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. Geoffroy 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. llis words are as follow: " Qu'est ce que ce nerf ? La monstruosite et In nouvelle theorie sur le point de depart des developpemens organiques assure ma appuye sur ces deux lanai's, aujourd'hui heti reusement import& dans l'histoire encore si obscure des premii‘._res formations je ne doute pas que ce ne soit he nerf optique. C'est ce nerf, parceque cet appareil, qui est an complet comme globe oculaire, a du se former de toutes ses dependences jusqu'A l'emplehement qui en arrke le cours. Cette determination ne sauroit Aire douteuse pour qui reconnait clue les or canes naissent a la cirronji.renre del etre, d'ou ils envoient leurs rameaux s'embrancher au plus priS dans le centre.* Amongst those who assert that the mole pos sesses a true optic nerve is Muller. 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. Ilenle has shewn me."t 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. ( Fig. 453.) a is the fifth nerve within the cranium, b the Gasseriao ganglion, c the inferior maxillary nerve, d the ophthalmic nerve going to the eye (e,) .1 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.
The chiasma is very distinct, without a trace of a nerve proceeding from it." mal. The cartilages of the nose are elongated into a tube or trunk, which extends far beyond the osseous basis, and is supported by a very delicate moveable hone, which is represented in the figure of the cranium of the mole at fig. 441. It is furnished with a muscular appa ratus of considerable complexity (see fig. 446), consisting of no less than four pairs of muscles, which arise from above the ears, and passing forwards are inserted by separate tendons into the circumference of the extremity of the carti laginous snout. This structure is of the utmost
advantage to the animal in its subterranean search after worms and insects.
There is no order of Mainmalia in which a greater contrast is exhibited in the external covering of the body than in the different groups of the Insectivora. The porcupine and the mouse, amongst the Radentia, do not offer a more remarkable contrast its this respect than do the two families of the Lrinaceades and the Talpidee. The habits of the hedgehog de pending for its defence upon the panoply of armour which it presents to its enemies, when rolled up in a compact ball by the muscular apparatus already described, it is furnished on all the upper and lateral parts of the body with hard sharp spines or quills. Fig. 454 The organ of hearing in the mole is so con structed as to afford the greatest possible deli cacy and acuteness of this important sense, and thus to counterbalance the deficiency in that of sight. It would indeed appear that the mass of compact bone constituting the petrous por tion of the temporal, which surrounds the laby rinth in most of the Mammalia, must more or less diminish the powers of hearing, for we find it deficient in many animals, which from their habits require this sense to be in the greatest perfection. Thus, in the mole, the semicircular canals are free and visible within the cranium, without any preparation; and the parietes of the cochlea itself are almost as cellular and loose as we find in birds.
The ear of the mole possesses no concha; it is small in the shrews; and in the water-shrews (Hydrosorex) the external meatus is closed at the will of the animal by means of the anti tragus ; a provision obviously essential to its aquatic habits.
NVhen it is considered that the sense of vision is only available on the rare occasions of the appearance of the mole on the surface, and then only for very limited objects, and that all its intimations of danger, and its only guide to the opposite sex, are by means of the sense of hearing, the necessity for this extraordinary deve lopement of that sense at the expense of that of sight becomes obvious. And as, from the nature and situation of its food and its means of pro curing it, the sense of smell is equally neces sary for effecting this object, we find that the olfactory organ is also of considerable volume. The structure of the nose itself is highly curious and admirably suited to the habits of the ani is taken from a drawing by William Bell, engraved in the Ilunterian Catalogue, vol. iii. from which also the following description is borrowed :—" On the cut edge of the skin may be seen the roots and sockets of the quills, extending to different depths from the surface, according to the period of their growth : the newly formed ones are lodged deep and ter minate in a broad basis, the pulp being large and active, and the cavity containing it of cor responding size; but as the growth of the quill proceeds, the reflected integument forming the socket contracts, and gradually draws the quill nearer the surface; the pulp is at the same time progressively absorbed, and the base of the quill in consequence gradually increases in size, so that it is at last seen to be attached to the surface of the skin by a very narrow neck, below which the remains of the socket and theca are seen in the form of a small bulb." ( Fig.454, a.) "A completely formed prickle or quill cut longitudinally and magnified, skewing that it is hollow and filled with a pithy sub stance, which is transversely disposed, so as to divide the cavity into many sections." But the defence of the animal against attack is not the only object of this modification of the hair. I have more than once seen a hedgehog run to the edge of a precipitous descent, and without a moment's hesitation throw itself over, rolling itself up at the same instant, and on reaching the bottom run off perfectly uninjured. It is unnecessary to point out how perfectly this habit is provided for by the elasticity of the spines, dependent upon their structure, as ex hibited in the figure. The under parts of the body and the limbs are covered with ordinary hair. The Tenrec and other Erinaccada re semble the hedgehog in these respects, and in some species the quills and hair are inter mixed.