CHAPTER III.

THE FUNCTIONS OF THE NERVES.

INASMUCH as I have already divided the nervous system into three principal portions, namely, the animal organs, sensorium commune, and the nerves properly so called, I shall also divide its functions into three classes, namely, into animal operations, operations of the sensorium, and operations of the nerves. The functions of the nerves are first to be considered; then the operations of the common sensory, and lastly, the animal operations.

SECTION 1.-ON THE ACTION OF THE NERVES IN PRODUCING SENSATION AND MOTION.

Since the nerves represent cords commencing in the cerebrum, medulla oblongata, and medulla spinalis, and thence extended throughout the whole body, two extremities are noted in each nerve; of these, the one is internal and continuous with the cerebrum, or medulla oblongata, or medulla spinalis, and termed the origin or beginning of the nerve; the other is external where the nerves terminate in various parts of the body, and termed, therefore, the end of the nerve. It is besides certain, that the nerves have the property of most readily receiving impressions, however great or of whatever kind they may be, and of transmitting them when received with great rapidity along their whole length. Consequently, if an impression be made at the termination of a nerve, which is termed an external impression, it is very rapidly transmitted along the whole length of the nerve quite to its origin; and vice versa, if the impression be made at the commencement of the nerve, which is termed an internal impression, it is transmitted with the same rapidity to the termination of the nerve. But if the impression be made midway on the trunk of the nerve, it is rapidly transmitted a the same moment to both its origin and termination. This

aptitude of the nerves to receive impressions, and when received of transmitting them either way with great rapidity, appears to be that called the vis nervosa of the nerves, which is also correctly designated, the sensibility or mobility of the nerves, and which is also well designated by Unzer, "corporeal sensation without concomitant perception."

This property of the nerves does not depend solely on their medullary pulp, (which possibly is capable of some slight vibration, or oscillation, although the nerves do not appear at all tense,) but it appears, as I have already observed in § 3 of the preceding Chapter, to be rather some other principle added to the medullary pulp, the conjunction of the two constituting the whole vis nervosa; and possibly the diligence of the very sagacious observers of nature may discover whether that principle be electricity, or phlogiston, or some species of air, or the matter of light, or a something compounded of these. That other principle, whatever it may be, seems to come to the nerves with the arterial blood, by means of the numerous blood-vessels which accompany the nerves of the whole body throughout their whole course; or to be attracted from the air through inorganic pores; or in both these ways, and not to be sent into the nerves from the brain, as its only source, although the brain itself appears to acquire a suitable portion of the same principle through its own vessels. For, as I have before shown, the nerves when separated from the brain have equally vis nervosa as the nerves in connection with the brain, and in proof hereof may be mentioned the nerves of acephalous fœtuses and of brainless animals, which are endowed with the vis nervosa, although they could not possibly derive it from a brain. Nevertheless, a certain cohesion of the medullary pulp of the nerves is necessary to the vis nervosa, because if we so injure the pulp of a nerve by strong compression, that the connection of its globules is destroyed, and their relations broken up, the vis nervosa ceases in the portion of the nerve thus compressed, neither can impressions be propagated through it, nor can that portion of the nerve produce motion or sensation, if pricked or irritated.

Although a nerve be necessary to sensation and motion, it does not excite motion or feel alone, but feels by means of the brain, which, when an impression made on a nerve is brought to it,

communicates the impression to the mind; and the nerve produces motion by means of a muscle, when an impression made on the nerve descends to the muscle, and excites it to movement. Consequently, a nerve separated from the brain, and no longer able to communicate impressions to it, can no longer produce sensation, just as a nerve separated by division from a muscle can no longer excite motion in the muscle, however much it may be irritated. Consequently, a nerve has a similar office in exciting sensation and motion, namely, to receive the impression of a stimulus, and to transmit it with the greatest rapidity along its whole length, which, when it arrives at the brain, produces the perception of a sensation, but when it arrives at a muscle, excites its contraction.

SECTION II.-THE ACTION OF THE NERVES ON THE VESSELS AND THEIR FLUIDS.

Another function of the nerves consists in a certain power over the blood-vessels, and specially the capillaries, in virtue of which, when the nerves are stimulated, they excite in that part to which they are distributed a much more copious accumulation of blood than would have taken place in the normal condition of the circulation. This phenomenon is termed congestion of the humours, afflux, derivation, abnormal direction, descent of the humours. Stahl termed it the tide of the microcosmic sea, or the ebb and flow of the blood.

