82 



ANATOMY OF THE CENTHAL XEKTOTJS SYSTEM. 



to follow the whole course in a single section; thus the accompanying 

 figure barely shows the relation between the arcuate fibers and the decussa- 

 tion. 



With this siirvey of the more important constituents in the cross- 

 section of the lower end of the medulla, it will be easy to understand the 

 opposite figiire (Fig. 43), which shows a similar section from the medulla of 

 an alligator. 



But what of the fibers of the anterior coliTmns which were found in 

 the spinal cord? In the section of the medulla of the ray (Fig. 41), they 

 occupy exactly the same position as in the spinal cord. A part of these 

 fibers pass, indeed, into the fillet, but most of them form a thick column, 

 very striking in most sections, which maintains the same location through 

 the oblongata, even increasing in volume. This tract — the Fasciculus 



Fig. 42. — Cross-section through the Oblongata of a mature Triton. 



longitudinalis posterior — may be followed to the base of the Thalamen- 

 cephalon. It appears to receive fibers from all motor nuclei throughout its 

 long course. Degeneration experiments on mammals teach that the fibers 

 are, for the most part, short ones. In fishes, amphibia, and reptiles this 

 fasciculus is one of the largest of the medulla. It occupies the whole floor 

 of the fourth ventricle, as you see in the sturgeon (Fig. 39). 



In the midst of this tract in aquatic animals are the previously 

 described thick fibers — Fihros acustico-spinales — from the acusticus region 

 to the caudal musculature. These fibers may be seen in Figs. 40 and 42, 

 Fig. 40 D showing the decussation and the large terminal cell {cellula M. 

 Fas.) from which on either side the fiber arises. 



"We have now located in the medulla most of the tracts previously 

 known in the cord. One important tract remains yet to mention: the Tr. 



