404 BIOLOGY. [Boox vi. 



acoustic, &c., nerves. In fact, every irritation which touches 

 either the course of these nerves or their central portion, after 

 section, only awakens in the nervous centres special sensations, 

 glimmerings, sounds, &c. In the same way in the vertebrates 

 the posterior roots of the rachidian nerves are sensitive ; further- 

 more, every excitation bearing upon any point whatever of their 

 course between the teguments and the nervous centres determines 

 sensitive perceptions, touch, pain, &c. 



There are then systems of fibres and cells physiologically quite 

 different from the motory cells and fibres, in spite of great analogy 

 in form and structure. Besides, this sensitive nervous system is 

 habitually armed, at the extremity of its fibrous irradiations, with 

 special apparatus destined to collect, to concentrate upon the ter- 

 minal nervous fibres excitations from without. These apparatus 

 are the organs of the senses, which we must now briefly describe 

 throughout the animal series. 



We shall merely mention in passing the protozoa. They have 

 no organs of the senses, any more than the rhizopods, the sponges, 

 the infusoria, since these rudimentary organisms are absolutely 

 destitute of nervous system, and the organs of the senses are 

 simply excitatory apparatus, means of reinforcing exterior im- 

 pressions, instruments by the aid of which the sensitive nervous 

 system palps the ambient medium. Can it be said that all the 

 special senses may be concentrated in one, that of touch 1 We 

 cannot deny that thei^e is some truth in this generalisation. 

 Under the apparent diversity of special sensations, there is always 

 the same cause, namely, the shock, the agitation of the sensitive 

 nervous extremities by molecules belonging to the exterior medium; 

 but as each sense selects from these multiplied excitations those 

 which correspond to its anatomical construction, we must admit 

 special senses. 



If certain physiologists have wished, with some appearance of 

 reastm, to reduce all the senses to one, others, on the contrary, 

 have capriciously tried to multiply their number. The ancients 

 admitted five senses; touch, tasjbe, smell, hearing, and sight. 



