STRUCTURE OF THE NERVE-CENTRES. 



519 



have been observed by Grandry in the substance of the nerve-cells. While 

 this fact, perhaps, shows that the substance contained in the cells and their 

 prolongations is like the substance of the axis-cylinder, it is possible that the 



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FIG. 186. Transverse section of the gray substance of anterior cornua of the spinal cord of the ox, 

 treated with silver nitrate (Grandry). 



markings may be entirely artificial, and that they do not indicate the exist- 

 ence of two distinct substances. 



Tracing the nerve-fibres toward their origin, they are seen to lose their 

 investing membrane as they pass into the white portion of the centres, being 

 here composed only of medullary substance surrounding the axis-cylinders. 

 They then penetrate the gray substance, in the form of axis-cylinders, losing 

 the medullary substance. In the gray substance, it is impossible to make 

 out all their relations distinctly, and it can not be stated, as a matter of 

 positive demonstration, that all of them are connected with the poles of 

 nerve-cells. Still, it has been shown in the gray matter of the spinal cord, 

 that many of the fibres are actual prolongations of the cells, others probably 

 passing upward to be connected with cells in the encephalon. 



Tracing the prolongations from the cells, it is found that at least one of 

 the poles in the gray substance gives origin to nerve-fibres, but that these 

 fibres do not branch after they pass into the white substance. Other poles 

 connect the nerve-cells with each other by commissural fibres of greater or 

 less length ; and it is probable that the cells are thus arranged in separate 

 and distinct groups, possibly connected with sets of muscles. 



Accessory Anatomical Elements of the Nerve-centres. In addition to the 

 cells of the gray matter and the axis-cylinder of the nerves, which are prob- 



