CHAP, ii.] OF CELLS AND OF NERVOUS FIBRES. 383 



and till there is more convincing proof it is wise to consider the 

 cell and the nervous fibre as the ultimate elements of the nervous 

 tissues. 



On the whole, every nervous system, invertebrated or verte- 

 brated, resolves itself into a number more or less great of cells, 

 and into a number more or less great of fibres, which connect the 

 cells or end therein. 



The regions of the system where the cells accumulate in great 

 number are the nervous centres. The parts almost wholly com- 

 posed of fibres form the nervous cords, and if we embrace at 

 a glance the ensemble of the kingdom, we see that the cellular 

 centres are the more voluminous and the less numerous, the 

 more exalted the animal is in the hierarchy. 



It is in the superior vertebrates and especially in man that the 

 cellular concentration attains its maximum (Fig. 65). It is here 

 also that unfold themselves in all their plenitude the special 

 properties of the nervous system, sensibility, motricity, and 

 thought. 



"We have not to describe here in their infinite details the 

 nervous centres of the superior mammifers. It suffices to figure 

 them to ourselves schematically. The spinal marrow (Figs. 69 

 and 71) is essentially a column of grey substance, that is to say 

 of multipolar cells, emitting or receiving three orders of fibres : 

 sensitive fibres radiated in all the organs, and especially 

 the skin ; motory fibres, setting forth from the cells, to arrive at 

 the muscles ; finally, intermediary fibres, serving solely to 

 connect the cells with each other, to solidarise them, to make 

 of all the grey substance of the marrow a harmonious whole, 

 able, in a certain degree, to vibrate in unison. 



As the mixed nerves, before arriving at the marrow, break 

 into posterior or sensitive bundles, and anterior or motory 

 bundles (Fig. 69), men of science have considered as sensitive the 

 posterior cells of the spinal marrow, which receive direct the 

 sensitive fibres. On the other hand they have regarded as 

 motory the anterior cells, whence go forth the motory fibres. It 



