ANATOMICAL SKETCH. 



495 



Any piece of protoplasm can conduct impulses, as is seen in the 

 rapid transmission of an impulse in animals and textures 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 im- 

 pulse, 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 conductor is a deli- 



FIG. 200. 



FIG. 201, 



FIG. 200. Highly magnified view of three medullated and two non-medul- 

 lated nerve fibres of frog, stained with osmic acid, which makes the medul- 

 lary sheath black. N. Nodes of Ranvier where the axis cylinder can be 

 seen to pass the gap in the medullary sheath. 



FIG. 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. 



cate protoplasmic fibril. Single, thin, thread-like fibrils are com- 

 monly found carrying impulses in the nerve centres. But in the 

 nerves distributed about the body one does not 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. 



