On the other hand, Mayo has found that the fifth nerve may be divided within the cranium in the cat and pigeon, and vision continue unaffected ; which circumstance shows that the apparent loss of vision in the rabbit was owing to the great contraction of the pupil, while according to Magendie's statement there does not remain any trace whatever of sensi bility to the impression of light in the eye after the section of the optic nerve. We must, then, conclude that the optic nerve is the proper medium of perception to visual im pressions, and that the co-operation of the fifth nerve is not even necessary to enable the optic nerve to fulfil its function. As the instrument of the general sensibility of the structures of the eye, however, the fifth nerve may be the channel through which impressions not visual, though perhaps excited by an agent of vision, viz. light, may be conveyed.
The conclusion thus drawn from experimental physiology is fully confirmed in man by the history of those cases in which the influence of the fifth nerve has been lost from disease : of these two have been adduced by Bell in the Philosophical Transactions for 1823, one from the observation of Mr. Crampton, the other from that of Dr. Macmichael, in which the surface of the eye was totally insensible, whilst vision was entire ; and another, still more remarkable, has been reported by Mr. Bishop,* in which the functions of the fifth nerve seemed altogether obliterated by the pressure of a diseased growth within the cranium, and yet the patient saw distinctly to the last, the only derangement which oc curred in the function of vision being the loss of the power of distinguishing colours, which appears sufficiently accounted for by a certain degree of pressure exerted by the tumour upon the optic nerve. Magendie endeavours to support his views upon this and other points connected with the properties of the nerve reference to a case reported by Serres, which appears very inadequate, and will be discussed by-and-bye.
Influence of the fifth nerve on hearing.— The great affinity between the sense of hearing and that of touch renders it more easy to conceive how hearing might be excited through the medium of the fifth nerve. As we have seen that the ocular nerve in certain animals is a branch of the fifth nerve, so is the auditory. Among the cartilaginous fishes there are several instances in which this occurs. The origin of the auditory nerve from the fifth in fishes was first announced by Scarpa,t and by him sup posed to apply to fish generally. This view
is combated by Treviranus:: it is admitted in part by Serres; lie states that in osseous fishes the auditory nerve is united at its in sertion with the fifth; in cartilaginous fishes, that the auditory is sometimes confounded with the fifth, sometimes separated distinctly enough, as in the raia clavata. From his own observations the writer would say, that in the bony fishes the two nerves cannot be said to be united or to arise the one from the other, but only to have a common superficial attach ment to the medulla oblongata ; and from the analogy of the same nerves in the higher classes of animals, he would not admit, without further proof, a common superficial attachment as establishing identity of ultimate connection with the encephalon. As to the cartilaginous fishes, it appears to him that Serres has fallen into an error with regard to the connection of the auditory nerve. It appears to the writer that the fifth and the auditory are con founded in the raia clavata as plainly as in any other individual of the class; the posterior ganglionic fasciculus of the fifth and the auditory nerve form one trunk for a distance of some lines after leaving the medulla ob longata; they are at all events enclosed within the same sheath :§ but whether they arc to be regarded as branches of a common trunk or not, it is difficult to decide. The weight of naalogy is certainly opposed to a conclusion in the affirmative; and, though this were ad mitted, a difference between the auditory and the other branches of the fifth (as supposed) must still be admitted, inasmuch as the auditory separates from the nerve before the occurrence of the ganglion, and has not itself a ganglion. On the other hand the auditory may be se parated from the rest of the nerve, after the division of the common investing membrane, with little or no laceration of fibres. Still it may be asked why, if they be distinct nerves, are they united into one trunk ? The opinion that the fifth nerve holds an important in fluence over the sense of hearing derives support from the circumstance, that in most, if not all, the cases of disease of the nerve, the sense of hearing becomes impaired, though not obliterated.
The last question proposed to be considered with reference to the functions of the fifth nerve is its connection with nutrition.