358 ANATOMY OF THE CENTRAL NERVOUS SYSTEJI. 



levels of the cord, which lie cephalad or caudad from them; hence they are 

 adapted to serve as a substratum for the long-accepted paths that connect 

 together different levels. 



To any irritation coming to the cord from the periphery there are offered a 

 large number of conduction-paths. There is, first of all, a number of posterior root- 

 fibers which extend directly into the anterior horn and arborize around its cells. 

 These are well adapted to "load" such cells Avith impressions, or after sufficiently high 

 irritation to call forth immediate discharge of motor reflexes. But, through cells 

 originally in relation with each other, and through others that are so as a result of 

 selective function, the motor cell-groups are functionally associated in such a man- 

 ner that a single impression is often sufficient to bring an entire group of cells to 

 discharge at the same time. So reflexes may consist of single muscular movements 

 and also of very complicated actions. Besides the dendrites the "tract-cells" with 

 their processes form the anatomic basis of these associations. It is not difficult to 

 accept the statement that an impulse, arriving at the spinal cord, spreads in this 

 way through these cells over different levels, and so unites in exciting motor cells 

 of the most varied positions to simultaneous action. (Exner and others.) 



All these fibers and cell-processes form an extraordinarily complicated 

 raesh-work in the gray matter of the cord. Its unraveling has been accom- 

 plished only by the use of all the methods ever conceived. In the adult 

 cordj colored after the Weigert method, this is impossible. 



All fibers crossing from one side to the other occupy either the anterior 

 or the posterior commissure. 



Although the component elements of these commissures have already been men- 

 tioned, each in its place, still it will be well to consider them again more topographic- 

 ally. 



In the anterior commissure, then, we find (Fig. 227) : — 



1. Belonging to the anterior roots: fibers from cells to the opposite root; col- 

 laterals of the direct pyramidal tract; numerous dendritic processes from neigh- 

 boring ventral horn-cells. 



2. Secondary sensory paths from those cells, around which arborize fibers from 

 tlje posterior nerve-roots. 



3. From the "tract-cells": numerous axis-cylinder processes to the opposite 

 anterior and lateral columns. 



4. Connecting fibers from the lateral column of one side to the anterior column 

 of the other. 



This tract, found by Schaffer in vertebrates of different classes, is, according to 

 him, made up of posterior root-fibers, which first enter the lateral column, and higher 

 up cross over into the opposite anterior column. It has been demonstrated, also, in 

 animals that have no medullary pyramidal fibers (reptiles) ; therefore it cannot be 

 an accessory pyramidal tract: a supposition which otherwise might seem true. 



Of the posterior commissure we know much less. Of a certainty it con- 

 tains meduUated fibers, and these surely arise from the posterior roots and 

 from tracts into which the posterior root-fibers have entered. 



