were subjected to a similar violence. The electricity produced by these animals, accordingly, is not supplied by the nerves, but by a special generating organ, the action of which is regulated by nervous influence.

Moreover, the experiments of Longet and Matteucci1 have shown that no electrical current is to be detected in a living nerve, even when in a state of activity. The electrical phenomenon, when it exists, is only a secondary effect, and is not the active force residing in the nervous tissue. This force is special in its nature, and is regulated by laws peculiar to itself.

I Longet, Traité de Physiologie. Paris, 1850, vol. ii. p. 130.

CHAPTER III.

THE SPINAL CORD.

WE have already seen that the spinal cord is a long ganglion, covered with longitudinal bundles of nervous filaments, and occupying the cavity of the spinal canal. It sends out nerves which supply the muscles and integument of at least nine-tenths of the whole body, viz., those of the neck, trunk, and extremities. All these parts of the body are endowed with two very remarkable properties, the exercise of which depends, directly or indirectly, upon the integrity and activity of the spinal cord, viz., the power of sensation and the power of motion. Both these properties are said to reside in the nervous system, because they are so readily influenced by its condition, and are so closely connected with its physiological action. We shall therefore commence the study of the spinal cord with an examination of these two functions, and of the situation which they occupy in the nervous system.

SENSATION. The power of sensation, or sensibility, is the power by which we are enabled to receive impressions from external objects. These impressions are usually of such a nature that we can derive from them some information in regard to the qualities of external objects and the effect which they may produce upon our own systems. Thus, by bringing a foreign body into contact with the skin, we feel that it is hard or soft, rough or smooth, cold or warm. We can distinguish the separate impressions produced by several bodies of a similar character, and we can perceive whether either one of them, while in contact with the skin, be at rest or in motion. This power, which is generally distributed over the external integument, is dependent on the nervous filaments ramifying in its tissue. For if the nerves distributed to any part of the body be divided, the power of sensation in the corresponding region is immediately lost.

The sensibility, thus distributed over the integument, varies in its acuteness in different parts of the body. Thus, the extremities of the fingers are more sensitive. to external impressions than the general surface of the limbs and trunk. The surfaces of the fingers which lie in contact with each other are more sensitive than their dorsal or palmar surfaces. The point of the tongue, the lips, and the orifices of most of the mucous passages are endowed with a sensibility which is more acute than that of the general integument. If the impression to which these parts are subjected be harsh or violent in its character, or of such a nature as to injure the texture of the integument or its nerves, it then produces a sensation of pain. It is essential to notice, however, that the sensation of pain is not a mere exaggeration of ordinary sensitive impressions, but is one of quite a different character, which is superadded to the others, or takes their place altogether. Just in proportion as the contact of a foreign body becomes painful, our ordinary perceptions of its physical properties are blunted, and the sense of suffering predominates over ordinary sensibility. Thus if the integument be gently touched with the blade of a knife we easily feel that it is hard, cold, and smooth; but if an incision be made with it in the skin, we lose all distinct perception of these qualities, and feel only the suffering produced by the incision. We perceive, also, the difference in temperature between cold and warm substances brought in contact with the skin, so long as this difference is moderate in degree; but if a foreign body be excessively cold or excessively hot, we can no longer appreciate its temperature by the touch, but only its injurious and destructive effect. Thus the sensation caused by touching frozen carbonic acid is the same with that produced by a red-hot metal. Both substances blister the surface, but their actual temperatures cannot be distinguished.

It is, therefore, a very important fact in this connection, that the sensibility to pain is distinct from the power of ordinary sensation. This distinction was first fully established by M. Beau, of Paris, who has shown conclusively that the sensibility to pain may be diminished or suspended, while ordinary sensation remains. This is often seen in patients who are partially under the influence of ether or chloroform. The etherization may be carried to such an extent that the patient may be quite insensible to the pain of a surgical operation, and yet remain perfectly conscious, and even capable of feeling the incisions, ligatures, &c., though he does not suffer from them. It not unfrequently happens, also, when opium has been adminis

tered for the relief of neuralgia, that the pain is completely abolished by the influence of the drug, while the patient retains completely his consciousness and his ordinary sensibility.

