474 PHYSIOLOGY. 



SIXTH CHAPTER. 



OF SENSATION AND THE SENSES. 



272. 



THE functions of the nervous system are certainly among the most 

 problematical of all the animal organs. Even in the higher animals, 

 in which observation is more easy, and it has to contend with fewer diffi- 

 culties, much still remains in impenetrable obscurity, notwithstanding 

 the light that has been given in modern times to this portion of phy- 

 siology ; it will therefore strike us as less singular if the most general 

 phenomena of the functions of the nervous system of the lower animals 

 have not been satisfactorily explained. We move here in a field where 

 simple experience frequently quits us, and a wider space is given to 

 the fancy for its hypotheses and inventions. Yet we will keep ourselves 

 as far as possible from this frequently misguiding conductress, and only 

 endeavour to explain what our own experience and that of others 

 enables us to do satisfactorily. 



It accordingly appears confirmed that the nervous system, and chiefly 

 the first chief ganglion or the brain, is the truly animating element 

 which sets all the other organs in activity, and retains them in it. 

 From the nervous system the muscle derives the irritability which puts 

 it in action ; by means of the nerves the intestinal canal is excited to 

 digestion, and by the impulse of the same organs the sexual parts 

 exercise the function appointed to them. Lastly, the nerve is the 

 recipient and conductor of all immediate perceptions of external objects, 

 and consequently the seat of sensation in general. Experience corro- 

 borates all these assertions. With respect to the effect of the nerves 

 upon the muscles, we know from Rengger's * experiments, that after 

 the nervous cord has been cut through at any part, the portion of the 

 body which lies beyond that spot can exhibit no more motion. Rengger 

 repeated this experiment in different kinds of caterpillars, some of 

 which he cut through at a higher and others at a lower part of the 



* Physiologischc Untcrsuckungen, &c. p. 41. 



