CHAPTER IX. 

 CIRCULATION. 



84. WE Lave already had occasion to mention that the 

 blood circulates through the body in a system of close vessels. 

 It is propelled by the heart through the arteries to a fine 

 capillary network, whence it returns to the heart again by 

 the veins. 



The blood which has circulated in the tissues requires to be 

 aerated to reconvert it from the dark to the scarlet condition, 

 before it can be allowed to go to the tissues again; and this 

 is managed in different ways in different animals. In fishes, 

 the blood returning from the system is propelled by the heart 

 into the gills, and from them right on into the system again; 

 passing through two sets of capillaries, one in the gills 

 and the other in the system, before it returns to the heart. 

 In amphibians, for example in the frog, and in reptiles, with 

 the exception of the crocodiles, the blood is propelled from 

 the heart partly into the respiratory organs, and partly into 

 the system, and returns from both these destinations to be 

 mixed in the heart; and this mixture of scarlet and dark 

 blood is what circulates again both in the system and respira- 

 tory organs. In crocodiles, none but dark blood is sent to 

 the lungs; but there is a communication by which part of 

 the dark blood may be carried back into the system along 

 with the scarlet stream. In warm-blooded animals, namely, 

 birds and mammals, the whole of the blood returning from 

 the system is sent from the heart to the lungs, and the whole 

 of the blood returned from the lungs is sent into the system. 



In fishes, the heart consists of one receiving chamber or 

 auricle, and one propulsive chamber or ventricle. In the 

 frog and the turtle, it has two auricles, one receiving dark 

 blood from the body, and the other red blood from the lungs, 



