206 VERTEBRATE ZOOLOGY 



the femoral vein, and the portal venous system through the portal 

 vein near the liver ; the veins, however, can be easily studied with- 

 out injection. Skin the inner surface of the leg from the trunk 

 to the knee. The femoral artery and vein will be seen lying along- 

 side of each other in the middle of the thigh, very near the surface ; 

 the vein is the larger of the two, and is filled with blood, while the 

 artery is usually a pale pink in color. 



If this is not done, it will be necessary, in order that the brain 

 may be studied when the time comes, to cut off the head of a fresh 

 cat and, after sawing off the top of the skull, to place it in a 5 

 per cent solution of formalin or, what is better, a solution con- 

 sisting of one part of 5 per cent formalin and three parts of 95 

 per cent alcohol, and to leave it there until the brain is thor- 

 oughly hardened and is needed for study. In removing the top 

 of the skull, care must be taken not to saw into the brain, and 

 also to remove the transverse bony partition between the cere- 

 brum and cerebellum without injuring either. 



Study the external characters of the animal. Just as a bird may 

 be told by its feathers, so the most distinguishing feature of the 

 mammal is its hair; all mammals have hair, and only mammals 

 have it. It is a highly specialized epidermal structure, whose prin- 

 cipal function is the retention of the bodily heat of the animal. 

 The warm air between the hairs is not easily removed ; radiation 

 is thus checked, and the animal keeps warm. It is due largely to 

 the development of hair that mammals have become animals with 

 a constant temperature, or warm-blooded animals. In cold-blooded 

 animals the temperature is in a high degree inconstant, varying 

 sometimes ioo° F. at different times, being approximately that of 

 the surrounding air or water. 



Observe the arrangement of the hair on the body. Almost the 

 entire body is covered, only the tip of the nose, the walking-pads 

 of the feet, and the tips of the nipples being bare. Note the 

 difference in the texture of the hair on different parts of the body. 

 The most highly specialized hairs are the whiskers, or vibrissae, 

 on the upper lip, the sides of the cheeks, and above the eyes. 

 These are tactile organs and are an important assistance to a 

 nocturnal animal. 



