OF THE SENSE OF TOUCH. 103 



205. Special Organs of the 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 

 terminate freely in the exterior, so as to receive directly the 

 contact of the producing agents of our sensations, but termi- 

 nate in positive instruments destined to collect, so to say, the 

 excitation, 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 organs that the 

 sensations 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. 



Having studied in a general way the phenomenon of the 

 sensibility, as well as the organs which are its seat, we ought 

 now to examine more in detail each of the forms under which 

 this property is manifested, or in other words, enter on the 

 particular history of each of the senses with which nature 

 has endowed animals. 



OF THE SENSE OF TOUCH. 



206. All animals possess a tactile sensibility more or 

 less delicate, and it is especially by the intermedium of the 

 membrane with which the surface of their bodies is covered, 

 that this faculty is exercised. To study it, it is, then, 

 above all necessary to examine what is the structure of the 

 skin. 



In man, the external surface of the body, and that of the 

 cavities hollowed out in the interior, but communicating 

 externally, such as the digestive canal, &c., are clothed with 

 a tegumentary membrane more or less thick, and quite 

 distinct from the parts which it covers. This membrane is 

 everywhere continuous with itself, and in reality it forms but 

 a continuous whole; but its properties are not everywhere 

 the same, and it is called by different names when it is 

 reflected internally to line the interior cavities, or when it is 



