MAMMALS 



189 



The skull is joined to the spinal column by two knobs 

 (or condyls), which fit into sockets in the first vertebra. 

 Compare the jaws with those of a bird and a reptile. 

 There is a prominent ridge in the temple to which the 

 powerful chewing muscles are attached. There is also a 

 ridge at the back of the head where the muscles which 

 support the head are attached (Fig. 348). 



Count the ribs. Are there more or fewer than in man ? 

 The breastbone is in a number of parts, joined, like the 

 vertebras, by cartilages. Compare it with a bird's ster- 

 num ; why the difference ? The shoulder girdle, by which 

 the front legs are attached to the 

 trunk, is hardly to be called a gir- 

 dle, as the collar bones (clavicles) 

 are rudimentary. (They often es- 

 cape notice during dissection, being 

 hidden by muscles.) The shoulder 

 blades, the other bones of this gir- 

 dle, are large, but relatively not so 

 broad toward the dorsal edge as 

 human shoulder blades. The clav- 

 icles are tiny because they are useless. Why does the cat 

 not need as movable a shoulder as a man ? The pelvic, or 

 hip girdle, to which the hind legs are attached, is a rigid 

 girdle, completed above by the spinal column, to which it 

 is immovably joined. Thus the powerful hind legs are 

 joined to the most rigid portion of the trunk. 



Mammals. — The cat belongs to the class Mammalia or 

 mammals. The characteristics of the class are that the 

 young are not hatched from eggs, but are born alive, and 

 nourished with milk (hence have lips), and the skin is 

 covered with hair. The milk glands are situated ventrally. 

 The position of the class in the animal kingdom was 



Fig. 338. — Claw of Cat 



(1) retracted by ligament, and 



(2) drawn down by muscle 

 attached to lower tendon. 



