SEC. 2. THE STBUCTUKE OF THE SPINAL COED. 



452. Before we proceed to discuss the functions of the 

 spinal cord it may be as well to call to mind some of the main 

 features of its minute structure. (Figs. 110, 111, 112.) The greater 

 number of the fibres of the anterior root are the clothed axis-cyl- 

 inder processes of cells lying in the anterior horn of the same side, 

 placed not far from the origin of the root and belonging to the 

 segment of the cord indicated by the root. The rest of the 

 fibres of the root are the clothed axis-cylinder processes of cells 

 lying in other parts of the grey matter, in the anterior horn of 

 the opposite side and elsewhere. In the case of all the fibres 

 of the root, the axis cylinder is the continuation of a process of 

 a cell lying in the grey matter not far from the origin of the 

 root. We may infer that changes started in these cells issue 

 as efferent impulses along the fibres of the root; those started 

 in the cells of anterior horn are motor impulses passing to the 

 skeletal muscles, the smaller number started in other cells are 

 probably efferent impulses destined for other structures, vaso- 

 constrictor impulses and the like. 



The fibres of the posterior root, coming from and dependent 

 for their nutrition on the ganglion of the root (we may neglect 

 the few exceptional fibres of a different nature and having 

 different relations), passing for the most part into the posterior 

 column so as to form the " root-zone " lying close to the median 

 side of the posterior horn, take the following course. Each 

 fibre, soon after its entrance into the cord, bifurcates, one 

 division running forwards towards the head, the other back- 

 wards towards the hind extremity. Each division gives off 

 "collateral" fibres, and the end of each collateral as well as the 

 final termination of each division is furnished by an arborescence 

 which embraces and is in contact with but not in continuity 

 with, the body of, and generally with the branches of the body of 

 some nerve-cell in the grey matter. But the course of the fibres 

 is not the same in all cases. Many of the fibres (and appar- 

 ently all the backward travelling divisions of all the fibres) are 

 thus by means of the collaterals or the main terminations brought 



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