THE SPINAL CORD AND ITS NERVES 127 



the walls of the tube are thickened. The nerve-cells retain their 

 primary position bordering the central canal, thus forming a 

 mass of central gray matter which is roughly H -shaped in cross- 

 section. This gray matter on each side is accumulated in the 

 form of two massive longitudinal ridges, a dorsal column 

 (columna dorsalis, or posterior horn), whose neurons receive 

 terminals of the sensory fibers of the dorsal roots, and a ventral 

 column (columna ventralis, or anterior horn) whose neurons give 

 rise to the fibers of the ventral roots. 



The white matter of the spinal cord is superficial to the gray 

 and is made up of sensory and motor root fibers of spinal nerves, 

 ascending and descending correlation fibers putting different 

 parts of the cord into functional connection, and longer ascend- 

 ing and descending tracts by which the spinal nerve-centers 

 are connected with the higher association centers of the brain. 

 In general, the shorter fibers lie near to the central gray and the 

 Ion7er tracts more superficially. 



The white matter which borders the gray in the spinal cord 

 is more or less mingled with nerve-cells and fine unmyelinated 

 endings, and thus shows under low powers of the microscope a 

 reticulated appearance. This is the reticular formation (pro- 

 cessus reticularis) of the cord (see pp. 65, 158, and Fig. 58). 

 Immediately surrounding the reticular formation and partly 

 embedded within it are myelinated fibers belonging to neurons 

 intercalated between the sensory and the motor roots, which 

 run for relatively short distances in an ascending or descending 

 direction for the purpose of putting all levels of the cord into 

 functional connection in the performance of the more complex 

 spinal reflexes. These fibers form the deepest layer of the 

 white matter and are termed the fasciculi proprii (dorsalis, 

 lateralis, and ventralis, see Fig. 59). These fascicles are also 

 called ground bundles and fundamental columns. 



In the narrow space between the ventral fissure and the cen- 

 tral canal (see Fig. 58) there is a bundle of nerve-fibers which 

 cross from one side of the spinal cord to the other. This is the 

 ventral commissure. A similar but smaller dorsal commissure 

 crosses immediately above the central canal. 



There is considerable confusion in the terminology in use in the further 

 analysis of the spinal white matter, and the usage which follows differs 



