156 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



(a) Those whose axons enter the white matter on the same side of 

 the cord, the tautomeric tract cells of Van Gehuchten. (b) Those 

 whose axons pass through the anterior white commissure and thus 

 reach the tracts in the anterior or lateral columns of the other side. 

 These are known as commissural cells or the heteromeric tract 

 cells of Van Gehuchten. They form one obvious means for crossed 

 conduction in the cord, (c) Those whose axons divide into two, 

 one passing into the white matter of the same side, the other pass- 

 ing by way of the anterior commissure to reach the white matter 

 of the opposite side the hecateromeric tract cells of Van Gehuch- 

 ten. (3) The Golgi cells of the second type that is, cells whose 



Dorsal 



Fig. 68. Schema of the structure of the cord. (After 'Lenhossek.) On the right the 

 nerve cells; on the left the entering nerve fibers. Right side: 1, Motor cells, anterior 

 horn, giving rise to the fibers of the anterior root; 2, tract cells whose axons pass into the 

 white matter of the anterior and lateral columns; 3, commissural cells whose axons pass 

 chiefly through the anterior commissure to reach the anterior columns of the other side; 

 4, Golgi cells (second type), whose axons do not leave the gray matter, 5, tract cells whose 

 axons pass into the white matter of the posterior column. Left side: 1, Entering fibers of 

 the posterior root, ending, from within outward, as follows: Clarke's column, posterior 

 horn of opposite side, anterior horn same side (reflex arc), lateral horn of same side, pos- 

 terior horn of same side ; 2, collaterals from fibers in the anterior and lateral columns ; 3, 

 collaterals of descending pyramidal fibers ending around motor cells in anterior horn. 



axons divide into a number of small branches like those of a 

 dendrite. The axons of these cells, therefore, do not become 

 medullated nerve fibers; they take no part in the formation of 

 the spinal roots or the tracts of white matter in the cord, but 

 terminate diffusely within the gray matter itself. (4) The pos- 

 terior root cells lying toward the base of the anterior horns. These 

 cells have been demonstrated in some of the lower vertebrates 

 (petromyzon chick embryo), but their existence in the mammal 

 is still a question in some doubt; their axons pass out from the 

 cord by the posterior root and they form the anatomical evidence 



