CHAP. n.l OF CELLS AND OF NERVOUS FIBRES. 



381 



form fibre : and it is the same with the fibres which regenerate 

 themselves in the adult after a lesion. According to Remak, 

 who discovered them, the gelatiniform fibres differ especially 

 from the others by the absence of the oily envelopment, the 

 medullary substance. Moreover they are constituted also by an 

 axile filament and a delicate envelopment. The lack of oily 

 envelopment is the cause of their grey colour. 



These diverse degrees of perfection in the structure of the 

 nervous fibre in the vertebrate, suggest naturally the idea of 



Fio. 70. 



Ganglionary network of the muscular membrane of the small intestine of the guinea pig ; 

 o, nervous network ; ft, ganglion ; c and d, lymphatic vessels. 



an evolution. No doubt some imagination is needed to find, as 

 Hseckel finds, all the genealogical links from man to the gastrula, 

 that is to say to the animal reduced to be nothing more than a 

 simple digestive pouch. But it is certain, nevertheless, that if 

 we classify hierarchically all the types of the animal kingdom, 

 we see, from the foot to the top of the scale, the vegetative life 

 adjoin to itself little by little the animal life, which gradually 

 expands in its turn. Now in the superior vertebrate all these 

 phases have left their impress behind, without speaking of the 

 embryological evolution which reproduces them all in epitome. 



