70 ANATOMY OF THE OENTEAL NEETOUS SYSTEM. 



fibers; namely, fibers from the anterior horn of one side to the anterior root 

 of the opposite side; also in mammals more central tracts. 



Among bony fishes and in part, also, among selachians, the individual 

 elements of the commissura ventralis are so far divided that one may often 

 recognize two quite distinct commissures. In the midst of the gray sub- 

 stance one may see everywhere long fascicles of medullated nerves passing 

 upward and downward. In part these nerves are elongated fascicles from 

 the roots, in part derivatives of the commissural cells, and in part fibers from 

 other sources. Among cyclostomes and some of the bony fishes there are so 

 many of these that it is not possible to trace a sharp line of division between 

 the gray matter and the peripheral white matter. 



But in most vertebrates we find the central gray matter of the spinal 

 cord surrounded by white fibrous tracts. The spaces in the H (or X) which 

 represent the outline of the gray may be named as follows: Dorsal or 

 posterior tracts, ventral or anterior tracts, and lateral tracts. That the 

 posterior tracts are mostly or entirely formed from ascending or descending 

 fibers of the posterior roots has already been mentioned. In the lateral and 

 anterior tracts we must look for those fascicles which arise from the com- 

 missural cells and which bring into association different levels of the cord. 

 These are usually called, in brief, the tracts of the cord. 



In most vertebrates, even as low as the fishes, there lie very long tracts 

 in the anterior column. In fishes and larval amphibians one may find, close 

 beside the gray matter, one or more very thick fibers: Mauthner's Fibers. 

 Arising in the cranium, near the origin of the eighth nerve — the nerve of 

 equilibration — from gigantic ganglion-cells, the axis-cylinder, surrounded 

 by enormous medullary sheaths, may be traced as far as the caudal vertebrae, 

 where they emerge with the last sacral nerves (Fritsch). When one recalls 

 the importance of the caudal musculature in the maintenance of equilibrium 

 in swimming animals, he will recognize that the musculature in question is 

 in. special relation with the nerves of the ampullfe. The apparent absence 

 of these fibers from many eel-like fishes (Haller) depends upon the different 

 method by which that form of body maintains its equilibrium. In Pig. 29 

 these fibers are designated as Pihrce Acustico-sacrales. 



Thus far the spinal cord has been represented as an independent center. 

 As is well IcQown, it is able to function as such under many circumstances; 

 all experiments on decapitated animals show this. They teach that in the 

 spinal cord are not only tracts for the simpler reflexes, but that even very 

 complicated movements may be innervated from the spinal centers and be 

 brought into activity reflexly. When the decapitated snake winds itself 

 about the proffered support; when the decapitated duck swims; or the 

 decapitated rabbit is able to make several normal leaps, it is not to be 

 otherwise accounted for than that there are in the spinal cord complete 



