764 



OPTIC NERVES. 



views were still more powerfully advocated by 

 Gall and Spurzheim, although they admitted 

 that the nerves derive a reinforcement from the 

 corpora geniculata externa and the tuber 

 cinereum. 



Tiedemann (although fully aware that some 

 filaments of the optic nerves are traceable to 

 the surface of the optic thalamus both in the 

 fetus and adult) yet believed the nates and 

 corpora geniculata externa to be the true origins 

 of the nerves under consideration, and in this 

 opinion he was strengthened by the Report on 

 the Memoir of Gall and Spurzheim, made to 

 the Institute by Cuvier, Portal, Sabatier, and 

 Pmel. 



According to Serres the tubercula quadri- 

 gemina are the proper sources of the optic 

 nerves, and by Leuret the second pair are traced 

 to a triple cerebral attachment, viz. the nates, 

 testes, and optic thalamus. 



It is proposed to examine in this place some 

 of the grounds on which the foregoing opinions 

 have been founded, and to this inquiry the aid 

 of comparative anatomy is indispensably re- 

 quisite. 



Fi SH- In these animals the optic nerves 

 are distinctly traceable to two of the ganglia 

 which compose the diminutive brain. The 

 ganglia in question are called " optic lobes," 

 from being the principal sources of the nerves 

 of vision ; they are hollow, and their position 

 in the brain is between the cerebral hemispheres 

 and the cerebellum (Jig. 407 .) The optic lobes 



Fig. 407. 



Fig. 409. 



Brain of a Hake, ( From nature.) Side view seen from 

 below. 



a a, optic nerves ; b, oblique crossing of ditto ; 

 c, ootic lobe of left side, being the. chief source of 

 the right optic nerve ; d d, two inferior lobes from 

 wliicn the nerves of vision ia fishes generally 

 derive roots. 



in fish very generally bear proportion to the size 

 of the optic nerves (a proof of their physiological 

 relations); and this proportion becomes par- 

 ticularly apparent in fish which possess either 

 unusually small organs of vision, as the Eel ; or 

 eyes of different dimensions, as the Pleuronectes. 



408. 



Brain of an Eel. (After 

 Solly.) Seen from above. 



a, optic nerve ; b b, op- 

 tic lobes, which are small, 

 being proportional to the 

 size of the optic nerves. 



In many kinds of fish the optic nerves de- 

 rive some of their filaments from a pair of 



Brain of a Halibut. (From nature.) 



A, seen from above. B, seen from below. 



c, large optic nerve in both ; (/, small optic nerve 

 in both ; e, large optic lobe in both ; f, small optic 

 lobe in both; qq, inferior lobes bearing the same 

 proportion to each other in size that the optic 

 lobes exhibit. 



N. B. The large optic nerves derive their roots 

 from the large lobes, and the small optic nerves have 

 their origin in the small lobes. 



Fig. 410. 



tubercles placed on the under surface of the 

 encephalon beneath the optic lobes (fig. 410). 

 The writer does not pre- 

 sume to decide whether 

 these tubercles are really 

 identical with the mam- 

 millary eminences of the 

 human brain as main- 

 tained by Desmoulins 

 and others; or with 

 the tuber cinereum, as 

 Carus, Spurzheim, &c. 

 have contended : but 

 that they have a share 

 in the origin of the optic 

 nerves is certain, since 

 in those fish which have 

 two optic nerves of un- 

 equal size, the tubercles 

 to which allusion is made 

 Brain of a Ray. ( From present corresponding 

 nature.) Seen from below, differences in dimen- 



aa, optic nerves; b, s j ons ffo, 4Q9, g ). 

 chiasina;cc, interior ,, ^ <? ,- 



v f \ i i ivt r 1 1 JjJjO* - ill till- 



lobes trom which the op- 

 tic nerves derive some of dass the optic nerves 



their roots ;eW,optic lobes are derived from optic 

 the principal sources of lobes very similar to 

 the optic nerves. those in fisn ; tne y are 



two in number and interposed between the 

 cerebral hemispheres and the cerebellum : their 

 size is proportional to the developement of the 

 optic nerves, and they are best seen at the upper 

 or dorsal surface of the brain (fig. 411, c). 



BIRDS. In birds the optic nerves originate 

 chieny in two lobes situated at the inferior and 

 lateral aspect of the brain, and called in this 

 class also " optic lobes." The size of these 

 lobes is in proportion to that of the optic nerves 

 and organs of vision, and they are therefore 



