226 ANIMAL FORMS 
Some rabbits and hares in the far north don a white coat 
in the winter season. 
212. Skeleton—As in other vertebrates, the external 
form of mammals is dependent in large measure upon the 
internal skeleton. This consists of relatively compact 
bones, the cavities of which are filled with marrow. Those 
forming the skull are firmly united, and, as in other verte- 
brates, afford lodgment for several organs of special sense 
and for the brain, which, like that of the birds, completely 
fills the cavity in which it rests. The vertebral column to 
which the skull is attached differs considerably in length, 
but it invariably gives attachment to the ribs, and to the 
basal girdles supporting one or two pairs of limbs. Gener- 
ally speaking, the number of bones in the head and trunk 
of all mammals is the same, so the variations we note in 
the species about us, for example, are simply due to differ- 
ences of shape and proportion. As we are aware, there is a 
great dissimilarity between the length of the neck of man 
and that of the giraffe, yet the number of bones in each 
is precisely the same. On the other hand, the variations 
occurring in the limbs are often due to the actual disap- 
pearance of parts of the skeleton. Five digits in hand 
and foot is the rule, and yet, as we well know, the horse 
walks on the tip of its middle finger and toe, the others 
being represented by small, very rudimentary, splint bones 
attached far up the leg. The even-hoofed animals walk on 
two digits, two smaller hoofed toes being often plainly 
visible a short distance up the leg, as in the pig. In the 
whales the hind limbs have completely disappeared, and in 
the seals, where the fore limbs are modified, as in the 
whales, into flippers, the hind limbs show many signs of 
degeneration. 
213. Digestive system.—Some mammals, such as man, 
monkeys, and pigs, are omnivorous; others, like the cud- 
chewers and gnawers, are vegetarians; and still others, 
like the foxes, weasels, and bears, are carnivorous. In 
