NERVOUS SYSTEM. 227 



lateral ventricles are continuous with the canals of the olfac- 

 tory nerves (95. D./,) as in all the higher classes. These 

 cerebral lobes, perhaps the analogues of the thalamic optici, 

 are developed in the direct ratio of the corpora pyramidalia 

 and crura cerebri, and their increased development in higher 

 animals corresponds also with the enlargement of the lateral 

 lobes or hemispheres of the cerebellum. 



Before the cerebral hemispheres are placed the most an- 

 terior pair of lobes, which here, as in higher classes, are ap- 

 propriated to the olfactory nerves, and vary in their form, 

 size, and situation more than any of the other parts of the 

 brain. These olfactory tubercles are generally in the osseous 

 fishes, (95. B./, C. p } ) in immediate contact with the cere- 

 bral hemispheres, and inferior to them in size ; in the an- 

 guilliform fishes (95. A. /,) they nearly equal the hemi- 

 spheres (95. A. e } ) and in the plagiostome fishes (95. D. g,g,) 

 they are placed on the course of the olfactory nerves at a 

 greater or less distance from the hemispheres (95. D. e, e,) 

 and present a great transverse development, exceeding in 

 magnitude the hemispheres themselves. In these last fishes 

 the rays and sharks, the large cineritious olfactory lobes are 

 situated at the end of thick peduncles and lie immediately 

 above the cribriform plate of the ethmoid which the olfac- 

 tory nerves perforate to be distributed on the extensive 

 pituitary membrane covering the laminae of the nose. 



The cerebellum forms only a minute transverse band on 

 the median plain in the cyclostome fishes, where it can be 

 perceived, and in the higher osseous fishes (95. B. c,) it still 

 consists merely of a simple median lobe, smooth on the sur- 

 face, destitute of lateral hemispheres, and analogous to the 

 vermiform median lobe first developed in the cerebellum of 

 the human embryo. It rises vertically in the osseous fishes 

 (Fig. 96. ,) compressed between the optic lobes (96, d,) and 

 the lobes of the medulla oblongata (96. b,) and generally ex- 

 tends backwards, tongue-shaped, over the fourth ventricle, but 

 is destitute of the laminated surface which it begins to present 

 in the plagiostome fishes. This median portion of the cere- 

 bellum, (95. D. e,) like the cerebral hemispheres, (e, e,) is 

 greatly developed in the muscular rays and sharks, extending 

 backwards over the medulla oblongata and forwards over the 

 small optic lobes (95. D. d,) and already presents not only a 



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