THE MEDULLA SPINALIS OR SPINAL CORD 



755 



Structure of the Gray Substance. The gray substance consists of numerous nerve 

 cells and nerve fibers held together by neuroglia. Throughout the greater part 

 of the gray substance the neuroglia presents the appearance of a sponge-like net- 

 work, but around the central canal and on the apices of the posterior columns 

 it consists of the gelatinous substance already referred to. The nerve cells are 

 multipolar, and vary greatly in size and shape. They consist of (1) motor cells of 

 large size, which are situated in the anterior horn, and are especially numerous in 

 the cervical and lumbar enlargements; the axons of most of these cells pass out to 

 form the anterior nerve roots, but before leaving the white substance they fre- 

 quently give off collaterals, which reenter and ramify in the gray substance. 1 (2) 

 Cells of small or medium size, whose 

 axons pass into the white matter, 

 where some pursue an ascending, 

 and others a descending course, but 

 most of them divide in a T-shape 

 manner into descending and ascend- 

 ing processes. They give off col- 

 laterals which enter and ramify in 



the gray substance, and the termi- 

 nations of the axons behave in a 

 similar manner. The lengths of 

 these axons vary greatly: some 

 are short and pass only between 

 adjoining spinal segments, while 

 others are longer and connect more 



Neuroglial cells 



Ependy'mal cells 



FIG. 667. Section of central canal of medulla 

 spinalis, showing ependymal and neuroglial cells, 

 (v. Lenhossek.) 



Collateral 



Ascending 



'Descending 



Arborisation 



FIG. 668. Cells of medulla spinalis. Diagram showing 

 in longitudinal section the intersegmental neurons of the 

 medulla spinalis. The gray and white parts correspond 

 respectively to the gray and white substance of the medulla 

 spinalis. (Poirier.) 



distant segments. These cells and their processes constitute a series of association 

 or intersegmental neurons (Fig. 668), which link together the different parts of 

 the medulla spinalis. The axons of most of these cells are confined to that side 

 of the medulla spinalis in which the nerve cells are situated, but some cross to the 

 opposite side through the anterior commissure, and are termed crossed commissural 

 fibers. Some of these latter end directly in the gray substance, while others enter 

 the white substance, and ascend or descend in it for varying distances, before finally 

 terminating in the gray substance. (3) Cells of the type II of Golgi, limited for the 



1 Lenhossek and Cajal found that in the chick embryo the axons of a few of these nerve cells passed backward through 

 the posterior column, and emerged as the motor fibers of the posterior nerve roots. These fibers are said to control the 

 peristaltic movements of the intestine. Their presence, in man, has not yet been determined. 



