312 



GENERAL ZOOLOGY 



Many of these organs have great range of sensibility the 

 eye can perceive about two hundred pure tints, the average 

 ear is able to discriminate some 11,000 different pitch 

 qualities. Effectors are nerve endings for setting off 

 organs which accomplish some particular work. They 

 activate voluntary muscles, visceral muscles, or glands. 

 The adjusters are groups of nerve cells and their branches, 

 which lie for the most part within the thickened wall of 

 the central nervous tube, or near it. They receive notice 

 of conditions from the receptors, send out stimuli to appro- 

 priate effectors, and make necessary coordinations. A 



neuropore 



cerebr 



um 



cerebrum 



cerede//um< 

 A 0, C 



FIG. 110. Development of the human brain. A, two weeks after fertilization ; 

 B, four weeks; C. during third month of foetal life; D, adult brain. (Largely 

 after Herrick.) 



slight external stimulus may set off a great many effectors 

 and thus cause a violent reaction, or a strong stimulus may 

 be suppressed and cause no response all such matters are 

 arranged by the adjusters. For example, a pioneer may 

 see a human form on the horizon if it is an Indian with a 

 gun, he will run or hide; if it is a member of his family, his 

 nervous system will make no response through effectors to 

 the image received through his eyes. 



Man has many of his important sense organs localized at 

 the anterior end of the body and it is but natural that the 

 great adjusters should be close at hand in the thickenings of 

 the wall of the neural tube which forms the brain. There 



