentorial processes. 

 On each side of the skull the supraoccipital, exoccipital, interparietal, 

 parietal, and temporal are separated by a large area of cartilage con- 

 tinuous below with the petrous portion of the temporal ; this cartilage 

 fills the posterior lateral fontanelle ; its lower part contains the semi- 

 circular canals. There is no true posterior fontanelle, as in man, at 

 the lambda between the supraoccipital and the parietals. 



In the temporal bone the squamous is ossified (from membrane), 



THE SKULL, BEFORE BIRTH. 



(Natural size.) 



