116 ANIMAL PHYSIOLOGY. 



amphibians and reptiles, while the heart is more complex, 

 the circulation is less perfect, there being a double waste of 

 power in sending part of the dark blood into the body, and 

 part of the red blood back to the lungs, which, though in 

 a manner accounted for as being a stage of progression 

 toward the more perfect organ found in higher animals, 

 might have been difficult to explain, if it could have been 

 noted by an observer before birds and mammals appeared on 

 the earth. 



Fig. 64. DIAGRAM OF HITMAN HEART AND VESSELS. To the sides 

 are the lungs represented in outline; and above and below are 

 the cut ends of the systemic vessels. The arrows indicate the 

 course of the blood. In the vessels left pale, pure blood circu- 

 lates; and in the darkened vessels, impure blood. 



85. The Heart in mammals, as will be seen from what has 

 been said, is divisible into a right and a left part, each of which 

 is comparable with a fish heart, consisting, as it does, of an 

 auricle and a ventricle; these parts are completely separate, 

 one from the other, from the time of birth, so far as the 

 blood contained in them is concerned; but they act syn- 



