HISTOLOGY OF NERVE-CELLS. 175 



der, which itself is a branch given off by a nerve-cell, in 

 some centre. The medullary sheath is interrupted half 

 way between each pair of nuclei at a point called the node, 

 which is the boundary between two of the enveloping cells. 

 In the course of a nerve-trunk its fibres rarely divide; when 

 a branch is given off some fibres merely separate from the 

 rest, much as a skein of silk might be separated out at 

 one end into smaller bundles containing fewer threads. 



Gray Nerve-Fibres. Some of these are merely white 

 fibres which near their peripheral ends have lost their 

 medullary sheaths ; others have no medullary sheath 

 throughout their whole course, and consist merely of an 

 axis cylinder (often striated) and nuclei, with 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 sym- 

 pathetic ganglia and the spinal nerves both white and gray 

 fibres are found; the former are cerebro-spinal fibres pass- 

 ing into the sympathetic system, while the gray fibres origi- 

 nate 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 sub- 

 division 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. So far as our knowl- 

 edge at present goes, the only structures known with cer- 

 tainty to be connected with the central ends of nerve-fibres 

 are nerve-cells, and these latter may therefore be regarded 

 as the central organs of the nerve-fibres. So many nerve- 

 fibres have been traced into continuity with nerve-cells,, 

 that it is pretty certain all arise in this way. 



At 1, Fig. 73, is shown a typical nerve-cell such as may 

 be found in an anterior horn of the gray matter of the 

 spinal cord. It consists of the cell body, or cell protoplasm, 

 in which is a large nucleus containing a nucleolus. From 

 the body of the cell arise several branches, the great ma- 



