2-26 NERVOUS SYSTEM. 



corresponds generally with that of the corpora olivaria, the 

 optic nerves, and the eyes, but is in the inverse ratio to that 

 of the cerebral and cerebellic hemispheres. The inner 

 medullary fibres of the optic lobes pass transversely and arch 

 upwards over the contained ventricles, but the exterior fas- 

 ciculi advance longitudinally to the optic nerves. The cine- 

 ritious portion predominates in these and other parts of the 

 brain, as in the human embryo ; and the wide canal extend- 

 ing through the middle of the spinal chord of fishes corres- 

 ponds also with the fretal condition of that part in mamma- 

 lia. These optic lobes, the first formed portions of the 

 brain anterior to the medulla oblongata, being analogous to 

 the supra-cesophageal ganglia which give origin to the optic 

 nerves in the invertebrata, are almost alone developed in the 

 cyclostome fishes ; they are large compared with the cerebral 

 hemispheres in most of the osseous fishes (95. C,) they are 

 comparatively small in the anguilliform fishes (95. A,) their 

 size is much reduced in the plagiostome chondropterygii (95. 

 D. d,) and they become proportionally smaller as we ascend 

 through the higher classes to man. They contain within 

 their cavity one or two pairs of tubercles and the large cor- 

 pora candicantia lie beneath them on the inferior surface of 

 the brain. The anterior and posterior commissures are al- 

 ready developed, and also a rudimentary fornix. 



Anterior to the hollow optic lobes of fishes are the proper 

 cerebral hemispheres) (95 . A. e,) which are scarcely percep- 

 tible in the cyclostome fishes, are very small in most of the 

 osseous fishes, (95. B. e, e, C. e, e,) equal the optic lobes in 

 the apodal fishes (95. A. e,) and have attained in the plagios- 

 tome species (95. D. e, e,) a much greater size than these 

 small optic lobes, (95. D, d.) In the osseous fishes, as in 

 the embryo condition of the human hemispheres, they are 

 destitute of internal ventricles^ smooth and cineritious on the 

 surface, without external convolutions, and they are com- 

 posed internally of radiating white fasciculi derived from the 

 corpora pyramidalia. In the rays and sharks (95. D. e, e,) 

 where the hemispheres attain a great size, they already pre- 

 sejit inequalities on the surface, they begin to extend back- 

 wards over the small optic lobes, (95. D. d,) and they already 

 manifest distinct ventricles in their interior, which continue 

 in almost all the higher animals to man. The cerebral or 



