362 BIOLOGY. [BOOK vi. 



which this tissue forms, present a . number of varieties in 

 structure and in form. In certain beings innervation is rudi- 

 mentary ; it reduces itself to the single faculty of exciting 

 movements and connecting them with each other ; that is to say, 

 to motridty. In others sensibility is joined to motricity. Finally, 

 the most perfect animals possess alike motricity, sensibility, 

 thought, in, however, very numerous degrees of perfection and 

 imperfection. 



In passing by for the moment the varieties of the nervous 

 elements, we can consider every nervous tissue or nervous system 

 as reducible to two histological types. Invariably, every nervous 

 apparatus is composed of fibres and cells. Always the fibres 

 continue direct with the cells, and always they connect those 

 cells with a motory or sensitive organ. We shall soon have to 

 describe the form and the function of the diverse nervous fibres 

 and cells. At present it is enough for us to mention that the 

 fibres are especially the conductors of impressions and of incita- 

 tions, while the cells act on the impression which is transmitted 

 to them, either to send it back, or to reflect it simply from one 

 fibre to another, or to transform it into phenomena of conscious- 

 ness. Finally, the cells can, in their turn, become centres of 

 excitation, and then, without any exterior impressiorJ, they 

 provoke movements or combine thoughts. 



Like all other organic systems, the nervous system complicates, 

 perfectionates, diversifies itself gradually in the animal series. 



No trace of nervous system in the rhizopods, the sponges, in 

 the monerions, the infusoria, unless we admit, with some romantic 

 naturalists, a diffused nervous tissue, disaggregated, or rather 

 not yet aggregated, invisible nervous molecules, infused and 

 latent in a living gangue not yet differentiated. But all this is 

 pure fancy. Neither is there any nervous system in the hydroids, 

 the lucernaries, the anthozoaries. In the medusae we meet with 

 a nervous ring formed of a cord, following the edge of the disk, 

 and offering, from distance to distance, cellular enlargements 

 called gcmglionaries. This is already the schema of all the 



