698 



THE NERVOUS SYSTEM. 



The gray commissure, which connects the two crescentic masses of gray mat- 

 ter, is separated from the bottom of the anterior median fissure by the anterior 



white commissure. It consists of transverse fibres, 



a, x"T~X with a considerable quantity of neuroglia between 



them. The fibres when they reach the lateral 

 crescents diverge : some pass backward to the 

 posterior roots ; others spread out, at various 

 angles, into the cervix cornu. 

 /CT~^\ Running through the gray commissure of the 



Post, commissure 



Epithelium oj 

 the central canal 



f 



Ant. com- 

 missure 



FIG. 404. Diagram of cross-sections of the cord at various levels, a, beginning of the cervical portion, b, 

 cervical enlargement, c, thoracic or dorsal region, d, lumbar enlargement, e, end of the same. /, end of the 

 cord. (Gegenbaur.) 



FIG. 40o. Cross-section of the cervical portion of the spinal cord of a six weeks' embryo. (Kolliker.) 



whole length of the cord is a minute canal, which is barely visible to the naked 

 eye in the human cord, but is proportionally larger in some of the lower verte- 

 brata. It is called the central canal, and opens above into the fourth ventricle, 

 and terminates below in a somewhat dilated extremity. It is lined in the foetus 

 by columnar ciliated epithelium, but in the adult very often the cilia have disap- 

 peared, and the canal is filled with their remains. The cells are supported on a 

 layer of neuroglia, which is sometimes called the substantia gelatinosa centralis. 



The mode of arrangement of the gray matter, and its amount in proportion to 

 the white, vary in different parts of the cord. Thus, the posterior horns are long 

 and narrow in the cervical region ; short and narrower in the dorsal ; short, but 

 wider, in the lumbar region. In the cervical region the crescentic portions are 

 small, and the white matter more abundant than in any other region of the cord. 

 In the dorsal region the gray matter is least developed, the white matter being 

 also small in quantity. In the lumbar region the gray matter is more abundant 

 than in any other region of the cord. Toward the lower end of the cord the white 

 matter gradually ceases. The crescentic portions of the gray matter soon blend into 

 a single mass, which forms the only constituent of the extreme point of the cord. 



Minute Anatomy of the Cord. The cord consists of an outer part, composed 

 of medullated nerve-fibres, which is the white substance ; and of a central part, the 

 gray matter, both supported in a peculiar kind of tissue, called neuroglia. 



The neuroglia consists of a homogeneous transparent matrix, of a network of 

 very delicate fibrillae, and of small stellate or branched cells, the neuroglia-cells. 



In addition to forming a ground substance, in which the nerve-fibres, nerve- 

 cells, and blood-vessels are imbedded, a considerable accumulation of neuroglia 

 takes place in three situations (1) on the surface of the cord, beneath the pia 

 mater ; (2) around the central canal ; and (3) in the posterior part of the posterior 

 horn, forming the substantia cinerea gelatinosa. 



