ANATOMY OF THE NERVOUS SYSTEM. 



179 



or without a primitive sheath. Such fibres are especially 

 abundant in the sympathetic trunks; and they alone form 

 the olfactory nerve. In the communicating branches between 

 the sympathetic ganglia and the spinal nerves both white 

 and gray fibres are found; the former are cerebro-spinal 

 fibres passing into the sympathetic system, while the grav 

 fibres originate in the sympathetic system and pass to the 

 membranes and blood-vessels of the spinal cord and spinal 

 column. Another group of gray nerve-fibres may be called 

 nerve-fibrils : they are extremely fine,, and result from the 

 subdivision of axis cylinders, close to their final endings in 

 many parts of the Body, after they have already lost both 

 primitive and medullary sheaths. Many fine gray fibres exist 

 in the nerve-centres. 



The Histology of Nerve-Cells. The only structures 

 known with certainty to be connected with the central ends 

 of nerve-fibres are nerve-cells, and so many nerve-fibres have 



FIG. 81. Nerve-cell from anterior horn of srey matter of spinal cord; a, axis- 

 cylinder process. 2, Cell from posterior horn of spinal cord. 



been traced into continuity with nerve-cells, that it is fairly 

 certain all arise in this way. The latter may therefore be re- 

 garded as the central organs of the nerve-fibres. 



