17 CENTRAL NERVOUS SYSTEM. 



without altering its essential structure, which is the same as in other rep- 

 tiles. By thus curving posteriorly it resembles the cerebellum of turtles 

 and serpents. Plates LXVII. & LXVI. In the green turtle, and other 

 marine species, also in river species, the organ forms a complete cover- 

 ing for the fourth ventricle, extending backwards over the ventricle twice 

 as far as in the alligator. 



In the frog, as shown by Plate LXVIII. its position is nearly verti- 

 cal, and so far as its form is concerned, independent of the optic lobes ; 

 while in tailed batrachians, Plate LXIX. the optic lobes seem to en- 

 croach upon its substance. 



Plates LXIV. & LXV. represent the direction in the curvature of 

 the cerebellum in all the saurians, which I have studied, with the excep- 

 tion of the alligator. See the admirable drawings of Rabl-Riickhard. 

 In heloderma, the organ curves forwards ; but it is not as closely applied 

 to the optic lobes as in other lizards. Whatever may be its shape, the 

 gray layer always lies towards the fourth ventricle, and the white layer 

 towards the optic lobes. 



Besides the nerve cells referred to on page 16 as belonging to the 

 motor root of the trigeminus, and the small and middle-sized cells which 

 are scattered though the pars commissuralis, underlying extension of 

 the medulla oblongata, there are other cells, of middle size, always to 

 be observed in sections of the cerebellum itself. These characteristic cells 

 are quite plainly shown, in some of the photographs, with most of the 

 layers given by Stiecla,* viz. i, Epithelium. 2, Nerve Fibres. 3, Gran- 

 ular substance with small nuclei. 4, Nerve Cells. 5, Superficial white 

 layer and pia mater. 



Plate LVIII. shows the cerebellum of the frog united to the pars 

 commissuralis of Reissner. 

 * Schildkrote, loc. cit., p. 58. 



