CHAPTER THIRD. 



ORGANS AND FUNCTIONS OF ANIMAL LIFE. 

 SECTION I. 



OP THE NEBYOTJS SYSTEM AND GENEBAL SENSATION. 



76. LIFE, in animals, is manifested by two kinds of 

 functions, viz.: First, the functions of animal life, or those 

 of relation, which include sensation and voluntary motion ; 

 those which enable us to approach, and perceive our fellow- 

 beings and the objects around us, and bring us into relation 

 with them : Second, the functions of vegetable life, which are 

 nutrition in its widest sense, and reproduction ;* those in- 

 deed, which are essential to the maintenance and perpetuation 

 of life. 



77. The two distinguishing characteristics of animals, 

 namely, sensation and motion ( 65), depend upon special 

 systems of organs, wanting in plants, and which are called the 

 nervous and muscular systems. The nervous system, therefore, 

 is the grand characteristic of the animal body. It is the 

 centre from which all the commands of the will issue, and to 

 which all sensations tend. 



78. Greatly as the form, the arrangement, and the volume 

 of the nervous system vary in different animals, they may all 

 be reduced to four principal types, which correspond, more- 

 over, to the four great divisions of the animal kingdom. 

 In the vertebrate animals, namely, fishes, reptiles, birds, 



* This distinction is the more important, inasmuch as the organs of 

 animal life, and those of vegetative life, spring from very distinct layers of 

 the embryonic membrane. The first are developed from the upper layer, 

 and the second from the lower layer of the germ of the animal. See 

 Chapter on Embryology. 



