CHAP. II.] 



A NERVOUS SYSTEM. 



33 



It is absent from the peripheral extremities of the nerves, 

 and it is ahsent also from their central extremities, at the 

 points where the fibres approach or depart from the nerve 

 cells. Both it and the membranous investing sheath have 

 been of late ascertained to be regularly interrupted at 

 comparatively short distances, so that such nerve fibres 

 have the appearance of being constricted in these situa- 

 tions (Ranvier). 



Nearly all visceral nerves, as well as the fibres of the 

 olfactory and some others, do not possess this medullary 

 sheath, to which the dead white colour of the great ma- 

 jority of nerve fibres is due. They are, therefore, semi- 

 translucent or grey 

 in tint, and are 

 commonly known as 

 the pale, gelatinous 

 or non-medullated 

 fibres (fig. 5). Their 

 average thickness is 

 about -Q-frQ o^h f an 

 inch ; and they dif- 

 fer from the dark 

 bordered fibres prin- 

 cipally in the ab- 



nf tliP 



FlG - 5. Gelatinous Nerve Fibres from the Calf 

 (Henle). Magnified about 400 diameters. A, Fibre 

 Sheath. They showing its constituent fibrillse (d) ; a, a, Nuclei in 



present a distinct mem1 8 8heath - 



appearance of fibrillation, are surrounded by a delicate 



membranous envelope, and the larger fibres are similarly 



formed by the running together of fibrils and smaller 



fibres. 



Nerve fibres thus compounded, both dark bordered and 

 pale, similarly tend to aggregate into cords or bundles oi 

 different sizes, the fibres of which run parallel to one 



D 



