344: ANATOMY OF THE CENTBAL NEHVOUS SYSTEM. 



neuroglia: the peripheral gelatinous layer (Fig. 219, to the right). Also on the point 

 of the dorsal horns there is found an especially- thick development of glia. Here 

 the entering posterior roots suffer, mostly at the expense of their medullary sheaths, 

 and appear very compact, as If tied together, in a section. (Obersteiner and Redlich.) 

 Fig. 5 explains the epithelium of the central canal in the fetus. In youth, 

 also, the cells lie in regular rows on the layer of neuroglia. They lose their cilia 

 probably soon after birth, although during life there remains a peculiar layer of 

 small, evenly placed nodules on the central edge of the cells, which, during the 

 fetal period, are found just at the bases of the cilia. With increasing age, the 

 resistance of the epithelial cells lessens. They become separated from each other, 

 neuroglia-fibers appear between them, they even loosen from their bases, and lie in 

 apparent confusion or in small groups between and among the developing neuroglia- 

 tissue. As a result, there is in the place of the original central canal a mass of cells 

 leaving either no lumen at all or perhaps several small lumina in the center of the 

 cord. 



This much has been ascertained by the examination of sections of the 

 adult cord. But in a knowledge of the minute structure of the cord, much 

 more has been learned. 



As one sees in the cross-section of the cord, it is divided into columns 

 by the spinal nerve-roots and by the longitudinal fissures, iledian to the 

 roots lie the anterior and posterior columns; lateral to them, the lateral 

 columns. 



The study of embryology, as well as that of the effects of interruption 

 of the fibers, and the examination of certain diseases of the spinal cord have 

 taught that these anterior, posterior, and lateral cokimns are not homogene- 

 ous, equivalent bundles of fibers, as might appear on casual inspection of a 

 cross-section of a healthy, adult cord, but that they are composed rather of 

 several divisions. 



It will be remembered that the tradus cortico-spinnlis. the pyramid, 

 passes from the cortex of the motor region down through the internal cap- 

 sule and the pes cerebri to the ventral portion of the pons. Where does it 

 pass in the medulla spinalis? It is not difficult to find. When anywhere in 

 its long course it is destroyed by a disease-focus, its fibers gradually dis- 

 appear, being displaced by connective tissue. This degeneratiqu, called 

 secondary degeneration, proceeds downward into the cord. It takes place 

 there in two locations; one in the median portion of the anterior column of 

 the same side as the cerebral lesion, and the other in a large portion of the 

 lateral column of the opposite side. Higher up, at the beginning of the bulb, 

 one sees that this crossed tract passes over to the direct tract, decussating 

 with the sound fibers of the opposite side. 



The tract, which is affected by the degeneration, is called in the cord, 

 as in the brain, the tradus cortico-spinalis, or the pyramidal trad. In the 

 cord it divides into the pyramid of the anterior column (median part of the 

 column) and the pyramid of the lateral column (in its posterior half). There 



