Sensation may be destroyed in a portion of the body while it still possesses the power of motion, and conversely a limb may lose the power of motion and still remain sensitive to external impressions. This independence of motion and of sensation revealed to the physiologists of antiquity the existence of two orders of nerves, one sensitive the other motor. Boerhaave and other modern anatomists accepted these doctrines, on which Lamarck based and promulgated theories nearly approaching the truth; but Sir Charles Bell was the first to distinguish, by actual experiment, the nerves of sensation from those of motion, and to show that they sprang from two distinct portions of the spinal cord.
The anterior fascicles of the cord and the anterior roots of the nerves which proceed from them are insensible, and produce muscular contraction. The posterior fascicles of the cord, and the posterior roots of the nerves, have no motor power, but they are sensitive. Each spinal nerve, formed by the union of the anterior and posterior roots, contains sensitive filaments and motor filaments placed side by side in its trunk and its ramifications. It follows, therefore, that these nerves and their subdivisions are mixed; as regards the composition of their fascicles, they are at once sensitive and motor. They are sensitive to mechanical irritation, and they excite muscular contraction under the influence of galvanism, for both these agents meet in the same nerve-filaments subject to their power. These filaments, separately considered, come from the centre to the periphery without division and without anastomosing, in the exact sense of the word, for what is called anastomosis of the nerves is a simple juxtaposition, proximity without exchange of their proper substance, and without intimate fusion.
It is to the continuity of the nervous filaments, and to their independence, that distinctness of tactile sensation and precision of motion is due. It is clear, therefore, that if two sensitive filaments should unite their proper substance, the impressions perceived by them previous to their union would be confounded, and would not be referred by the brain to distinct points. If, for example, two filaments running to the index and middle fingers, were united, instead of being simply placed side by side, at any point between the fingertips and the brain, they would carry to the brain but one single sensation for both fingers, and it would be impossible to tell upon which one the impression had been made. The result would be the same if two filaments of a motor nerve running to these fingers were united, instead of isolated in their proper substance; the motor impulse would be transmitted to both fingers, alike, and the brain could not move either expressly.
In persons who have suffered amputation, phenomena, are produced which are explained by the fact that all the filaments of a nerve exist at its origin which are found at its extremity. The man who has lost an arm or a leg, feels pains, which he refers not to the stump which remains, but to the hand or the foot which he has lost It is the nervous filaments primitively destined to these parts which are the seat of the pain, and which now transmit it as coming from the organ to which they formerly gave sensibility. The same effect is produced when a piece of skin has been transplanted from the forehead to the nose by autoplasty; if the patient be touched on the nose he feels the impression on his forehead.
We shall see in treating of the senses, that tactile impressions may be distinct on the tips of the fingers, at a distance of one-fiftieth of an inch from each other; which implies that there are two filaments at this interval from each other, running directly to the brain, but we err if we reckon in this way the extent of nerve subdivision, for every point in the skin, however small, is sensitive to the touch. It is by innumerable ramifications, each of which contains at least one nervous filament, that the nerves terminate in the organs, those of motion to excite muscular contraction, and those of sensation to receive and transmit impressions.