NERVOUS SYSTEM. 225 



(Fig. 95. A,) where these three pairs of lobes are nearly 

 equally developed and similar in form. The posterior pair, 

 (95. A. c,) immediately before the cerebellum (95. A. b,) are 

 the optic lobes or corpora quadrigemina, which are hollow in- 

 ternally, as in the human foetus, and give origin to the prin- 

 cipal fibres of the optic nerves. The second or middle pair 

 of lobes (95. A. e,) are the cerebral hemispheres, which are 

 here, as in the human embryo, destitute of internal ventricles 

 and without external convolutions. The anterior pair (95. 

 A. /,) are the olfactory tubercles, which are entirely appro- 

 priated to the olfactory nerves (95. A. g, a.) In the trigla 

 lyra, (Fig. 95. C,) where the medulla oblongata (b, b,) is 

 marked by ganglionic enlargements, and the cerebellum (d,) 

 is proportionally small, the optic lobes (e, e,) are much larger 

 than the cerebral hemispheres (/,) and the olfactory tuber- 

 cles (g,} are much inferior in size. In the perch, (Fig. 95. 

 B,) the medulla oblongata (a,) forms two broad lobes at its 

 anterior termination (b } ) over which the elevated cerebellum 

 (e,) arches backwards. The optic lobes (95. B. d, d,) have 

 an elongated form, the cerebral hemispheres (e, e,) much 

 smaller than the optic lobes, are extended vertically, and the 

 olfactory tubercles (/,) form two slight spherical enlarge- 

 ments at the commencement of the olfactory nerves (g, g.) 



In most fishes, as in the earliest condition of the human 

 brain, the optic lobes are larger than the hemispheres 5 they 

 are smooth and cineritious on the outer surface, and destitute 

 of the transverse sulcus which gives them a quadrigeminous 

 appearance in the adult mammalia ; they are hollow within 

 and have their inner parietes lined with white medullary 

 fibres. The ventricles of the optic lobes communicate freely 

 with each other, and they open behind, by a narrow aquiduct, 

 into the fourth ventricle beneath the cerebellum. The interior 

 white medully parietes of these two lateral cavities meet above 

 on the median line, and form an extended commissure like 

 the corpus callosum of the hemispheres ; they descend along 

 the median line to form a prominent ridge, but not a complete 

 septum, between the ventricles. The optic lobes of fishes, 

 like their medulla oblongata, are larger in proportion to the 

 cerebral hemispheres than in any of the higher vertebrata, 

 and they present the same great proportions the earlier we 

 observe them in the human embryo. Their development 



PART in. Q 



