f/vi-iioJL.^v. t-u*vrtj 



SPINAL CORD 



845 



roots. Certain nerve cells situated in the cervix of the ventral horn are believed 

 to send splanchnic efferent axones in company with the motor axones in the ven- 

 tral root, while very few similar efferent axones leave by means of the dorsal root. 

 (6) Nerve cells whose axones pass into the white substance, usually bifurcating 

 into a shorter descending and a longer ascending branch. Two kinds of cells 

 are distinguished: 



1. Strand or tract cells whose axones (ascending branches) traverse the cord, 

 to come into relation with higher centres in the brain, -f- 



2. Association cells whose axones, after a comparatively brief course in the 

 jvvhite substance, ree'nter the gray substance and serve to coordinate different 



levels of the cord. 



FIG. 614. Diagram showing the order of myelinization of the various tracts in the spinal cord (cervical 

 level). The tracts are named on the right side; the Roman numerals on the left side correspond with the 

 enumeration given in the text. H. Olivospinal tract of Helweg. M. Marginal tract. 



The tract cells may be further divided into two categories homolateral and 

 contralateral tract cells. Homolateral cells are those whose axones enter the white 

 columns of the same side; contralateral cells are those whose axones traverse the 

 white (ventral) commissure to the other side. Tract cells exist in all parts of the 

 gray substance, and are termed, according to their situation, ventral, lateral, and 

 dorsal horn cells. The contralateral tract cells preponderate in the dorsal horn, 

 its cervix, and in the intermediate zone, and, on account of their course, are also , 

 called commissural cells. 



(c) Xerve cells of Golgi's type II, or cells with short, multibranched axones. 



The motor ventral horn cells and the splanchnic efferent cells differ, therefore, 

 from the other categories in that they alone send their axones out of the central 

 axis to the periphery. The tract cells, commissural cells, and the Golgi type II 

 cells are strictly confined to the central axis; the tract cells serve to coordinate 

 the separate units of the spinal neurone system with higher centres; the asso- 

 ciation cells maintain the paths of conduction between higher and lower cell 

 complexes; while the cells of Golgi's type II are limited to a narrower field of 

 nerve activity as nerve links in the chaining together of neurones. 



White Substance. The white substance consists essentially of axones the great 

 majority of which are disposed longitudinally. These axones comprise: 



