FIFTH PAIR OF NERVES.—This title is derived from the relation which the nerve bears numerically to the other encephalic pairs; it is the fifth nerve met with on the base of the brain counting from before backwards. The fifth is also called the trigentinal (Winslow) and the trifacial (Chaussier) nerve. It is the nerve upon which the general and tactile sensi bility of the face and its cavities, as well as the voluntary power of certain muscles of these parts, depends.
The following account of this nerve is meant to apply especially to the human subject; but as a knowledge of its structure and distribution in other animals must contribute very much to enlighten us in regard to its true character and properties in man, occasion has been taken to mention those particulars by which it is dis tinguished throughout the animal series.
The fifth nerve is connected at its one ex tremity with the medulla oblongata, whilst its other end is distributed to the eye and its appendages, to the nostrils, to the palate, the mouth and tongue, to the salivary glands, to the ear, to the integuments and muscles of the face, forehead, and temple, and to the muscles which move the lower jaw in mas tication, the temporal, pterygoid, and mas seter muscles. The general distribution of the nerve throughout the animal series corres ponds to that in man ; but, in certain animals and classes, varieties are presented, which claim our attention equally, whether as matters of curiosity or of physiological interest. In some individuals of the class Mammalia, the eyes possess a very inferior degree of develop ment; a distinct optic nerve either does not exist or its existence is a matter of doubt, and its place is supplied, in part or alto gether, by a branch of the second division of the fifth nerve: thus, in the Mole, accord ing to AI. Serrcs,* the optic is altogether absent, and its place is supplied by a branch of the fifth ; but, according to Treviranus,f that animal is provided with an optic nerve, as large as a human hair, and according to Carus/ it joins an optic branch from the fifth, and the two concur to form the retina. In other animals of the same class the optic seems decidedly absent, and its place is supplied al together by the fifth. Among Reptiles also in stances occur, in which the optic nerve is wanting. According to both Treviranus* and Serres,t the fifth nerve takes the place of the optic in the Proteus Anguinus. A va riety in distribution, still more remarkable, is presented in the disposition of the fifth nerve in Fishes. Among the Rays the audi tory appears to be, not a distinct nerve, but a branch of the fifth :/ the special organs, with which they are provided, likewise, in many instances, derive their nerves from the fifth pair ; thus, in some the electrical § organs are supplied by that nerve, and also the albu mino-gelatinous organs : lastly, in many the nerve is distributed in a manner and to an extent for which there is no analogy among other animals, the fins being throughout fur nished with branches from the fifth. hence
in Fish, in which the distribution of the nerve is so much more extended than in other animals, both the size of it is propor tionally greater, and it consists of a greaterc number of divisions; these, which in the three other classes of vertebrate animals are only three, amounting with them to from three to six. See sketch of nerves in the Ray and Cod. (Figs. 144, 145.) The size of the fifth nerve is very great, it being by far the largest of those proceeding from the medulla oblongata. In this respect it pre sents much variety according to the animal or its class. M. Serres states that, the nerves being proportioned always to the volume of the organs from whence they proceed, the extent of the face and of the organs of the senses taken together gives the size of this nerve in the different classes of vertebrate animals. Among the Mammalia the extent of the face and of the organs of the senses increases pro gressively from Man to Apes, the Carnivora, the Ruminantia, and the Rodentia, and, ac cording to him, the size of the fifth nerves follows in a general manner the same pro-. gression. Birds are remarkable for the atrophy of the muscles of the face and of several of the organs of the senses, and their fifth nerve is far from presenting the developement to be observed in the inferior Marnmalia. Reptiles are still lower than Birds with regard to the dimensions of the nerves of the fifth pair ; while in Fish*• the size of the nerve is very great, and even surpasses in some the volume it presents in the other classes.ff However just the estimate of the comparative volume of the nerve in different animals, as here stated, may he, the data, from which it is professedly drawn, may be reasonably objected to. In the first place the volume of the organ cannot be assumed as being alone the measure of that of the nerve supplying it the degree of ner vous endowment, whether general or special, which the organ enjoys, must be also taken into account ; and in the second, the extent of the organs of the senses cannot be admitted as a measure of the volume of the fifth nerve, which is not connected with them all; thus the greaterpart of the organs of touch is inde pendent of that nerve. It appears to me that the extent of distribution and amount of endowment conjointly determine the volume of the nerve, and that the latter cannot be inferred a priori.