The causes that determine a more copious derivation of the humours into any part of the body, are usually considered to be twofold; the one, a mere mechanical cause, consists in a diminished resistance of the vessels of the part, so that the humours contained in the vessels being forced on by the power of the heart and the vessels themselves, flow to the point of least resistance, according to the laws regulating other fluids, and cause congestion of the humours; for this reason, when a vein or artery is divided, the blood rushes from the adjoining vessels, even against its natural direction and gravity; for this reason, also, congestion takes place, when vessels are relaxed by emollient cataplasms and pediluvia. Thus also the blood is con

1 Haller, De Part. Corp. Hum. Fabr., tom. iv, pp. 93, 289.

gested under a cupping-glass when the usual atmospheric pressure is removed from the part; a local derivation of the humours takes place also when a compression of the vessels occurs in any part, and the blood is repelled into other parts; as occurs, for example, when the stomach is filled, by which the abdominal vessels are compressed and the lungs forced into less room, and, consequently, a greater quantity of blood goes to the head, rendering the face redder. The other cause is a stimulus to the nerves; which when applied to the nerves excites a more copious flow of humours. Innumerable phenomena of daily occurrence show this. Thus a stimulus applied to the nerves is the cause why the cheeks, ears, and nose, become intensely red, and a sense of heat is felt when exposed to a cold wind in winter. No one is ignorant how much the stimulus of sinapisms and blisters cause derivation of the humours to the stimulated part; an acrid smoke or fine powder getting into the eyes excites a copious flow of tears, and the vessels of the conjunctiva, previously invisible, become distended with blood. The smoke of tobacco, or any other acrid aroma, retained in the mouth, excites a copious flow of saliva; purgatives and emetics bring off much gastric and intestinal mucus; titillation of the nipple of the mamma causes it to become turgid and erect; the touch, or the stimulus of the semen or urine, or a gonorrhoea, cause the penis to be distended and erected by exciting a more copious flow of blood into the corpora cavernosa. These phenomena take place if the nerves be stimulated locally; but the same thing happens when the nerves are excited not directly, but indirectly, through the brain. We know, that thus the face is suffused with the blush of modesty; grief causes a copious flow of tears, congestion of the vessels of the conjunctiva, and redness and swelling of the whole face. The sight of agreeable food provokes a flow of saliva; it is not unusual for some persons to vomit, or be purged by only seeing a medicine; a lascivious idea erects the penis, &c.

Although it is placed beyond a doubt, that stimulated nerves cause congestion and derivation of the humours to the part stimulated, the mode in which the nerves accomplish this is as yet unknown. Distinguished writers have advanced various conjectures, by which they have attempted to explain this influence of the nerves on the vessels. Some have supposed that

there are nervous loops and nooses at the termination of the arteries and roots of veins, similar to those which are seen to surround the larger arteries in many places; and they opined that these loops could be tightened or relaxed, and so be able to admit blood to the part or retain it. Haller, together with some of his disciples, followed Willis in adopting this opinion; but when he learnt, from experiments, that the nerves do not contract when stimulated, he rejected the doctrine. Some located muscular sphincters at the terminations of the arteries and roots of the veins, which constricting the vessels, and causing the blood to accumulate above the constriction, so inundated the lateral vessels: Boerhaave in particular propounded this opinion in his theory of obstruction, and also founded his theory of inflammation upon it. But many and weighty objections have been raised against this production of accumulation and inflammation by obstruction and constriction only; for obstruction of a vessel does not cause such an accumulation of fluid, anterior to the obstruction, because it easily finds an exit through the lateral vessels so obvious in every part of the body; and the comparison of a river swelling from an obstruction, and inundating the adjoining parts, does not apply to our vessels; for if one, or even many of them, be obstructed, there still remain innumerable lateral vessels, through which the fluids find a free outlet. For this reason, Haller found that the trunk of an artery, when tied, became swollen indeed for a moment, between the ligature and the heart, and manifested one or two pulsations; but so far is it from the fact, that the impetus of the fluids is directed against the ligature, that rather the canal is contracted, and it impels the blood into the communicating arteries, until that which was tied is quite empty. The same thing is shown by the umbilical arteries, which also become empty, and impervious. Winterl2 fully sets forth these and other arguments of distinguished men, and proves that the fluids do not rush towards an obstruction, but rather prefer to pass away by the lateral and unobstructed vessels; consequently, no congestion and no inflammation can arise from an obstruction only, but the stimulus of the nerves is the cause, which immediately excites the fluids to accumulate more copiously in the vessels subjected to them. Moreover, 1 Aphoris., 113. 2 Nova Theoria Inflammationis, p. 19.