In some of the nervous centres, however, no visible change of any kind takes place upon the irritation of the nervous matter, nor does the ani mal seem to suffer pain. Such is the case when the hemispheres of the brain are the subject of experiment. We are not to infer from this that the nervous force is not developed in these centres, but that they have no direct connexion with the muscular system, nor have they that peculiar organization which would enable them when irritated to excite painful sensations.
There are certain nerves which when stimu lated excite neither muscular motion nor coin mon sensation or pain, but a sensation peculiat to themselves. Thus if the optic nerve be sti mulated by a mechanical or galvanic stimulus, a sensation of light is produced ; if the auditory nerve be stimulated in like manner, a sensation of sound is produced.
These facts prove not only that a peculiar force is generated by the nervous matter, but they also show that the nerve fibres in the cen tres, as well as in the nerves, possess special endowments depending,in all probability, upon their central as well as upon their peripheral connexions. Thus nem-fibres connected with muscles are capable of exciting muscular con traction, and are therefore called motor or mus cular nerves. Nerve-fibres, which are distri buted to a sentient surface, as the skin or mu cous membmne and have a certain relation with that part of the nervous centre which con stitutes the centre of sensation, (vide p. 711,) are when stimulated capable of exciting a feel ing which may be agreeable or painful, accord ing to the degree of stimulation. These are called sensitive nerves, or nerves of common sensation. To the class of sensitive nerves belong those which, owing no doubt to a peCu liarity in their connexion with the centre, as well as to their relation to a special apparatus at their periphery, develope peculiar sensations, as the nerves of sight, hearing, taste, &c., and they have been distinguished as nerves of special sensation.
Very many sentient nerves are implanted in the nervous centre near to certain motor nerves, so that a stimulus applied to the former is capable of reacting upon the latter, and of ex citing motion through their connexion with the muscles. Dr. M. Hall, however, ingeniously supposes that this power resides only in a par ticular class of nerve-fibres (and not in the ordinary sentient nerves through their closeness ' of relation with the ordinary motor nerves). A
nerve of this kind would constitute an arc, consisting of an incident and a reflex portion, which are united at the nervous centre. The stimulus is conveyed to the centre by the inci dent portion, and is then reflected into the reflex or rnotor portion. Such nerves, Dr. Hall designates excito-motor. We shall examine further on the grounds of this hypothesis.
It is an important fact, which Sir C. Bell was the first clearly to prove that nerve-fibres of different endowments may be bound together in one sheath, forming, in anatomical language, one nerve. Thus a nerve may contain sentient and motor fibres as the median nerve in the arm, or if we admitted Dr. Ilall's hypothesis, it might contain sentient, motor, and excito motor fibres. And most nerves in the diffe rent regions of the body are of this description, e. compound nerves, made up of sentient and motor fibres bound together in the same sheath, in very different proportions. In many of these nerves, as in the spinal nerves, and the fifth pair, the separation of the fibres of motion from those of sensation exists at the implantation in the centre, and there the fibres of each endow7 ment are collected into a separate bundle, which possesses the endowment proper to its constituent fibres. These are the roots of these nerves, of which one has been satisfactorily proved to be sentient, the other motor, the former being genemlly the larger, and having the peculiar feature of a ganglion being formed upon it.
Thete is scarcely a nerve in the body, which, in strictness, ought not to be regarded as a com pound one ; the physiological character of each nerve must depend on the endowment of the ma jority of its fibres, and the nerve will be called sensitive or motor, according to the predomi nance of motor or sensitive fibres in it. For example, the facial nerve, or portio dura of the seventh pair, is called motor, because it is almost wholly cornposed of motor fibres ; but it contains, besides, in very much smaller nutnber, some sensitive filaments which it derives from anastomoses with neighbouring nerves. The third, fourth, and sixth nerves are of similar constitution to the facial. In the ramifications of the fifth nerve, on the other hand, the filaments of sensation are predomi nant; those of motion being much fewer, and confined to the ramifications of its inferior maxillary division.