650 



NERVOUS SYSTEM OF THE HORSE 



groove. These grooves indicate the division of the white matter of the cord 

 into columns to be described later. 



Examination of cross-sections of the spinal cord shows that it is a bilaterally 

 symmetrical structure, incompletely divided into right and left halves by a ventral 

 fissure and a dorsal septum. The ventral median fissure (Fissura mediana ven- 

 tralis) is narrow and penetrates nearly to the middle of the dorso-ventral diameter 

 of the cord. It is occupied by a fold of pia mater. The dorsal median septum 

 (Septum medianum dorsale) is a partition which descends from the posterior median 

 groove to about the middle of the cord. It apparently consists of condensed neu- 

 roglia with an admixture of pial tissue. The two halves of the cord are connected 

 by commissures of gray and white matter. The gray commissure (Commissura 

 grisea) is a transverse Imnd of gray matter at the ventral end of the dorsal septum. 

 It is divided into dorsal and ventral parts by the central canal of the cord. The 



Fig. 497. — Cross-section of Spinal Cord m situ, in Part Schematic. 

 1, Dorsal column; 2, lateral column; 3, ventral column; 4, dorsal horn; 5, ventral horn; 6, gray commis- 

 sure; 7, white commissure; 8, central canal; 9, dorsal septum; 10, ventral fissure; 11, dorsal nerve-root; 12, 

 ventral nerve-root; iS, spinal ganglion; i 4. nerve; i5, intervertebralforamen; i6, arch of vertebra; i7, epidural 

 space; 18, dura mater (represented a little too thick); 19, hgamentum denticulatum; SO, 20, longitudinal venous 

 sinuses; 21, middle spinal artery. The subdural and subarachnoid spaces (black) are traversed by delicate trabe- 

 cular. The outer part of the arachnoid and the pia mater are not shown. 



white commissure (Commissura alba) is a bridge of white matter which connects 

 the ventral columns of the cord over the dorsal end of the ventral median fissure, 

 and constitutes a conducting path from one side to the other. 



The central canal of the cord (Canalis centralis), the spinal vestige of the 

 lumen of the emljryonal neural tube, is a minute passage which tunnels the gray 

 commissure. It opens at its anterior end into the posterior part of the fourth 

 ventricle of the brain, and its terminal part in the conus medullaris forms a slight 

 dilatation, the ventriculus terminalis. It is lined by epithelium and is surrounded 

 by a layer of modified neuroglia (Substantia grisea centralis). 



The gray matter of the spinal cord as seen in cross-sections resembles roughly 

 a capital H> fhf" cross-bar of the letter being formed by the gray commissure. 

 Each lateral part is considered as consisting of dorsal and ventral gray columns or 

 horns (Columna grisea dorsalis, ventralis). In the greater part of the cord the 



