766 NERVOUS SYSTEM OF THE HORSD 
mm. in width, a little lateral to the ventral median fissure, and no groove is found 
here. In the greater part of the cervical region and the anterior part of the thoracie 
region there is a shallow dorsal intermediate groove (Sulcus intermedius dorsalis) a 
short distance lateral to the median 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 dorsal 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 
Fia. 628.—CrRoss-section oF Sprnat Corp in situ, ENLARGED AND IN Part ScHEMATIC. 
1, Dorsal column; 2, lateral column; 3, ventral column; 4, dorsal horn; 45, ventral horn; 6, gray commissure; 7, 
white commissure; 8, central canal; 9, dorsal septum; 10, ventral fissure; 11, dorsal nerve-root; 12, ventral nerve-root,. 
13, spinal ganglion; 14, spinal nerve; 15, intervertebral foramen; 16, arch of vertebra; 17, epidural space; 18, dura 
21, middle 
mater (represented a little too thick); 79, hgamentum denticulatum; 20, 20, longitudinal venous sinuse 
spinal artery; 22, body of vertebra. The subdural and subarachnoid spaces (black) are traversed by delicate trabec- 
ule. The outer part of the arachnoid and the pia mater are not shown, 
by commissures of gray and white matter. The gray commissure (Commissura 
grisea) is a transverse band 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 
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 spinal cord (Canalis centralis), the spinal vestige of the 
lumen of the embryonal 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 
