ORGANS AND FUNCTIONS OF ANIMAL LIFE. 2 J 



termining (6) muscular contraction and motion and 

 changes in the external world. The skeleton acts as 

 levers to make the motion more rapid, precise, and ef- 

 fective. Thus the sense organs may be regarded as re- 

 ceptive organs of sensation and consciousness, and the mus- 

 cles and skeleton as executive organs of the will, and 

 the whole as an instrument of action and reaction be- 

 tween the external and the internal world. 



Therefore the necessary parts of an instrument of 

 communication between the outer and the inner world 

 are (i) two kinds of cells in the nerve center, viz., a 

 sensory cell and a motor cell with connecting fiber be- 

 tween ; (2) two kinds of transmitting fibers, the one 

 sensory, transmitting inward, the other motor, trans- 

 mitting outward ; and (3) two kinds of nerve-fiber end- 

 ings, one in a sensitive surface or a sense organ, the 

 other in a contractile tissue or muscle. Each cell, sen- 

 sory or motor, with its fiber and its ending is called a 

 neurone or neurocyte. The connection between a sensory 

 and a motor neurone, until recently, was supposed to be 

 continuous and permanent, as represented in the figure; 

 but now it is believed to be by contact of branching pro- 

 cesses, and perhaps only during stimulation. This will 

 be explained more fully hereafter. 



Of the four systems mentioned as concerned in ani- 

 mal functions, viz., nervous system, sense organs, mus- 

 cles, and skeleton, the fundamental one is the nervous. 

 The others may be regarded as appendages of this one. 

 We therefore take this first. 



Order of Treatment. — There are two modes or 

 orders of taking up the subject of comparative physiol- 

 ogy and morphology. We may begin with the lowest 

 and go up the scale ; this is the order of evolution. 

 Or we may begin with man and pass down the scale. 

 If our subject were mainly morphology the former 



