ACCESSORY ELEMENTS LN" THE NERVE-CENTRES. 53 



of the medullary substance surrounding the axis-cylinder. 

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

 cylinders, losing here the medullary substance. In the gray 

 substance, it is impossible to make out of all their relations 

 distinctly, and we cannot assume, as a matter of positive dem- 

 onstration, that all of them are connected with the poles of 

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

 of the spinal cord, that many of the fibres are actual prolon- 

 gations of the cells, the others probably passing upward to 

 be connected with cells in the encephalon. 



Tracing the prolongations from the cells, we find that 

 one or more of the poles branch and subdivide in the gray 

 substance, and give origin to 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 ; but it has never been posi- 

 tively demonstrated that the cells are thus connected into 

 separate and distinct groups, though this is possible. 



The accompanying figure, taken from the excellent mono- 

 graph on the lumbar enlargement of the spinal cord, by Dean, 

 shows the mode of connection between certain of the cellular 

 prolongations and the fibres of the anterior roots, and the 

 commissural fibres by which the cells are connected with each 

 other. 



Accessory Anatomical Elements in the Nerve-centres. 

 While we must regard the cells of the gray matter and the 

 axis-cylinder of the nerves as probably the only anatomical 

 elements concerned in innervation, there are other struct- 

 ures in the nervous system which it is important for us to 

 study. These are : 1, outer coverings surrounding some of 

 the cells ; 2, intercellular, granular matter ; 3, peculiar cor- 

 puscles, called myelocytes ; 4, connective-tissue elements ; 

 5, blood-vessels and lymphatics. 



Certain of the cells in the spinal ganglia and the ganglia 

 of the sympathetic system are surrounded with a nucleated 



