THE GENERAL ANIMAL FUNCTIONS. 71 



pletely bound to one another. It becomes necessary not only 

 that they receive impulses from such parts as are favorably 

 situated for the reception of stimuli, but that a degree of 

 coordination of the interrelated parts be secured, in order that 

 just such response shall be made as will best meet the needs 

 of the organism. This power of coordinated response to ex- 

 ternal stimuli makes it possible for an organism to become 

 suited to its environment. 



103. In the higher forms, the work above described de- 

 mands five classes of structures (see Fig. 34); (i) end or 

 sense organs, which are specially sensitive to stimuli of dif- 

 ferent orders, as mechanical (touch), chemical, ethereal 

 (light), etc.; (2) conductive tracts (afferent nerves), which 



FIG. 34- 



FIG. 34. Scheme showing the essential relations of the parts of a nervous system: I, 

 the sensory end organ (epithelial); 2, afferent nerve tract; 3, central nervous cells 

 (ganglia); 4, efferent nerves, leading to 5, muscle, gland, etc. g, ganglion cells; gl., 

 gland; m, muscle fibre; n.f., nerve fibre; s.e., sensory epithelium. 



Questions on the figure. What seems to be the function of the 

 various parts or elements in this scheme? Your reasons for your view? 



connect (i) with (3) central nervous structures (ganglion- 

 cells) where the impulse is received and suitable responsive 

 impulses are originated; (4) conductive tracts (efferent 

 nerves), which make the work of the central organs of value 

 by carrying an impulse which produces corresponding activi- 



