NERVOUS TISSUE. 



109 



FIG. 81. 



1 Nucleus 



cells, whose outlines can be made out by treatment with silver 

 nitrate. This layer is known as Henle's sheath or the endo- 

 neural sheath (Retzius). 



Nerve fibres of such complicated structure as that described 

 are found in the cerebrospinal nerves. They are of variable 

 thickness (1-20 p in diameter). Usually the longest fibres are 

 also the thickest. The division of a 

 medullated fibre into two, often three or 

 four, branches takes place always at the 

 nodes of Ranvier. As it approaches the 

 nerve-ending it loses its sheaths. 



The medullated nerve fibre may lack 

 the sheaths of Schwann and Henle, as is 

 the case in the central nervous system, 

 where the fibre consists of only the 

 medullary sheath and axis cylinder. 



Non-medullated or sympathetic nerve 

 fibres (Remak's fibres) are characterized 

 by the absence of the medullary sheath 

 (Fig. 81). In adult vertebrates such 

 fibres are found only in the sympathetic 

 nervous system. They are only 1 to 2 p 

 in thickness, and are direct continuations 

 of axones of sympathetic ganglion cells. 

 Each fibre is surrounded by a covering 

 resembling Schwann's sheath. It pos- 

 sesses at various places nuclei surrounded 

 by granular protoplasm. This sheath 

 seems to be a continuation of the thin Non-meduiiated ( Remak's) 



1,1 i . i , T fibre from the cervical sympa- 



capsule that surrounds the sympathetic thetic of the rabbit, x 300. 

 cells, and is of connective-tissue origin. 



The olfactory nerve fibres are of a still simpler type. They 

 are very fine (less than 0.5 ^ in diameter) fibres which consist 

 of a naked axis cylinder. More often than other fibres, they 

 are varicose and thickened in places. The bundles of these 

 fibres are surrounded by a homogeneous sheath containing 

 nuclei. This does not resemble Schwann's sheath, because 



