766 



OPTIC NERVES. 



occur in the form of two masses ; they persist 

 as such during two-thirds of fetal existence ; 

 they are hollow at first and not covered by 

 the cerebral hemispheres, and their size is im- 

 mense in proportion to the bulk of the ence- 

 phalon. As developement advances, a trans- 

 verse groove appears on the surface of the 

 future tubercula quadrigemina ; this divides 

 them into /'our eminences, which are now for 

 the first time really entitled to be called " qua- 

 drigemina :" nervous matter is gradually depo- 

 sited from within on their walls, in conse- 

 quence of which they henceforth become solid; 

 their growth in size is arrested, and the cere- 

 bral hemispheres having grown backwards, 

 overlap and conceal them from view. This 

 overlapping occurs in all except a few of the 

 lowest families of the mammalia, in which the 

 tubercula quadrigemina remain permanently 

 uncovered. 



From the foregoing exposition it appears 

 that during their developement the tubercula 

 quadrigemina in man and mammalia assume 

 for a time all the characters which the optic 

 lobes of birds, reptiles, and fish exhibit in the 

 permanent condition ; and hence it can scarcely 

 be questioned that the nates and testes of the 

 former class are identical with the optic lobes 

 of the latter animals ; but since the optic 

 nerves in the oviparous Vertebrata are trace- 

 able to the optic lobes and manifestly derive 

 from them the greater proportion of their roots, 

 there is so far prima facie evidence that the 

 optic nerves in man have their origin in part 

 from the tubercula quadrigemina. In further 

 confirmation of the same view it may be re- 

 marked that some of the roots of the optic 

 nerves in certain orders of the mammalia are 

 seen to spring from these bodies ; for example, 

 in the horse a large proportion of the nerves 

 can be traced distinctly to the nates. In Ro- 

 dentia and Carnivora numbers of the fibres of 

 the nerves emanate obviously from the same 

 pair of tubercles, and in the Ruminants a simi- 

 lar anatomical arrangement prevails. 



As an additional proof Tiedemann asserts 

 that although much difficulty is encountered in 

 attempts to follow the optic nerves to the tuber- 

 cula quadrigemina in the adult human subject, 

 he has succeeded in tracing them to the nates 

 in foetuses of the third month, and at the fourth 

 and fifth months he has frequently repeated 

 the same observation. 



Human pathology would seem to furnish 

 some corroborative facts : thus in every case of 

 long-continued atrophy of the optic nerve, 

 where the wasting had involved the tractus 

 opticus, Gall and Spurzheim found the nates 

 of the side corresponding to the diseased tract 

 diminished in size; and the experiments insti- 

 tuted upon living animals with a view to deter- 

 mine the functions of the several constituents 

 of the brain by the successive removal of the 

 different parts of the organ and careful obser- 

 vation of the disturbance thereby produced, 

 lead also to the belief that the optic nerves have 

 an origin in the tubercula quadragemitia. Of 

 course great allowance must be made for inac- 

 curacy in the result of such mutilations, but 



Flourens, Magendie, Desmoulins,and Hertwig, 

 all agree that destruction or mutilation of the 

 nates and testis of one side invariably produces 

 blindness of the opposite eye. 



The writer agrees fully with Cruveilhier in 

 the belief that the optic nerves in the human 

 subject can be rarely traced to the tuber- 

 cula quadrigemina satisfactorily ; but never- 

 theless with the above facts before them, 

 anatomists can scarcely refuse to allow that 

 the optic nerves in man derive a share of 

 their roots from these eminences. 

 The tubercula quadrigemina probably fulfil 



other purposes besides that of affording origin 



to the optic nerves. 



This may be inferred from the fact that the 

 optic nerves are not invariably developed in 

 direct proportion to the tubercles ; thus in 

 certain mammals which are either devoid of 

 optic nerves altogether, or in which they 

 are so excessively diminutive as to be with 

 difficulty discovered, the tubercula quadri- 

 gemina are as large and perfect as in other 

 allied species possessed of well-marked organs 

 of vision. 



Fig. 414. 



Brain of a mole. (From na- 

 ture.) Seen from above. 

 The upper part of each 

 cerebral hemisphere has 

 been removed by a hori- 

 zontal section. 

 a, a, olfactory lobes ; 

 b, b, cut surface of cerebral 

 hemispheres ; c, c, nates ; 

 d, d, testes ; e, cerebellum. 

 The nates and testes, or tu- 

 bercula quadrigemina, are 

 of immense size ; the cere- 

 bral hemispheres are small. 



The common mole, for example, has eyes so 

 diminutive and imperfect in structure, and its 

 subterranean habits bespeak so little necessity 

 for organs of vision, that many excellent anato- 

 mists believe it to have no optic nerves; never- 

 theless the tubercula quadrigemina in this 

 animal are of immense size. (Fig. 414 ) Serres 

 never could satisfy himself that the mole pos- 

 sesses optic nerves, although he examined thirty 

 or forty specimens for the express purpose, and 

 if they do exist (as has been maintained by 

 Carus and Treviranus) their minuteness must 

 be almost microscopic. (See INSECTIVORA, 

 vol.ii.7zg. 453.) 



Other examples confirmatory of the same 

 views are afforded by the mammalia; it is 

 stated on the authority of Serres that in the 

 rat-mole of the Cape, and the Zanni, or blind 

 rat-mole, there is no appearance whatever of 

 proper optic nerves, (the rudimental eyes being 

 supplied by the fifth pair,) and yet in these 

 animals the tubercula quadrigemina exist in 

 great perfection. 



The human optic nerve probabh/ derives roots 



from the optic thalamus. 

 The writer is of opinion that modern anato- 

 mists have fallen into error in supposing that 

 none of the roots of the second pair are derived 

 from the optic thalamus, although the argu- 

 ments by which that supposition has been 

 sustained are sufficiently imposing, viz. : 



