66 ANATOMY OF THE CENTRAL XEETOUS SYSTEM. 



column in question contains, besides the small ganglion-cells whicli produce 

 the motor nerves, — by chance not shown in the figure, — a mass of immense 

 spherical ganglion-cells: the Nucleus nervorum eledricorum. These cells 

 certainly correspond to a group of motor cells in other animals; but, as you 

 know, the electric organ of Gymnotus is shown by location and structure to 

 be derived from muscular tissue. 



After their entrance into the spinal cord most of the posterior root- 

 fibers soon enter the gray matter; in part, however, they reach the gray 

 matter only after a longer or shorter course in the posterior tracts. It has 

 been found that a small number of the root-fibers actually pass through the 

 gray matter, crossing over to the dorsal column of the other side to end 

 immediately, or to end only after passing along the posterior tract of that 

 side for a certain distance. These crossed sensory nerves pass from one side 

 to the other in the Commissura dorsalis medullce spinalis. They are very 

 unequally developed in different animals, even in animals of related species. 

 Besides that, the commissura dorsalis is unequally developed at difEerent 

 levels of the cord. The number of fibers in it depends upon the size of the 

 posterior roots, and from the proximity of this to the point of section. 



A certain portion of the posterior roots is not at once lost in the net- 

 work of the gray matter, but proceeds farther ventrally to the region of the 

 ventral column. This will be discussed later. 



Finally it must be mentioned that in mammals a part of the sensory 

 tract comes into connection with cells which stand in direct connection with 

 the cerebellum through their axis-cylinders. These cells arranged in long 

 columns — the column of Stilling-Clarke — have only been demonstrated in 

 mammals. Their presence in fishes, reptiles, and birds is, however, at least 

 probable, though their certain identification has not yet been successful. 



Eemember that the dorsal columns or posterior horns owe their exist- 

 ence to the entering roots, and that the posterior tracts are little more than 

 the continuation of root-fibers. The same thing is true for a part of the 

 "lateral columns, varying with different orders of animals. 



Such are the characteristics of the apparatus through which the im- 

 pressions of the outer world are conducted to the central system. Before 

 tracing the course of the afferent impulses within the central system, the 

 origin of the motor nerves will be described. That may be readily done, 

 since the essential facts have already been presented. Eemember that in the 

 ventral column (anterior horn) of the gray matter, lie long columns of 

 ganglion-cells whose neuraxons for the greater part emerge from the an- 

 terior roots of the same side, while the smaller remaining parts cross over 

 and emerge from the opposite root. As motor nerves, they proceed on 

 their course. At quite regular intervals the ventral columns form ventrally 

 projecting prominences of the spinal gray matter: the anterior liorns. They 



