INSECTIVORA. 



1003 



cult, and require intense ardour and perseve- 

 rance to effect this object. 



The most remarkable and interesting pecu- 

 liarities which are to be met with in the struc- 

 ture of any of the Mammalia are to be found 

 in the eye of the mole, and doubtless of the 

 nearly allied forms of Chrysochloris and the 

 other subterranean species. In these, in order 

 to meet the requirements of their habits, the 

 organ of sight is reduced to a mere rudiment, 

 whilst those of hearing and of smell are deve- 

 loped to an extraordinary degree; and the 

 theory of the balance of organs can scarcely 

 boast of a stronger support in the whole range 

 of animal organization than in this instance. 



The question whether the mole possesses vi- 

 sion has been long and often debated. With- 

 out entering into an unnecessary examination 

 of all that has been said on both sides of the 

 question, it may be well to observe that the 

 principal argument which has been urged on 

 the negative is derived from the absence of an 

 optic nerve. That this animal, at least our 

 common species, does possess the faculty of 

 vision to a certain degree cannot, however, be 

 disproved, whatever may be the means by 

 which the sense is communicated, that is to 

 say, whatever the nerve may be which supplies 

 the place of the true optic nerve ; and the ex- 

 periments of Henri le Court and Geoffroi St. 

 Hilaire* would well nigh go to prove the 

 affirmative of the proposition. It has been 

 urged then that an optic nerve is absolutely 

 necessary to the existence of vision; and that, 

 therefore, either the mole has a true optic nerve, 

 or that it does not possess vision. There have 

 indeed been three classes of observers on this 

 point ; those who maintain that the mole sees, 

 and that it possesses an optic nerve ; those who 

 contend that it is blind, and possesses no such 

 nerve ; and others have ventured to agree with 

 the former as to its power of vision, and with 

 the latter in denying the existence of the optic 

 nerve. 



The eye of the mole is extremely small, but 

 differs not materially in its structure from that 

 of other animals. The pupil is elliptic and 

 vertical; the cornea even more convex than that 

 of birds; there is a sclerotic and a true choroid. 

 The crystalline lens is perfect, and much more 

 convex than in most of the Mammalia ; and 

 the eye-ball, emptied of its contents, exhibits 

 at the base a lining of a whitish colour. " In 

 an injected subject," says Geoffroi, " the cen- 

 tral artery of the retina was distinctly seen." 

 Whether there be or not a true retina will be 

 variously solved according to the views of dif- 

 ferent physiologists. If the nerve, whatever it 

 may be, which takes here the place of the 

 optic nerve, be really the seat of the sense of 

 vision, there seems to be no objection to con- 

 sider its expansion, upon which the pictures of 

 objects are impressed, as a retina ; giving the 

 name rather to the physiological than to the 

 anatomical character of the part ; but be this 

 as it may, from the posterior part of this minute 



eye there passes a distinct branch of a nerve. 

 Perhaps, after all, the name and analogy of 

 this nerve will be viewed differently according 

 to the general views of organic developement 

 entertained by different physiologists. Geoffrey 

 considers it as the optic nerve, notwithstanding 

 it has no connexion with that part of the brain 

 which in all other cases is in immediate com- 

 munication with it; and he founds this opinion 

 upon the theory that all organs are developed 

 from without. His words are as follow: 

 " Qu'est ce que ce nerf ? La monstruosite et 

 la nouvelle theorie sur le point de depart des 

 developpemens organiques assure ma marche : 

 appuye sur ces deux fanaux, aujourd'hui heu- 

 reusement importes dans 1'histoire encore si 

 obscure des premieres formations je ne doute 

 pas que ce ne soit le nerf optique. C'est ce 

 nerf, parceque cet appareil, qui est au complet 

 comme globe oculaire, a du se former de toutes 

 ses dependances jusqu'a I'empechement qui en 

 arrete le cours. Cette determination ne sauroit 

 etre douteuse pour qui reconnait que les or- 

 ganes naissent a la circonj'erence de Vetre, d'ou 

 ils envoient leurs rameaux s'embrancher au plus 

 pres dans le centre.* 



Amongst those who assert that the mole pos- 

 sesses a true optic nerve is Miiller. His state- 

 ment is so brief, and, it may be said, so unsa- 

 tisfactory, that it can scarcely be considered as 

 sufficient to outweigh the carefully formed opi- 

 nion of many who differ from him. His state- 

 ment, as given in Dr. Baly's translation of his 

 admirable book, is as follows: " Some ani- 

 mals, though provided with eyes for instance, 

 the mole and the proteus anguinus have been 

 said to want the optic nerves; the sense of 

 vision being then placed in the ophthalmic 

 branch of the fifth nerve. This statement has 

 arisen, in the case of the mole, from inaccuracy 

 in the anatomical examination ; and the same 

 is the case probably in the Proteus. The mole 

 has an uncommonly small optic nerve, as Dr. 

 Henle has shewn me."f On the other hand, 

 Serres, Gall, Desmoulins, and others have 

 agreed in delaring that there is no nerve passing ' 

 from the optic lobes to the eye ; and Dr. Todd 

 has recently verified this statement, and traced 

 the nerve which does supply this organ, which 

 he finds to be the ophthalmic branch of the 

 fifth pair. I give an enlarged view of a dissec- 

 tion of this nerve, drawn by Mr. Bowman. 

 fF^.453.) 



a is the fifth nerve within the cranium, b 

 the Gasserian ganglion, c the inferior maxillary 

 nerve, d the ophthalmic nerve going to the eye 

 (e,) ./'the superior maxillary nerve, g branch of 

 the ophthalmic, supplying the side of the nose. 

 Dr. Todd in a letter to me says, " I have 

 been lately looking at the point about the optic 

 nerve in the mole. I can see an optic commis- 

 sure, but no optic nerve beyond it, and I can 

 very distinctly trace a branch [the ophthalmic] 

 of the fifth to the eye. I carefully searched to 

 see if a second nerve were bound up in the 

 cellular sheath with it, but could find none. 



* Geoff. St. Ililaire, Cours d'Hist. Nat. des Mam. 

 lev. 16, 



* Conrs d'Hist. Nat. des Mammif. 16 Ie9. p. 27. 

 t Miiller f s Phys. by Baly, p. 767. 



