14 



ANIMAL PHYSIOLOGY. 



It follows from this that every part of a complex body, 

 like that of man, requires a supply of nourishment to be 

 brought to it, and a channel of escape for waste products ; 

 and to meet these requirements there are many different 

 organs. The immediate source of nourishment of all the 

 tissues is the blood, but to recruit the supply of this fluid 

 new materials have constantly to be taken in ; and to fulfil 

 this end, the alimentary canal receives the food and digests 

 it, that is to say, reduces the newly received materials to 

 such a condition that they can be taken up by processes of 

 absorption from the cavity of the canal, and carried to the 

 blood. To complete the elaboration of the blood, and free it 

 from impurities constantly resulting from waste, a variety of 

 organs are engaged, of which the principal are the lungs, 

 spleen, liver, kidneys, and skin. Lastly, to convey the 

 blood to and from the tissues which it nourishes and the 

 organs in which it is purified, a system of ramifying vessels 

 is required, and a heart with muscular force sufficient to 

 propel the blood through them. 



Fig. 1. -DIAGRAM OF CAPILLARY NETWORK, with termination of an 

 Artery and commencement of a Vein. 



It may be here mentioned that the system of blood-vessels 

 is completely closed in all vertebrate animals, the blood 

 being distributed from the heart by arteries, circulating 

 through the tissues in a network of minute vessels called 

 capillaries, and returning to the heart by veins. But the 



