2o8 Journal of Comparative Neurology and Psychology. 



munis components in the palatine branch of the seventh nerve, 

 and there is accordingly no communis component in the 

 trigeminus and no viscero-sensory center in this segment. In 

 the exceptional case of Amia (3) a large number of communis 

 fibers enter the trigeminus from the facialis root, to be dis- 

 tributed to end buds on the surface of the head. Even here the 

 communis fibers do not belong to the trigeminus segment. 



The central nervous structures related to the nerves of this 

 segment include the cerebellum and the nuclei of the spinal V 

 tract, the motor nucleus of N. V which lies a short distance in 

 front of the nucleus of N. VII in the oblongata, and the 

 nucleus of N. IV which lies in the base of the mid-brain. From 

 the adult relations of centers and nerve roots it would be diffi- 

 cult to decide with what neuromere they are associated. This 

 is especially noticeable when attention is called to the fact that 

 the root of N. V is connected with the cerebellar segment in 

 some forms, while in others it is distinctly further caudad. If we 

 look to embryology to determine its primitive location we find 

 the same difference in different classes of vertebrates. Con- 

 sidering the cerebellar segment as Locv's neuromere vi, the 

 trigeminus root is found to have the following location. In both 

 the embryo (76) and the adult (68) of Petromyzon it is connected 

 with neuromere vi. In selachians (124, 106, 58. 96, Fig. 6, 97) it 

 is connected with neuromere vi, in the embryo and shifts to neuro- 

 mere vii. The actual shifting has been seen and clearly described 

 by ZiMMERMANN. It appears also in Neal's (97) figures 1 5 to 2 1 , 

 In teleosts(52) it has the same relations. In a Salmo embryo of 

 23 days it is connected with the cerebellar neuromere; at 3 i days it 

 has shifted to the neuromere following. In amphibia (118) it is 

 connected with neuromere vi in the embryo. In the lizard (56, 

 99, 87), chick (52) and pig (92) the trigeminus is connected with 

 neuromere vii in the embryo. Giglio-Tos (40) describes a 

 shifting of the ophthalmic and trigeminal roots in the human 

 embryo from the mesencephalon to the hind-brain. From this 

 series it must be thought that there has been a shifting back- 

 ward of the trigeminus during the phylogenetic history of 

 vertebrates. The reason for this shifting is probably to be found 



