THE SPINAL CORD. 



711 



which supply the upper extremities. The lumbar enlargement is situated opposite 

 the last two or three dorsal vertebrae, and corresponds with the origin of the nerves 

 which supply the lower extremities. Below the lumbar enlargement the cord 

 gradually tapers to form a cone, the conus medullaris, the apex of which is con- 

 tinuous with the filum terminale. In form, the spinal cord is a cylinder, flattened 

 before and behind. 



Fissures. It presents on its anterior surface, along the middle line, a longi- 

 tudinal fissure, the anterior median Jissure, and on its posterior surface another 

 fissure, which also extends along the entire length of the cord, the posterior median 

 Jissure. These fissures penetrate through the greater part of the thickness of the 

 cord, and incompletely divide the cord into symmetrical halves, united in the 

 middle line by a transverse band of nervous substance, the commissure. 



The Anterior Median Fissure is wider, but of less depth, than the posterior, 

 extending into the cord for about one-third of its thickness, and is deepest at the 

 lower part of the cord. It contains a prolongation from the pia mater, and its floor 

 is formed by the anterior white commissure, which is perforated by numerous 

 blood-vessels passing to the centre of the cord. 



The Posterior Median Fissure is not an actual fissure, as the space between the 

 lateral halves of the posterior part of the cord is crossed by connective tissue and 

 numerous blood-vessels, so that no actual hiatus exists, and there is consequently 

 no prolongation of the pia mater into it. It extends into the cord to about one- 

 half its depth, and its floor is formed by the posterior gray commissure. 



Opposite middle of cervical region. 



Opposite middle of dorsal region. 



Opposite lumbar region. 



FIG. 381. Transverse sections of the 

 cord. 



FIG. 382. From a transverse section through the spinal 

 cord of a calf. Magnified about 180 diameters, showing part 

 of the central canal and the tissue immediately around it, 

 viz., the central gray matter. (Klein and Noble Smith.) 

 The canal is lined with epithelium, composed of ciliated, 

 more or less conical, cells ; in most instances a filamentous 

 process passes from the cell into the tissue underneath. 

 This tissue contains, in a hyaline matrix, a network of 

 fibrils ; most of these run horizontally ; others have a longi- 

 tudinal course, and appear therefore here cut transversely 

 i. e., as small dots. The nuclei correspond to the cells of 

 the neuroglia, the cell-substance not being shown. Both 

 the nuclei of the neuroglia-cells and those of the epithe- 

 lium contain three or more large disc-shaped particles. 



Lateral Fissures. On each side of the posterior median fissure, along the line 

 of attachment of the posterior roots of the nerves, a delicate fissure may be seen, 

 leading down to the gray matter which approaches the surface in this situation ; 

 this is called the postero-lateral fissure of the spinal cord. On the posterior sur- 

 face of the spinal cord, between the posterior median fissure and the postero-lateral 

 fissure on each side, is a slight longitudinal furrow (posterior intermediate furrow), 

 marking off two slender tracts, the postero-median and postero-lateral columns. 

 These are most distinct in the cervical region, but are stated by Foville to exist 

 throughout the whole length of the cord. On each side of the anterior median 

 fissure the anterior roots of the spinal nerves emerge from the cord, not in one 

 vertical line, but by separate bundles which occupy an area of some width. This 

 is called, by some anatomists, the antero-lateral Jissure of the cord, although no 

 actual fissure exists in this situation. 



Columns of the Cord. Each half of the spinal cord is thus divided into four 

 columns : an anterior column, a lateral column, a posterior column, and a postero- 

 median column. This division, hoAvever, is very imperfect, since the limit between 



