OUE LIFE. 51 



by every portion of his frame, we must expect him to 

 share his characteristic functions with the rest of the 

 mammals. Such we find to be the case. The circu- 

 lation of the blood and respiration are accomplished 

 in man under precisely the same laws and in the 

 same manner as in all the other mammals — and in 

 these alone ; they are determined by the peculiar 

 structure of their heart and lungs. In mammals only 

 is all the arterial blood conducted from the left 

 ventricle of the heart to the body by one, the left, 

 branch of the aorta, while in birds it passes along 

 the right branch, and in reptiles along both branches. 

 The blood of the mammal is distinguished from that of 

 any other vertebrate by the circumstances that its red 

 cells have lost their nucleus (by reversion). The 

 respiratory movements are effected largely by the 

 diaphragm in this class of animals alone, because 

 only in them does it form a complete partition 

 between the pectoral and abdominal cavities. Special 

 importance, however, in this highest class of animals, 

 attaches to the production of milk in the breasts 

 (mammce), and to the peculiar method of the rearing 

 of the young, which entails the supplying of the 

 offspring with the mother's milk. As this nutritive 

 process reacts most powerfully on the other vital 

 functions, and the maternal affection of mammals 

 must have arisen from this intimate form of rearing, 

 the name of the class justly reminds us of its great 

 importance. In millions of pictures, most of them 

 produced by painters of the highest rank, the 

 "madonna with the child" is revered as the purest 

 and noblest type of maternal love — the instinct which 

 is found in its extreme form in the exaggerated 

 tenderness of the mother-ape. 



