THE STRUCTURE OF THE SPINAL CORD 



563 



The gray matter on each side of the cord is divided into an anterior and a 

 posterior horn. In the lower cervical and in the upper thoracic regions of the 

 cord, as well as in the lumbar region, the lateral portion of the anterior horn 

 becomes partly separated off as a lateral horn (o). 



Among the nerve cells of the gray matter we distinguish: (1) The cells 

 arranged in groups in the anterior horn; (2) the cells of the so-called column 

 of Clarke (s) situated at the median side between the anterior and posterior 

 horns, and extending from the end of the cervical enlargement to the beginning 

 of the lumbar cord; (3) the cells of the substantia gelatinosa Rolandi capping 

 the posterior horn ; (4) the rest of the cells in the posterior horn. 



The fibers of the anterior nerve roots are the axis-cylinder processes of the 

 cells in the anterior horn of the same side, or, less frequently, of the opposite 

 side, the latter fibers crossing via the anterior commissure before they gain the 

 anterior root. The cells of the posterior horn on the contrary do not connect 

 directly with fibers of the posterior root, since the latter have their origin in 

 the cells of the spinal ganglia. 



These cells of the spinal ganglia, for the most part, are unipolar i. e., they 

 have but one process which, however, after a short course, splits into two 

 branches. One of the two proceeds toward the 

 periphery and joins the anterior nerve root in 

 a mixed nerve trunk; the other enters the cord 

 by the posterior nerve root. Practically all of 

 the fibers which enter the cord divide into an 

 ascending and a descending branch, both of 

 which give off collaterals, and sooner or later 

 end like the collaterals about the cells of the 

 gray matter of the cord. The peripheral nerve 

 fibers, therefore, have their origin either in the 

 cells of the anterior horn or in the cells of the 

 spinal ganglia. 



The nerve fibers from the anterior horn 

 cells are all efferent in function, and the nerve 

 fibers arising from the cells in the spinal gan- 

 glia are mainly afferent fibers. 



With regard to the further connection of 

 the two kinds of fibers we distinguish: (1) a 

 secondary efferent path; (2) a secondary affer- 

 ent path; and (3) the paths by which afferent 

 pass over into efferent impulses. 



Those tracts which connect the nerve cells 

 of the anterior horn with the higher centers 

 we designate as secondary efferent paths. They 

 are the paths by which impulses liberated in the 

 cells of the higher centers are conveyed to the 

 motor cells of the anterior horn. 



The fibers entering the cord from the spinal 



ganglia are brought into relation with the cells of the posterior horn whose 

 axis-cylinder processes constitute the secondary afferent paths. It is by these 

 paths that impulses are transmitted to the higher centers. 



The transition from afferent to efferent paths may occur in several ways. 

 The simplest instance is when an afferent nerve fiber or one of its collaterals 

 connects directly with the motor cell of an efferent nerve fiber or with some of 

 its processes by means of fibrils. 



FIG. 255. Schema, after Kolliker 

 and Lenhosse'k. A, motor cells 

 with root fibers ; B, spinal ganglion 

 cell with its processes; C, a sen- 

 sory collateral ; D, column cell with 

 T-shaped branching processes; E, 

 collaterals of the same. 



