SPINAL CORD 411 



median fissure, which is bounded on either side by the bulgings just de- 

 scribed. Into each of these two swellings the gray substance projects, 

 forming the ventral "horns" or columns (columna anterior or ventralis). 

 The term "horn" refers to the appearance in sections, and "column" 

 applies to their true form, taken as a whole. Corresponding with the 

 ventral columns of gray substance, there are two dorsal columns, which 

 arise somewhat later, and cause the gray substance, as seen in sections, 

 to assume the form of a letter H. With many variations this appearance 

 is characteristic of the entire spinal cord in mammals generally. As 

 seen in Fig. 424, there are secondary swellings on the sides of the "H" 

 which are called lateral columns; at certain levels they are ill-defined or 

 absent. 



Instead of forming a dorsal median fissure, the medullary tube pro- 

 duces a dorsal median septum. The lower or ventral part of the septum 

 is apparently formed by the coalescence of the lateral walls of the medul- 

 lary tube, thus leaving the ventral portion of the original lumen as the 

 central canal of the adult. Occasionally this small cavity, 0.5-1.0 mm. 

 wide, is entirely obliterated. The dorsal portion of the septum consists 

 of neuroglia fibers extending from the roof of the central canal to the per- 

 iphery of the cord. Thus in the adult the cord is divided into right and 

 left halves, except for the transverse connections or commissures near the 

 central canal. These include a dorsal commissure, a ventral gray commis- 

 sure, and a ventral white commissure. 



The white substance of each half of the cord is subdivided into three 

 longitudinal Juniculi, each of which includes several smaller bundles or 

 fasciculi, otherwise known as "fiber tracts." The funiculi are dorsal, 

 lateral, and ventral respectively, and their boundaries are seen without 

 magnification. The dorsal or sensory roots enter the cord along a groove 

 known as the dorso-lateral sulcus, and the ventral or motor roots emerge 

 along the ventro-lateral sulcus. All the white substance between these 

 two sulci is included in the lateral funiculus. The dorsal funiculus ex- 

 tends from the dorso-lateral sulcus to the median dorsal septum; and the 

 ventral funiculus extends from the ventro-lateral sulcus to the mid- 

 ventral fissure. 



The fasciculi of which each funiculus is composed cannot be studied 

 profitably in normal specimens. They have been followed chiefly by 

 observing the effects of local injury and disease, for if a group of nerve cells 

 is destroyed, all the fibers proceeding from it will degenerate. In this 

 way it has been shown that the fibers of the funiculi are not arranged in- 

 discriminately, but occur in definite tracts, which in some respects are 

 radically different in different animals. Thus the fibers of voluntary 

 motion which descend from the cerebral hemispheres to the motor cells of 

 the cord, forming the cerebro-spinal fasciculi, are found in the dorsal fun- 



