THE OBLONGATA AND THE NUCLEI OF THE CRANIAL NEEVES. 81 



tives of that branch. In this connection compare Fig. 41 with Fig. 230 

 {Processus reticularis). It is evident that we have here to deal with the 

 anatomical basis of very similar fvinctions. Through this tract of associa- 

 tion-fibers the fibers which pass from the spinal cord to the cerebellum, the 

 Mesencephalon, and the Thalamencephalon are pushed laterally toward the 

 periphery. Note in Fig. 41 the location of the Tr. cerebello-spinales and the 

 Tr. tecto-thalamo-spinales. 



The Tr. tecto-spinales contain, as was previously shown, crossed tracts 

 from the terminal nuclei, into which the sensory fibers of spinal roots 

 enter. They are, indeed, a secondary sensory tract from those nuclei to the 

 roof of the Mesencephalon. In the medulla they are joined by the much 

 larger bundle from the terminal nuclei of the cranial nerves and the now 

 much enlarged bundle, which may be subdivided into numerous parts 

 properly designated by the term Tr. tecto-spinales et bulbares, or more 

 briefly the English name Fillet. The Fillet undergoes a further increase 

 in the medulla. It comes from the nuclei of the spinal and cranial nerves 

 of the opposite side. Bear in mind that in many vertebrates a large part of 

 the posterior root-fibers does not end in the gray horns, but ascends to 

 the medulla in the posterior columns. It enters the nucleus of the pos- 

 terior column, which replaces the column at the lower end of the medulla. 

 In birds and mammals in which the nucleus is very large one may de- 

 termine with certainty that from this nucleus a system of arcuate fibers 

 arise, which traverse the medulla to the raphe, cross to the other side, 

 and join the Tr. tecto-spinales et tecto-bulbares, which pass toward the 

 brim on either side of the median line. 



The fibers in question — Fibrm arcuatm internee medullce — are nothing 

 else than a secondary tract from the points where the posterior root-fibers 

 end. They are the last of the sensory nerves to cross to the opposite side of 

 the central system and unite with those that crossed in the spinal cord. 



The fillet, or Lemniscus, thus increased occupies a large field in the 

 ventral and lateral part of the medulla. 



So we have to deal here with tracts which, coming from the midbrain, 

 reach the termination of the sensory spinal nerves by crossing to the opposite 

 side of the medulla as arcuate fibers. Keep this in mind, for in the medulla, 

 where the nerves have such large nuclei, we shall frequently find such 

 arcuate fibers and be able to trace them into the fillet. 



In the oblongata of the adult Triton (Fig. 43), which is chosen because 

 it is closely related to the salamander larva shown above, you see the 

 arcuate fibers from the region of the nucleus of the posterior columns well 

 developed, and you recognize also, that they pass to an area on either side 

 of the middle line, which is filled with severed fibers, while many of them 

 cross to the opposite side in a ventral decussation. It is not usually possible 



