SENSORY ORGAXS. 113 



cause, and that what it does at any moment would be 

 as clearly intelligible, if we only knew all the internal 

 and external conditions of the case, as the striking of a 

 clock is to any one who understands clockwork. 



The adjustment of the body to varying external con- 

 ditions, which is one of the chief results of the working 

 of the nervous mechanism, would be far less important 

 from a physiological point of view than it is, if only 

 those external bodies which come into direct contact 

 with the organism * could affect it ; though very delicate 

 influences of this kind take effect on the nervous apparatus 

 through the integument. 



It is probable that the seta, or hairs, which are so 

 generally scattered over the body and the appendages, 

 are delicate tactile organs. They are hollow processes of 

 the chitinous cuticle, and their cavities are continuous 

 with narrow canals, which traverse the whole thick- 

 ness of the cuticle, and are filled by a prolongation of 

 the subjacent proper integument. As this is supplied 

 with nerves, it is likely that fine nerve fibres reach the 

 bases of the hairs, and are affected by anything which 

 stirs these delicately poised levers. 



* It may be said that, strictly speaking, only those external bodies 

 \vhich are in direct contact with the organism do affect it as the 

 vibrating ether, in the case of luminous bodies ; the vibrating air or 

 water, in the case of sonorous bodies ; odorous particles, in the case of 

 odorous bodies : but I have preferred the ordinary phraseology to a 

 pedantically accurate periphrasis. 



9 



