80 



ANATOMY OF THE CBNTEAL NEBVOtJS SYSTEM. 



therefore, with some hesitation that I designate this structure as the 

 "nucleus of the posterior columns." 



But the greatest difference between this section and one of a typical 

 spinal cord is the fact that in the space between anterior and posterior horns 

 innumerable commissural cells have made their appearance, cells whose 

 large neuraxons, arranged in small fasciculi, pass upward through the 

 medulla even into the Mesencephalon and Thalamencephalon. This is prob- 

 ably a great system of association-fibers which connect certain levels of the 

 cranial segment of the central system to each other and with the anterior 

 end of the spinal cord. This system, which is similarly located in all ani- 

 mals, is characteristic of the Medulla, and is well adapted to be the organ 



. r a a'v 







Fig. 41. — Section through the medulla oblongata of a Ray: 

 the Cephaloptcra Iwnpus. 



of those most intimately co-ordinated functions whose seat is in the MedtiUa. 

 In the figure this region is designated as Tradus irevis. 



Without doubt we have to deal here with an increase of that structure 

 already described with the spinal cord as the cellulas eommissurales and the 

 tracts arising from them. As in the spinal cord, so here there exist fibers 

 of short course, crossed and uncrossed. The ventral commissure, small in 

 the spinal cord, naturally becomes much increased incident to the increase 

 of the whole system. It is known from this point up to the Corpora Quad- 

 rigemina as the raphe decussation. Within this decussation, as in the 

 spinal cord, are cross-fibers of other categories than those which arise from 

 the cellulas eommissurales. But these will be described later. 



The area of association-fibers — i.e., the Tr. brevis of the oblongata — is 

 just as well developed in the lowest vertebrates as in the highest representa- 



