ANATOMICAL SKETCH. 



497 



which have no special conducting elements or nerve fibres. Thus 

 in the hydra all the cells act as nerves, and in the higher animals 

 an impulse, producing a wave of contraction, can pass on from 

 one muscle cell to the other directly, as is seen in the ureter, or 

 in the frog's heart. The only essential part of a nervous con- 

 ductor is a delicate protoplasmic fibril. Single, thin, thread-like 

 fibrils are commonly found carrying impulses in the nerve cen- 

 tres. But in the nerves distributed about the body one does not 



FIG. 200. 



FIG. 201. 



Fio. 200. Highly magnified view of three medullated and two non-medullated nerve 

 fibres of frog, stained with osraic acid, which makes the medullary sheath black. N. 

 Nodes of Rauvier, where the axis cylinder can be seen to pass the gap iu the medullary 

 sheath. 



Fio. 201. Transverse section of nerve fibres, showing the axis cylinders cut across, 

 and looking like dots surrounded by a clear zone, which is the medullary sheath. Fine 

 connective tissue separates the fibres into bundles. 



meet these single protoplasmic threads (except where the fibrils 

 are interwoven to form terminal networks, as seen in the cornea), 

 but the fibrils are clustered together in large bundles, so as to 

 make one nerve fibre. This bundle of protoplasmic fibrils is, 

 in the peripheral nerves, always covered, and is called the axis 

 cylinder of the nerve fibre. In some nerve fibres there is but one 

 very thin, transparent covering, termed the primitive sheath, while 

 in others there is a thick layer of doubly-refracting fluid inside 





42 



