376 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



two chambers — a receiving chamber and a pumping 

 chamber. This latter chamber must be strong, for it 

 urges the whole circulation. In the animal body the 

 receiving chamber is called the auricle and the pumping 

 chamber the ventricle. 



(4) But there are two distinct objects to be subserved 

 in animal circulation — viz., the nutrition of the tissues 

 and carrying away of waste, on the one hand, and aera- 

 tion of the blood on the other. In the highest animals 

 these two are wholly differentiated and jealously kept' 

 separate. There are, therefore, two complete circula- 

 tions and two hearts, each with its two chambers (auricle 

 and ventricle) and its three kinds of blood vessels (ar- 

 teries, veins, and capillaries). But for convenience the 

 two hearts are put together and form a/^r-chambered 

 heart, but separated by a dead wall between. The blood 

 starts out from the heart on its journey in one circula- 

 tion and comes back ; then starts again on the other 

 circulation and comes back, and so on continuously. 

 The first is called the greater or systemic circulation, and 

 its function is to nourish the tissues and carry away 

 their waste. The second is the lesser or pulmonic circu- 

 lation, and its function is to aerate the blood or ex- 

 change the C0 2 of the blood for O of the air. Of the two 

 hearts, that on the left side is the systemic and that on 

 the right the pulmonic. Through each of these circula- 

 tions the whole blood passes, and so jealously are they 

 kept separate that although the whole of the blood 

 passes through the lungs in the pulmonic circulation, 

 yet none of it is used for the nutrition of the lungs, but 

 this work is done by a small branch from the systemic 

 circulation. 



General Course of the Circulation. — Fig. 258 

 gives a very generalized idea of this course. The ar- 

 rows show the course of the current. Commencing at 



