Johnston, Morphology of the Head. 209 



in the place of ending of the cutaneous fibers within the brain, 

 although shifting of other structures (to be spoken of below) 

 may have aided or at least offered favorable conditions. In 

 Petromyzon (68) a part of these fibers go directly to the cere- 

 bellum, others bifurcate and send one branch to the cerebellum 

 and the other backward, and the remainder turn directly backward 

 to end in the acusticum or the nucleus funiculi. In selachians (63, 

 28) and in ganoids [f>'j) the destination of the fibers is practi- 

 cally the same. In amphibia (123) a part of the trigeminus 

 fibers enter the cerebellum. In higher vertebrates few if any 

 sensory V fibers go to the cerebellum; they run regularly to 

 the nuclei funiculi. Here, then, is a shifting of the place of 

 central ending of these fibers corresponding to the shifting of 

 the root. The writer would interpret these facts as follows. 

 The general cutaneous fibers of the trigeminus, as of the spinal 

 nerves, typically bifurcated on entering the brain; in primitive 

 vertebrates the ascending fibers ended in the cerebellum or far- 

 ther forward, the descending fibers passed backward varying 

 distances. The reduction of general cutaneous area in the region 

 of the IX and X nerves due the disappearance of postauditory 

 myotomes, and the developement of the lateral line system re- 

 sulted in the general cutaneous centers in the cerebellum and 

 oblongata coming gradually into the service of the special 

 cutaneous sense organs (neuromasts and ear). Concurrent with 

 this was the specialization of the cerebellum as a center of co- 

 ordination. These processes account for the gradual shifting of 

 the general cutaneous fibers of the trigeminus which we observe 

 in vertebrates, until they are all collected in the spinal V tract 

 leading to the nuclei funiculi at the caudal end of the oblongata. 

 Along with these processes, then, and probably as a result of 

 them, we find the root of the nerve shifting back one segment 

 nearer the place of ending of the fibers. The shifting of the 

 whole root of the profundus from the mesencephalon to join the 

 trigeminus is a more conspicuous example of the same process 

 carried out earlier. Both of these shiftings are repeated in the 

 ontogeny in lower vertebrates, as we have seen. On the ground 

 of the source of the V ganglion and the position of its root in 



