716 



THE NERVOUS SYSTEM. 



its extremity is a lamina or layer of gelatinous material, which covers the head 

 like a cap, and from this it tapers almost to a point, which approaches the surface 

 of the cord at the postero-lateral groove. 



The gelatinous substance is a peculiar accumulation of neuroglia (Klein) 

 similar to that found around the central canal (page 713), and has been named 

 by Rolando the substantia cinerea gelatinosa. It probably takes its origin from 

 the columnar cells which line the posterior part of the embryonic spinal canal. 



The anterior horn of the gray substance in the cervical and lumbar swellings, 

 where it gives origin to the motor nerves of the extremities, is much larger than 

 in any other region, and contains several distinct groups of large and variously 

 shaped cells. 



In addition to this, a lateral horn is found projecting outward from the 

 lateral region of the gray matter on a level with the gray commissure in the upper 

 part of the dorsal region of the cord ; in the cervical and lumbar regions this 

 lateral horn blends with the anterior horn, which thus becomes broad and ex- 

 panded. From the concavity of the crescent, between the anterior and posterior 

 horns, processes of gray matter extend into the white substance, where they 

 divide and anastomose to form a network, termed iheformatio reticularis. 



Posterior Roots. 



Posterioi 

 Column. 



Anterior 

 Column. 



FIG. 386. Longitudinal section of the 

 white and gray substance of the spinal 

 cord, through the middle of the lumbar 

 enlargement. Magnified 14 diameters. 



FIG. 387. Transverse section through the 

 white matter of the spinal cord of a calf. 

 Magnified about 300 diameters. (Klein and 

 Noble Smith.) 



In the upper part are shown two isolated 

 flattened nucleated cells of the neuroglia, 

 under a somewhat higher power than the 

 rest. In the bulk of the figure the nerve- 

 fibres are seen in transverse section. They 

 are of different sizes, and possess a laminated 

 medullary sheath surrounding the axis- 

 cylinder, 'which was deeply stained in the 

 preparation, and is here represented by a 

 black dot. The nerve-fibres are embedded 

 in the neuroglia. Among the neuroglia are 

 also seen two branched connective-tissue 

 cells neuroglia-cells. 



The gray commissure contains the central canal, and is situated behind 

 the white commissure, which separates it from the bottom of the anterior median 

 fissure. 



The gray substance of the cord consists of (1) nerve-fibres of variable but 

 smaller average diameter than those of the white columns; (2) nerve-cells of 

 various shapes and sizes, with from two to eight processes ; (3) blood-vessels and 

 connective tissue. 



The nerve-fibres of the gray matter of the posterior horn are for the most part 

 composed of a dense interlacement of minute fibrils, intermingled with nerves 



