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! 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 Cholepus). The different vertebre, 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. 
