THE SKELETON 39 



of the skull receive much impress from the real shape of the cavity 

 containing the brain. The size and form of the mouth, and the 

 modifications of the jaws for the support of teeth of various shape 

 and number, the ridges and crests on the cranium for the attachment 

 of the muscles necessary to put this apparatus in motion, and out- 

 growths of bone for the enlargement of the external surface required 

 for the support of sense organs or of weapons, such as horns or 

 antlers (which outgrowths, to prevent undue increase of weight, are 

 filled with cells containing air), cause the principal variations in the 

 general configuration of the skull. These variations are, however, 

 only characteristically developed in perfectly adult animals, and are 

 in many cases more strongly marked in the male than the female 

 sex. Throughout all the later stages of growth up to maturity the 

 size and form of the brain-case remain comparatively stationary, 

 while the accessory parts of the skull rapidly increase and assume 

 their distinctive development characteristic of the species. 



The hyoidean apparatus in mammals (Fig. 6) supports the tongue 

 and larynx, and consists of an inferior median portion termed the 

 basihyal, from which two pairs of half arches, or cornua, extend up- 

 wards and outwards. The anterior is the more important, being 

 connected with the periotic bone of the cranium. It may be almost 

 entirely ligamentous, but more often has several ossifications, the 

 largest of which is usually the stylohyal. The posterior cornu 

 (thyrohyal) is united at its extremity with the thyroid cartilage of 

 the larynx, which it suspends in position. The median portion, 

 or basihyal, is sometimes, as in the Howling Monkeys, enormously 

 enlarged and hollowed, admitting into its cavity an air-sac connected 

 with the organ of voice. 



Vertebral Column. — The vertebral column consists of a series of 

 distinct bones called vertebra?, arranged in close connection with 

 each other along the dorsal side of the neck and trunk, and in the 

 median line. 1 It is generally prolonged posteriorly beyond the 

 trunk, to form the axial support of the appendage called the tail. 

 Anteriorly it is articulated with the occipital region of the skull. 

 The number of distinct bones composing the vertebral column 

 varies greatly among the Mammalia, the main variation being 

 due to the degree of elongation of the tail. Apart from this, in 

 most mammals the number is not far from thirty, though it may 

 fall as low as twenty-six (as in some Bats), or rise as high as 

 forty (Hyrax and Cholcepus). The different vertebrse, with some 

 exceptions, remain through life quite distinct from each other, 

 though closely connected by means of fibrous structures which 

 allow of a certain, but limited, amount of motion between them. 

 The exceptions are the following: — (1) near the posterior part 



1 For the sake of uniformity, in all the following descriptions of the vertebral 

 column, the long axis of the body is supposed to be in the horizontal position. 



