14 ZOOLOGY. 



Since the blood in the course of its circulation gives 

 up some of its nutriment to the various parts of the 

 body, it would in the end become useless for the pur- 

 poses of nutrition if it did not receive a fresh supply 

 of food-stuffs from the gut, either directly or indirectly 

 (through the lacteal system). But apart from this, 

 the blood would ultimately become useless, and that 

 very quickly, if it did not traverse the lungs, kidneys, 

 and sweat-glands. It is well known to every one that 

 a man or animal cannot live without air, or at any 

 rate without a certain gas, oxygen, that is contained 

 in air. This oxygen must be able to penetrate into 

 the minutest particles of the body, and the blood, in 

 the corpuscles of which it is contained, carries it 

 everywhere. In the smallest particles (molecules) of 

 the body an oxidation (combustion) of body-substance 

 takes place, which not only causes an evolution of 

 heat, but also renders the body capable of doing work. 

 But if now the blood passes from the capillaries into 

 the veins, it contains too little oxygen. And besides, 

 it has taken up from the molecules of the body several 

 substances, developed in those molecules, which would 

 be fatal to the animal if they were not removed from 

 the body. Now, when the blood streams through the 

 lungs, it gets rid of the poisonous gaseous matter, and 

 when it traverses the kidneys and sweat-glands it 

 parts with the injurious liquid and solid substances. 

 But in the lungs the blood takes up at the same time 

 fresh oxygen ; and since in this way the air in the 

 lungs becomes poor in oxygen, the movements of 

 breathing (respiration) provide for the passage of a 

 fresh suppty of oxygen into the lungs. Only the 

 higher Vertebrates breathe by means of lungs ; fishes 

 and numerous aquatic Invertebrates breathe by gills, 

 and insects by air-tubes (trachese). 



While Nutrition is the life-process which shields 

 the individual from death. Reproduction serves to 

 maintain the species. It is familiarly known that 



