294 BACKBONED ANIMALS. 
Class VII.—MAamMaLia (Milk-givers). 
General Characteristics—We now come to the highest 
and most perfect animal forms. They are covered with 
hair instead of scales. The young are born alive,* and 
nourished by a fluid called milk, secreted in the mammary 
glands. About twenty-one hundred species of living 
mammals are known, three hundred and ten inhabiting 
North America. 
Skeleton.—The skeleton, that in the majority of birds 
is extremely light, is in the mammals solid, and the limb- 
cavities filled with 
marrow. Taking 
the cat (Fig. 321) as 
an example, we first 
notethecranium,cra, 
orskull, thatis united 
to the backbone or 
vertebral column by 
two occipital con- 
dyles. Thelowerjaw 
is composed of two 
Fic. 321.—Cat, with bones of right side drawn. pieces, and is joined 
Cra, cranium; sc, scapula or shoulder- directly to the skull 
blade; 1, humerus; 2, radius and ulna; y 7 
3, carpus; 4, phalanges; 5, femur; 6,tibia and not to the quad- 
and fibula; 7, tarsus; 8, metatarsus; 9, rate bone, as we have 
phalanges ; 7, innominate bone, a number seen in the birds and 
of bones combined, forming the pelvic ' 
arch; v, vertebral column. (After Morse.) r eptiles. The back- 
bone is divided into 
five divisions: First, the cervical or neck region, where 
the vertebrae generally number seven. In the cat they are 
small, in the whale they are pressed together, while in the 
long-necked giraffe each bone is lengthened out. Sec- 
ond, the dorsal or back region, the vertebrz of which 
generally number from ten to fifteen; they support the 
* See note on page 297. 
