CHAPTER XI. 



THE ORGAXS OF SPECIAL SENSE, ETC., IN THE IXVERTEBKATA. 



As we have already seen, all nerves have not the power of 

 transmitting sensations to the brain or its equivalent ; some, 

 on the contrary, are clearly nerves of motion, whether acted 

 on by will or excited by other means. Some nerves, as the 

 optic, transmit only the impressions received from colours— - 

 i.e., due to the action of light ; to other stimulants this nerve 

 is insensible. The olfactory nerve is sensible to various 

 odours, but it is insensible to the action of light or sound. 

 To these modifications of the sensibility of nervous elements 

 are due the phenomena of special senses. The senses of 

 touch, taste, smell, hearing, and seeing, are so many distinct 

 faculties putting the animal kingdom in relation with the 

 various qualities of the external world. 



The apparatus or mechanism of the sensibility is not com- 

 posed only of the different parts of the nervous system, whose 

 use we have already alluded to ; for the sense-nerves do not 

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

 contact of the producing agents of sensations, but terminate 

 in various mechanisms destined to collect the excitation, and 

 to prepare it in such a way as to assure its action. These 

 mechanisms are the sense-organs, and it is essentially by the 

 intermedium of these organs that the sensations reacli the 

 brain or its erjuivalent ; but it may be remarked that they are 

 not indispensable for the exercise of all the special senses ; 

 the tactile sensibility may be called into play everywhere, 

 where nerves exist adapted to conduct the ordinary sensa- 



