1 Fig. 4.28 represents the encephalon, optic nerve, and organ of vision in a kitten at the full , period of gestation, (the subject of the same 1 form of monstrosity,) which lately came into the writer's possession : the preparation is pre served in the Museum of the Richmond llos I pital School, Dublin. In all essential particu ' lars this specimen bears the closest resemblance to the human monsters of which the dissection lhas just been described.
The fundamental defect in these monstrous fcetuses consists in the total absence of the organ of smell, in consequence of which de ficiency the symmetrical organs at either side become united in the middle line and actually engrafted upon each other : the two eyes are conjoined so as to form but a single organ of vision, and the very same metamorphosis occurs in the two orbits, the two optic foramina, the two optic nerves, &c.
That this is the rationale can scarcely admit of doubt, since in some parts of the organs the fusion remains incomplete ; thus two crys talline lenses still exist in the interior of the solitary eye-ball : a. double set of muscles with their corresponding nerves are provided for the globe of the eye ; andfinir eye-lids protect the organ in front, causing, the aperture of the lids to assume a quadrangular form.
General development of the optic nerves in the higher classes of animals.
Fish.—In fish as a general rule these nerves are highly developed, and exhibit a marked preponderance in size when contrasted with the corresponding nerves in many animals holding a more exalted position upon the scale. This rnay be explained by the nature of the medium which the fish inhabits; for some of the light incident on the surface of the water is reflected, and another part, after penetrating the water, becomes absorbed, in consequence of the continual disturbance to which the transparency of this fluid is subject ; so that fishes necessa rily require a greater developement of visual apparatus,than would suffice land animals for an equal amount of vision.
Birds.—In birds the sense of sight exists in' great perfectiod; and the optic nerves exhibit corresponding developement.
Maramalia the faculty of vision ceases to preponderate, and accordingly the proportions of the optic nerves in this class are no longer excessive.
Many facts in comparative anatomy war rant the conclusion that the senses of smell and vision are at times supplemental to each other ; for example, the mole either possesses no optic nerve, or if any such exist it is so diminutive as to be most difficult of recogni tion, but the olfactory lobes of the brain and the whole olftctory apparatus of the animal exist in great perfection, and its subterranean habits enable it to turn this latter function to 'account, whilst a highly finished organ of vision would have been an useless appendage. In certain fish which frequent the mud or slimy waters (as for instance the eels), the visual apparatus is poorly developed, and the optic nerves are particularly small ; but tbe olfactory nerves preponderate, and there can be little doubt that the superiority of the sense of smell in them serves in great measure to supersede the necessity for highly•wrought organs of vision, and in probably the majority of the Mammalia the olfactory nerves preponderate in size over the optic, but the corresponding faculty by its acuteness makes amends for any inferiority in vision. Thus the keen scent of many Carni vora renders the eye of secondary importance in the pursuit of their prey, and the vegetable feeders are much indebted to the perfection of their sense of smell for the discrimination they evince in their choice of nutriment.