78 



ZOOLOGY 



1 06. In the higher forms, the work above described de- 

 glands five classes of structures (see Fig. 36) : — (i) end or sense 

 gorans, which are specially sensitive to stimuli of different orders, 

 as mechanical (touch), chemical, ethereal (light), etc.; (2) con- 

 ductive tracts {afferent nerves), which connect (i) with (3) cen- 

 tral nervous structures {ganglion-cells) where the impulse is re- 

 ceived and suitable responsive impulses are originated; (4) con- 

 ductive tracts {efferent nerves), which make the work of the 

 central organs of value by carrying an impulse which produces 



Pig 36. 



i 



-s.t 



Pig. 36. Schemeshowingtheessentialrelationsof 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 s, muscle, gland, etc. g, ganglion cells; gl., gland; m, muscle fibre; n./., 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? 



corresponding activities in (5), some form of actively related 

 cells, — muscular, glandular, or nervous. It is readily apparent 

 how increase of volume and differentiation of the other parts will 

 make necessary a more complicated nervous system. The spe- 

 cial arrangement of these parts of a complete system differs very 

 much in various animal groups, yet it may be said that there is 

 a progressive accumulation of the central nervous matter at the 

 anterior end of the body as we ascend the scale of animal life. 

 When this concentration is well advanced the mass of nervous 

 matter is called the central nervous system which always includes 

 the brain. The nerves passing to and from the central part and 

 their endings, taken collectively, are described as the peripheral 

 nervous system. 



