GENERAL ANIMAL FUNCTIONS. 



9 I 



As organisms increase in the number of cells, and as 

 these specialize in the various ways indicated in this 

 chapter, they would lose this power of quick response 

 to their conditions, unless there is also developed some 

 specially sensitive conductors to bind the deeper with 

 the outer cells, so that all may act more harmoniously. 



FIG. 1 6. Scheme showing the essential relations of the parts of a nervous system . 

 i, the sensory end organ (epithelial) ; 2, afferent nerves; 3, central nervous cells (ganglia) ; 

 4, efferent nerves, leading to 5, muscle, gland, etc.; g, ganglion cells; gl, gland; m t 

 muscle fibre; s.e., sensory epithelium. 



Questions on the Figure. What seems to be the special function of 

 each of the parts or elements in this scheme? Your reasons for 

 your answer? 



In a complex animal, there are five sets of structures 

 which are commonly found to do this work of co-ordinat- 

 ing the parts of the organism to each other and to the 

 outside world. These are shown in diagrammatic form 

 in Fig. 16. They are: (i) the end organs of sense, 

 whose business it is to be especially sensitive and to re- 

 ceive the various types of external stimuli, as light, 

 sound, contacts, chemical changes, and the like; (2) 

 conductive tracts (afferent nerves) w r hich conduct an 



