THE NERVE-TRACTS. 717 



of a larger size. This interlacement is formed partly by the axons and dendrites 

 of the cells of the gray matter, and partly by fibres which enter the gray matter 

 and which come from various sources. 



The nerve-cells of the gray matter are collected into groups as seen on trans- 

 verse section, but they really form columns of cells placed longitudinally ; or else 

 they are found scattered throughout the whole of the gray matter. 



In the anterior horn the cells consist of two chief groups : one mesial, the 

 more constant, near the anterior column ; the other lateral, near the lateral column. 

 A second lateral group is present in the cervical and lumbar enlargements. 

 At the base of the posterior horn on its inner side, adjoining the gray commissure, 

 is a group of nerve-cells, called Clarke's posterior vesicular column, which extends 

 from the eighth cervical to the second lumbar nerve. 



At the junction of the anterior and posterior cornu, in the outer portion of the 

 gray matter, is a third group of cells, the lateral cell column ; this is best seen in 

 the dorsal region. In certain regions of the cord these cells extend in among the 

 fibres of the white matter of the lateral column, and give rise to the lateral horn. 

 In addition to these groups a few large scattered cells are found in the posterior 

 horn and in the substantia gelatinosa of Rolando. 



Origin of the Spinal Nerves. The roots of the spinal nerves are attached to the 

 surface of the cord, opposite the horns of gray matter. 



The posterior nerve-root enters the cord in two bundles, mesial and lateral. The 

 mesial strand consists of coarse fibres which enter the outer part of the column of 

 Burdach. The lateral strand is sometimes divided into a middle and an external 

 bundle. The former contains large fibres, and passes through the gelatinous sub- 

 stance of Rolando into the posterior horn. The external bundle consists of fine 

 fibres which assume a longitudinal direction in Lissauer's tract. All the posterior 

 root-fibres divide into ascending and descending branches on entering the cord, and 

 these in their turn give off collaterals. The fibres and their collaterals terminate 

 by forming arborescences, some around the cells in the posterior horn, and others 

 around the cells of Clarke's column, while the long ascending branches pass up in 

 the columns of Goll and Burdach, and end by arborizing around the cells in the 

 gracile and cuneate nuclei. Some of the fibres, however, pass to the gray matter 

 of the opposite horn, and others to the anterior horn of the same side of the cord. 



Anterior Nerve-roots. The majority of the fibres of the anterior nerve-roots are 

 the continuations outward of the axons of the large or small multipolar cells in the 

 anterior horn of gray matter. Some, however, appear to pass across in the anterior 

 white commissure to the cells in the anterior horn of the opposite side, while others 

 extend backward to the posterior horn and outward to the lateral column of the 

 same side. 



The Nerve-tracts. 



The anatomy of the various parts of the central nervous system having been 

 described, a short account will now be given of the course taken by its more impor- 

 tant nerve-tracts, and of the direction in which impulses pass along them. Before 

 doing so, however, it is necessary to refer to the methods employed in elucidating 

 this complex subject. All nerve fibres may be regarded as outgrowths from nerve- 

 cells, and it is found that if a nerve-fibre be cut, the portion of it which is severed 

 from the cell undergoes degeneration and becomes atrophied. Until recent years 

 it was believed that the cell itself showed no change under such circumstances. 

 This, however, is not the case, for if a nerve, the sciatic for instance, be divided in 

 an animal, and after an interval of some weeks the animal be injected with methy- 

 lene-blue and killed, it will be seen, on examining sections of the lumbar region 

 of the spinal cord, that the cells are stained imperfectly or not at all, owing to a 

 diminution, or, it may he, an entire disappearance of the chromatin, a substance 

 which, in a normal cell, shows marked affinity for staining reagents. Further, the 

 body of the cell is swollen, the nucleus displaced toward the periphery, and the 

 part of the axon still attached to the altered cell is diminished in size and some- 



