226 VETERINARY PHYSIOLOGY 



(Ly.), from which it gains certain necessary constituents, and 

 finally bring it back to the central pump. 



The fluid, which has not passed out of the capillaries into 

 the tissues, has been deprived of many of its constituents, and 

 this withdrawal of nutrient material by the tissues is made 

 good by a certain quantity of the blood being sent through the 

 walls of the stomach and intestine (Al.C.), in which the 

 nutrient material of the food is taken up and added to the 

 blood returning to the heart. At the same time, the waste 

 materials adiled to the blood by the tissues are partly got rid 

 of by a certain quantity of the blood being sent through the 

 liver and kidneys (Liv. and Kid.). 



The blood is then poured back, not at once into the great 

 pump which sends it through the body, but into a subsidiary 

 pump the pulmonic heart (P.H.) by which it is pumped 

 through the lungs, there to obtain a fresh supply of oxygen, 

 and to get rid of the carbon dioxide excreted into it by the 

 tissues. Finally the blood, with its fresh supply of oxygen 

 from the lungs, and of nourishing substances from the ali- 

 mentary canal, is poured into the great systemic pump 

 the left side of the heart again to be distributed to the 

 tissues. 



Thus the circulation is arranged so that the blood, exhausted 

 of its nourishing material by the tissues, is replenished in the 

 body before being again supplied to the tissues. 



The sectional area of this irrigation system varies enormously. 

 The aorta leaving the heart has a comparatively small channel. 

 If all the arteries of the size of the radial were cut across and 

 put together, their sectional area would be many times the 

 sectional area of the aorta. And, if all the capillary vessels 

 were cut across and placed together, the sectional area would 

 be about 700 times that of the aorta. 



From the capillaries, the sectional area of the veins and 

 lymphatics steadily diminishes as the smaller branches join 

 with one another to form the larger veins and lymphatics ; but, 

 even at the entrance to the heart, the sectional area of the 

 returning tubes, the veins, is about twice as great as that of 

 the aorta (fig. 132, p. 288). 



The circulatory system may thus be compared to a stream 

 which flows from a narrow deep channel, the aorta, into a 



