ACTION OF THE NERVOUS SYSTEM. 6S 



must conclude that it is- a nervous activity which 

 immediately controls the machinery of growth, and 

 thus determines the growth or development of any 

 and every particular part. 



The facts concerning the nervous system, which 

 have been referred to above, have been attained 

 chiefly by the study of man and the higher verte- 

 brate animals, but few will doubt that they hold true 

 equally of all the lower animals possessing a nervous 

 system. For, in the first place, their tissues are 

 similar in composition and form of elements ; and, 

 moreover, the investigations of the physiology of 

 the snail, ^ and other invertebrate animals, show that 

 they are affected similarly by various drugs, and 

 that in their physiological economy they are gen- 

 erally the same as the higher animals. The differ- 

 ent functions of their organs have also been shown 

 to be under the control of a central nervous system. 

 But there is a great difference in the extent of 

 control which the visible central nervous system 

 exercises over the bodies of different animals. As 

 a general rule, the lower the animal in the scale of 

 organisation the less centralised is the control exer- 

 cised by the nervous system over the component 

 organs. The organs may be able to perform their 



' Contribution a I'histoire fihysiologii/tie de Vescargot {Helix poma- 

 iia), par Emile Yung. Bruxelles, 1887. 



