SENSORY ORGANS. 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 sete, oy 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 
which 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. 
