LESSON 84.] NERVOUS SYSTEM IN EEPTILES. 271 



cerebellum (6), are the optic lobes, which are hollow internally, as in 

 the human embryo, and give origin to the principal fibres of the 

 optic nerves. 



1217. The second or middle pair of lobes (e) are the cerebral 

 hemispheres, which here, as in the human embryo, are destitute of 

 internal ventricles, and without external convolutions. 



1218. The anterior pair (f) are the olfactory tubercles, which 

 are entirely appropriated to the olfactory nerves (g, g). 



1219. In the Trigla lyra, where the medulla oblongata (Fig. 384, 

 b, b] is marked by ganglionic enlargements, and the cerebellum (c, d) is 

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

 cerebral hemispheres (/), and the olfactory tubercles (g) are much 

 inferior in size. 



1220. In most fishes, as in the earliest condition of the human 

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

 smooth and gray on the outer surface, and destitute of the transverse 

 sulcus (a furrow), which gives them a four-lobed (quadrigeminous) 

 appearance in the adult mammalia ; they are hollow within, and 

 have their inner walls lined with white medullary fibres. The ven- 

 tricles of the optic lobes communicate freely with each other, and 

 they open behind by a narrow aqueduct, into the fourth ventricle 

 beneath the cerebellum. 



1221. The interior white medullary walls of these two lateral 

 cavities meet above on the median line, and form an extended com- 

 missure ; they descend along the median line to form a prominent 

 ridge, but not a complete septum, between the ventricles. 



1222. Finally, the optic lobes of fishes, like their medulla oblon- 

 gata, are larger in proportion to the cerebral hemispheres than in any 

 of the higher vertebrata, and they present the same great propor- 

 tions the earlier we observe them in the human embryo. 



LESSON LXXXIY. 



NERVOUS SYSTEM IN REPTILES. 



1223. In the Amphibia, and in the larva state of those which 

 lose the gills, the spinal chord, the medulla oblongata, and the cere- 

 bral parts contained within the cranium, present the same proportions 

 and general conditions which we observe as permanent characters in 



