242 COMPARATIVE ZOOLOGY 



do much of this ; but the lungs do something else. 

 They expose the blood to the air, and introduce oxygen, 

 which, we shall find, is essential to the life of every 

 animal. 



These centripetal and centrifugal movements in the 

 body throwing in and throwing out are so related 

 and involved, especially in the lower forms, that they 

 can not be sharply defined and classified. It has been 

 said that every dog has two lives, a vegetative -and an 

 animal. The former includes the processes of digestion, 

 circulation, respiration, secretion, etc., which are com- 

 mon to all life ; while the functions included by the latter, 

 as motion, sensation, and will, are characteristic of animals. 

 The heart is the center of the vegetative life, and the brain 

 is the center of the animal life. The aim of the vegeta- 

 tive organs is to nourish the individual, and reproduce 

 its kind ; the organs of locomotion and sense establish 

 relations between the individual and the world without. 

 The former maintain life ; the others express it. The 

 former develop, and afterward sustain, the latter. The 

 vegetative organs, however, are not independent of the 

 animal; for without muscles and nerves we could not 

 procure, masticate, and digest food. The closer the 

 connection and dependence between these two sets of 

 organs, the higher the rank. 73 



All the apparatus and phenomena of life may be in- 

 cluded under the heads of - 



NUTRITION, 

 MOTION, 

 SENSATION, 

 REPRODUCTION. 



These four are possessed by all animals, but in a 

 variety of ways. No two species have exactly the same 

 mechanism and method of life. We must learn to dis- 



