THE MEDULLA SPINALIS OR SPINAL CORD 



eceptors). The medial processes of the ganglion cells grow into the medulla spinalis 



s the posterior roots of the spinal nerves. 



The posterior nerve root enters the medulla spinalis in three chief bundles, 

 edial, intermediate, and lateral. The medial strand passes directly into the fas- 

 ciculus cuneatus : it consists of coarse fibers, which acquire their medullary sheaths 

 about the fifth month of intrauterine life; the intermediate strand consists of coarse 

 fibers, which enter the gelatinous substance of Rolando; the lateral is composed 

 of fine fibers, which assume a longitudinal direction in the tract of Lissauer, and 



o not acquire their medullary sheaths until after birth. In addition to these 

 medullated fibers there are great numbers of non-medullated fibers which enter 

 with the lateral bundle. They are more numerous than the myelinated fibers. 



They arise from the small cells of the spinal ganglia by T-shaped axons similar 

 to the myelinated. They are distributed with the peripheral nerves chiefly to the 

 skin, only a few are found in the nerves to the muscles. 1 



Having entered the medulla spinalis, all the fibers of the posterior nerve roots 

 divide into ascending and descending branches, and these in their turn give off 

 collaterals which enter the gray 

 substance (Fig. 676). The de- 

 scending fibers are short, and 

 soon enter the gray substance. 

 The ascending fibers are grouped 

 into long, short, and intermedi- 

 ate: the long fibers ascend in 

 the fasciculus cuneatus and fas- 

 ciculus gracilis as far as the me- 

 dulla oblongata, where they end 

 by arborizing around the cells of 

 the cuneate and gracile nuclei; 

 the short fibers run upward for 

 a distance of only 5 or 6 mm. 

 and enter the gray substance; 

 while the intermediate fibers, 

 after a somewhat longer course, 

 have a similar destination. All 

 fibers entering the gray sub- 

 stance end by arborizing around 

 its nerve cells or the dendrites 

 of cells, those of intermediate 



length being especially associated 



.7 i ,, i j i 



With the Cells OI the dorsal nU- 

 i 



The long fibers of the posterior nerve roots pursue an oblique course upward, being 

 situated at first in the lateral part of the fasciculus cuneatus : higher up, they occupy 

 the middle of this fasciculus, having been displaced by the accession of other 

 entering fibers; while still higher, they ascend in the fasciculus gracilis. The upper 

 cervical fibers do not reach this fasciculus, but are entirely confined to the fascic- 

 ulus cuneatus. The localization of these fibers is very precise: the sacral nerves 

 lie in the medial part of the fasciculus gracilis and near its periphery, the lumbar 

 nerves lateral to them, the thoracic nerves still more laterally; while the cervical 

 nerves are confined to the fasciculus cuneatus (Fig. 673). 





. FIG. 676. Posterior roots entering medulla spinalia and dividing 

 into ascending and descending branches. (Van Gehuchten.) a. 

 Stem fiber. 6, 6. Ascending and descending limbs of bifurcation. 

 c. Collateral arising from stem fiber. 



1 Ransom, Brain, 1915, 38. 



