CHAP, ii.] OF CELLS A3S T D OF NERVOUS FIBEES. 379 



envelopments seem limited by two dark parallel lines. For 

 this reason the name of fibres with double contour has been given 

 to them (Fig. 68). 



It is these fibres which, united in bundles with a general 

 fibrous envelopment, constitute all the nervous cords, the nerves 

 properly so called, at least those of the animal life, those which 

 in man and the superior vertebrates distribute themselves to 

 the skin, to the organs of the special senses, to the muscles. 

 Seen with the naked eye, all the regions or portions of the 

 nervous system in which they dominate are white. This white 

 tint is due to the oily substance of the fibres with double contour. 



Physiology has demonstrated that certain of these fibres seem 

 to transmit to the muscles motory excitations arising in the 

 nervous centres : that certain others, on the contrary, carry from 

 the periphery to the nervous centres the sensitive excitations. 

 On this account, the first have been called motory fibres, the 

 second sensitive fibres. There is however no decided anatomical 

 difference between these two orders of fibres. But there is seen 

 on the course of the nervous fibres, shortly before they reach 

 the nervous centres, a ganglionary expansion, a nervous cell 

 (Fig. 69 p). 



In most of the nervous cords, motory fibres and sensitive 

 fibres are mingled and confounded under the general neuro- 

 lemma. "We have however to except the nervous cords, specially 

 transitive, which proceed to the organs of the senses. 



We have seen that usually the special sensitive nerves proceed 

 direct to the cerebroidal ganglion of the invertebrates and to the 

 brain of the vertebrates. The others, the mixed nerves, take 

 their course, in the arthropods, to the ganglions of the abdominal 

 chain, and in the vertebrates to the spinal marrow (Fig. 69, q, 

 and Fig. 71). But in this last case the two orders of fibres 

 separate a little, before arriving at the marrow, into an anterior, 

 cylindrical root and into a posterior ganglionary root, that is to 

 say, expanded at a point where are found united the cells of all 

 the sensitive fibres of the nervous cord (Fig. 69). 



