i6 



MAMMALS 



of a frog's foot (Fig. ii), where the corpuscles can be seen 

 following one another through the veins in an endless 

 procession. 



.6 



Fig. II. — Caph.lary Circulation in the Web of a Frog's Foot, x ioo. 



a, b, small veins ; d, capillaries in which the oval corpuscles are seen to follow one 

 another in single series ; c, pigment cells in the skin. 



In the mammals and higher animals in general, the 

 blood vessels are of two kinds, arteries and veins. The 

 former take blood from the heart, while the latter conduct 

 it back to that great reservoir. The veins have curious 

 valves (Fig. 12). 



The mammals breathe by means of lungs (Fig. 13), which 

 are elastic, spongelike organs permeated with air cells. 

 Air, by the breathing process, is taken in at the nostrils or 

 mouth, and reaches the blood in the lungs, there aerating it. 



The brain of mammals is larger than that of many 



