15 CENTRAL NERVOUS SYSTEM. 



group of cells, either in reptiles or batrachians. Plate LI. seems to in- 

 dicate the larger cells as the vagus centre, although some of the fibres 

 of the hypoglossus can be followed as far as the same group. 



The cells of origin of the abducens nerve, have been demonstra- 

 ted, in all the reptiles referred to in this work. Plate LVII. shows 

 their position with reference to the floor of the ventricle and to the fibres 

 of the nerve. 



An upper and a lower group of cells, both of which appear in sec- 

 tions, to be related to the acoustic nerve, can be clearly distinguished in 

 most cases ; the upper group consisting of small, and the lower group 

 of large cells. I have very seldom found both divisions in chelonia, 

 while the group of large cells is readily shown, in all the classes, except 

 that of batrachians. The large-cell group is especially conspicuous in 

 ophidians ; in the skinks and iguanidae among saurians, and in all turtles. 

 In the alligator, this group is not numerous and the cells are relatively 

 smaller. 



The small-cell group is well represented, as seen in cross section, 

 by Plates XLIX. & CIV. and in longitudinal section, by Plate LIII. 

 These elements of the "eminentia acustica" or "tuber nervi acustici,"* 

 by their position and great number, are peculiar to the alligator. 



Deitersf in support of, and perhaps biased by his theory about 

 nerve cells, states : "So far as the large nerve cells at the origin of the acus- 

 ticus are concerned, they have nothing to do with this nerve, but belong 

 to the crura cerebelli ad medullam oblongatam, which is partly surround- 

 ed and partly traversed by the acusticus." In view of this positive as- 



* Names given to a prominence developed on the lateral borders of the fourth 

 ventricle, at the exit of the acoustic nerves. Rabl-Riickhard, loc. cit. p. 349. 



t Untersuchungen iiber Gehirn u. Riickenmarks des Menschen u. Saugethiere. 

 1865 p. 85. 



