42 COMPARATIVE ZOOLOGY. 



all animals, the functions are performed under conditions 

 essentially the same. Thus, respiration in the Sponge, the 

 Fish, and in Man has one object and one means, though 

 the methods differ. A function, therefore, is a group of 

 similar phenomena effected by analogous structures. 



The life of an animal consists in the accumulation and 

 expenditure of force. The tissues are storehouses of 

 power, which, as they waste, is given off in various forms. 

 Thus, the nervous tissue generates nerve-force ; the mus- 

 cles, motion. If we contemplate the phenomena presented 

 by a Dog, the most obvious fact is his power of moving 

 from place to place, a power produced by the interplay of 

 muscles and bones. We observe, also, that his motions 

 are neither mechanical nor irregular; there is method in 

 his movement. He has the power of willing, seeing, hear- 

 ing, feeling, etc. ; and these functions are accomplished by 

 a delicate apparatus of nerves. 



But the Dog does not exhibit perpetual motion. Sooner 

 or later he becomes exhausted, and rest is necessary. Sleep 

 gives only temporary relief. In every exercise of the 

 muscles and nerves there is a consumption or waste of 

 their substance. The blood restores the organs, but in 

 time the blood itself needs renewal. If not renewed, the 

 animal becomes emaciated, for the whole body is laid un- 

 der contribution to furnish a supply. Hence the feelings 

 of hunger and thirst, impelling the creature to seek food. 

 Only this will maintain the balance between waste and 

 repair. We notice, therefore, an entirely different set of 

 functions, involving, however, the use of motion and will. 

 The Dog seizes a piece of meat, grinds it between its 

 teeth, sends it into the stomach, where it is digested, and 

 then into the intestine, where it is further changed; there 

 the nourishing part is absorbed, and carried to the heart, 

 which propels it through tubes, called blood-vessels, all 

 over the body. In this process of digestion, certain fluids 