In all cases, however, if the influence of the narcotic be pushed to its extreme, both kinds of sensibility are suspended together, and the patient becomes entirely unconscious of external impressions.

MOTION. Wherever muscular tissue exists, in any part of the body, we find the power of motion, owing to the contractility of the muscular fibres. But this power of motion, as we have already seen, is dependent on the nervous system. The excitement which causes the contraction of the muscles is transmitted to them by the nervous filaments; and if the nerve supplying a muscle or a limb be divided or seriously injured, these parts are at once paralyzed and become incapable of voluntary movement. A nerve which, when irritated, acts directly upon a muscle, producing contraction, is said to be excitable; and its excitability, acting through the muscle, produces motion in the part to which it is distributed.

The excitability of various nerves, however, often acts during life upon other organs, beside the muscles; and the ultimate effect varies, of course, with the properties of the organ which is acted upon. Thus, the nervous excitement transmitted to a muscle produces contraction, while that transmitted to a gland produces an increased secretion, and that conveyed to a vascular surface causes congestion. In all such instances, the effect is produced by an influence transmitted by the nerve directly to the organ which is called into activity.

But in all the external parts of the body muscular contraction is the most marked and palpable effect produced by the direct influence of nervous excitement. We find, therefore, that so far as we have yet examined it, the nervous action shows itself principally in two distinct and definite forms; first, as sensibility, or the power of sensation, and second, as excitability or the power of producing motion.

DISTINCT SEAT OF SENSATION AND MOTION IN THE NERVOUS SYSTEM. Sensation and motion are usually the first functions which suffer by any injury inflicted on the nervous system. As a general rule, they are both suspended or impaired at the same time, and in a nearly equal degree. In a fainting fit, an attack of apoplexy, concussion or compression of the brain or spinal cord, or a

wound of any kind involving the nerves or nervous centres, insensibility and loss of motion usually appear simultaneously. It is difficult, therefore, under ordinary conditions, to trace out the separate action of these two functions, or to ascertain the precise situation occupied by each.

This difficulty, however, may be removed by examining separately different parts of the nervous system. In the instances mentioned above, the injury which is inflicted is comparatively an extensive one, and involves at the same time many adjacent parts. But instances sometimes occur in which the two functions, sensation and motion, are affected independently of each other, owing to the peculiar character and situation of the injury inflicted. Sensation may be impaired without loss of motion, and loss of motion may occur without injury to sensation. In tic douloureux, for example, we have an exceedingly painful affection of the sensitive parts of the face, without any impairment of its power of motion; and in facial paralysis we often see a complete loss of motion affecting one side of the face, while the sensibility of the part remains altogether unimpaired.

The above facts first gave rise to the belief that sensation and motion might occupy distinct parts of the nervous system; since it would otherwise be difficult to understand how the two could be affected independently of each other by anatomical lesions. It has accordingly been fully established by the labors of Sir Charles Bell, Müller, Panizza, and Longet, that the two functions do in reality occupy distinct parts of the nervous system.

If any one of the spinal nerves, in the living animal, after being exposed at any part of its course outside the spinal canal, be divided, ligatured, bruised, or otherwise seriously injured, paralysis of motion and loss of sensation are immediately produced in that part of the body to which the nerve is distributed. If, on the other hand, the same nerve be pricked, galvanized, or otherwise gently irritated, a painful sensation and convulsive movements are produced in the same parts. The nerve is therefore said to be both sensitive and excitable; sensitive, because irritation of its fibres produces a painful sensation, and excitable, because the same irritation causes muscular contraction in the parts below.

The result of the experiment, however, will be different if it be tried upon the parts situated inside the spinal canal, and particularly upon the anterior and posterior roots of the spinal nerves. If an irritation be applied, for example, to the anterior root of a spinal