108 ZOOLOGY. 



the eighth pair, is the instrument by which the sensation of 

 sounds is conveyed to the brain ; and by means of a branch 

 of the fifth pair we perceive the sapid qualities of objects ; 

 whilst the spinal nerves distributed to the fingers and toes, 

 endow their extremities with fine tactile powers. 



203. The roots of the spinal nerves being double, a 

 double function had been long suspected; this was put 

 beyond dispute by Bell and Magendie,* who showed that by 

 the posterior roots the sensations travel to the brain, whilst 

 by the anterior the power of motion travels to the muscles. 

 Some differences have been observed in various parts of the 

 spinal marrow ; the sensibility is acute in the dorsal aspect of 

 the organ, and much more feeble anteriorly. 



[Some recent experiments performed by M. Brown-S^quard 

 raise doubts as to the correctness of this view when taken too 

 exclusively, but they do not affect the chief results. R. K.] 



204. Ganglionary System. This system is but little, 

 if at all, sensible to ordinary stimulants : neither cutting nor 

 pinching affects it. In the healthy state, the organs to which 

 the nerves of this system proceed are in like manner but little, 

 if at all, sensible; but when diseased, their sensibilities 

 become highly exalted. 



205. Special Organs of tlie Senses. The apparatus of 

 the sensibility is not composed only of the different parts of 

 the nervous system whose uses we have already pointed out ; 

 the nerves furnished with the faculty of transmitting to the 

 brain the sensations reaching us from without, do not termi- 

 nate freely in the exterior, so as to receive directly the con- 

 tact of the producing agents of our sensations, but terminate 

 in positive instruments destined to collect, so to say, the exci- 

 tation, and to prepare it in such a way as to assure its action. 

 These instruments are the organs of the senses, and it is 

 essentially by the intermedium of these^rgans that the sen- 

 sations reach us; but they are not indispensable for the 

 exercise of all these faculties : the tactile sensibility may be 

 called into play everywhere where nerves exist adapted to 

 conduct the ordinary sensations, and it is only by the special 

 senses, that is to say, by the taste, smell, hearing, and sight, 

 that this intermediate organ between the nerve and the 

 external world is a necessary condition. 



* First by Mr. A. Walker. K. K. 



