The spinal nerves (exclusive of the spinal accessory nerve, which, from the fact that it emerges from the skull, is usually ranked among the cranial nerves) are thirty-one on either side, there being a pair for each pair of intervertebral foramina (whose formation is described in the article VoilittEnan AND VERTEBRAL Comt:tx)„ and for the foramina between the atlas (the first or highest vertebra) and the occipital bone at the base of the skull. Every spinal nerve arises from the cord by two roots, an anterior and a posterior, of which the latter is distinctly the larger. Each root passes out of the spinal canal by a distinct opening in the data mater. Immediately after its emergence, a ganglion is seen on the posterior root, and in the anterior surface of this ganglion the anterior root lies imbedded. Just beyond the ganglion, but not at all previously, the nervous fibers of both roots intermingle, and a compound nerve results. The trunk thus formed sepa rates immediately after it has passed through the intervertebral canal into two divisions theanterior and posterior—each of which contains filaments from both roots, and possess ing, as will be immediately shown, perfectly different functions. These diVisions, of which the anterior is considerably the larger, proceed to the anterior and posterior parts of the body respectively, and are distributed to the skin and the muscles. The anterior branch communicates with the sympathetic nerve, as is shown in the figure. The mode of cmnection of the roots of the nerves with the cord is noticed in the article SPINAL CORD. These nerves are arranged in classes, according to the regions of the spine in which they originate, and we thus speak of eight cervical, twelve dorsal, five lumbar, and six sacral nerves on either side.
The discovery of the separate functions of the anterior and posterior roots of the spinal nerves, which has been characterized as the first important step towards a right under standing of the physiology of the nervous system, was made by our distinguished countryman sir Charles Bell, although there is reason to believe that Mayendie, without any knowledge of Bell's experiments, arrived at similar conclusions at nearly the same time. The original experiments consisted in laying open the spinal canal in rabbits, and irritating or dividing the roots of the spinal nerves. It was observed that irritation of the anterior roots caused muscular movement, and that the posterior roots might he irri tated without giving rise to any muscular action; while division of the posterior roots did not impair the voluntary power over the muscles. Hence it was inferred that the anterior roots were motor (or conveyed motive power to muscles), and the posterior roots not motor; but,i,t, was•not, fully cictowniOed:- what degree...of •sensibility,remnined in parts supplied divided foots. Niatierous physiologistS arrived at similar results to those of Bell; but the must conclusive experiments are those of 3111ller, who operated on frogs, in which, from the great width of the lower part of the spinal canal, the roots of the nerves can be exposed with great facility. In these experiments, it was found that
irritation of the anLerior root always excited muscular contraction, while no such effect followed irritation of the posterior root; that section of the anterior root caused paraly. sis (or loss of power) of motion, while section of the posterior root caused paralysis of sensation; and that when the anterior roots of the nerves going to the lower extremity were cut on one side, and the posterior roots on the other, voluntary power without sen sation remained in the latter, and sensation without voluntary motion in the former. The obvious conclusion to be derived from these experiments is, that the anterior root of each spinal nerve is motor, and the posterior sensitive. (In place of the terms sensitive and motor, the terms afferent and efferent are now frequently used. The functions of tho nerves being to establish a communication between the nervous centers and the various parts of the body, and vice an afferent nerve communicates the impressions made upon the peripheral nervous ramifications to the centers, while nerve conducts the impulses of the nervous centers to the periphery.) The cranial nerves, although twelve in number on either side, were arranged by Wil lis (Cerebri _Inatome,. oil access& ...Vcrvorum Descriptio et Usits, 1664), whose system is still generally adopted, in nine pairs, which, taken from before backwards in the order in NWhiCh they are transmitted through the foramina at the base of the skull, stand as fol lows: 1st, olfactory; 2d, optic; 3d, motores oculorum; 4th, pathetic; 5th, trifacial; 6th, abducentes; 7th, portio lava or facial, portio monis or auditory; 8th, glossopharyngeal, par vague or pneumogastric, spinal accessory; 9th, hypoglossai.
They may be subdivided into three groups, according to their functions—viz. nerves of special sense—the olfactory (see 1CosE), optic (sec EYE), and auditory (q.v.); nerves of motion, or efferent nerves—the motores oculorum, pathetic, abducentes, facial, and hypoglossal; and compound nerves—the trifacial, giossopharyngeal, piacumegastric, and spinal accessory.
The reason why no nerve of taste is included in the above arrangement amongst the nerves of special sense will be subsequently seen; and we proceed briefly to notice the functions of the motor cranial nerves.
The 3d, 4th, and 6th pairs—the motores oculorem, pathetic, and abdueentes—together make up the apparatus by which the muscles of the orbit (the four recti, the superior and inferior oblique, and the levator palpebras) are called into motion, and are sufficiently noticed in the article EYE.