As to the internal sensations, they have phenomena much more obscure than the preceding. The brain is undoubtedly the centre of these sensations, as well as those which take place without it; in fact, if the action of this organ is suspended by wine, opium, or any other means, though acute pains may affect the internal organs, these pains are not perceived. Thus when the brain has received a concussion, though the impression of sounds, of light, of odours is made as usual upon the ear, the eye and the nostrils which are uninjured, yet there is neither hearing, seeing or smelling. But how do the impressions made upon the internal organs get to the brain? Here are different phenomena, that it is impossible to conceive of well, by supposing that the nerves are charged with transmitting these impressions exactly like those which are experienced by the external organs.
1st. There are organs that have the most acute sensibility upon the slightest touch, and which however receive very few apparent nerves; such is the medullary membrane of the long bones. 2d. Certain organs in which the cerebral nerves evidently enter, as the liver, the lungs, &c. can be irritated in animals, without seeming to give them much pain. 3d. The muscles of animal life, in the structure of which so many nerves enter, in which also the branches of these perform so great a part as it relates to animal contractility, do not occasion much pain when their texture is cut without entangling the nervous filaments that penetrate them. 4th. The ligaments, that no nerve enters, are the seat of acute pain when they are distended, as my experiments have proved. It is the same as it respects the tendons, the aponeuroses, &c. 5th. All the organs with the structure of which the nervous system has manifestly no connexion, transmit however to the brain the most painful impressions when they are inflamed, &c. &c.
I could bring many other facts, which the opponents of Haller have carefully collected; but these are such, that we cannot hesitate to admit, that the opinion of this celebrated physiologist should not be entirely acceded to.
All that we know upon the internal sensations is, that, 1st. there is an organ in which the cause of sensation is seated; 2d. that this organ transmits to the brain the particular modifications that it experiences in its vital forces. But we are wholly ignorant of the medium of communication of one with the other. Hence, why, in my division of the vital forces, I have avoided a systematic basis. The distinction of the two kinds of sensibility, of the three kinds of contractility, rests wholly upon the observation of facts. Such is the obscurity of the phenomena of life, that I doubt if we shall ever be able to establish divisions from a knowledge of the nature and essence of the vital forces.
I observe that there is a great difference between animal sensibility and contractility; that in the first, the nerves are in certain cases the evident agents of communication between the organs that receive the impression and the brain that perceives it, but that in other cases, we know not the kind of relation; whilst in the second, it is always manifestly by the nerves that the brain communicates with the muscles, and that the organs can never execute a voluntary motion without the influence of the cerebral nerves.
Let us confine ourselves to this general view, which is from accurate observation; let us abandon reasoning, where experiments are not the basis of it. Some modern authors have been less judicious; they have admitted a nervous atmosphere extending more or less remotely, and acting at a determinate distance; so that though an organ may not receive a nerve, it is sufficient that it should be in the atmosphere of a nervous cord to be the seat of sensations. This ingenious idea of Reil, should be placed at the side of a great number of those that Bordeu has scattered in his works, and which are rather proofs of an ingenious author, than an accurate and judicious mind, hostile to every opinion not founded on rigorous experiment. In fact, what is this atmosphere? Is it an emanation that is constantly made at the exterior of the nerves? Is it a fluid that is independent of them, and that nature has placed around each nervous cord, as it has placed the air around the earth? Is it a power that has been given to the nerves to act at a distance without intermediate bodies? Some galvanic experiments seem to prove something similar to this in the nerves; but these experiments have no relation to the transmission of animal sensibility. Moreover, when pain takes place in the middle of a very thick tendon, in the centre of a large articulation, as that of the knee for example, it would be necessary that the atmosphere of nervous activity should extend sometimes even an inch. Why is there not suffering produced, by the irritation of an insensible part that is at the side of a nerve, or even connected with it, whilst the pain is very acute in an inflamed part, though it is at a distance from every nervous cord? Would the nerves then have also, a sphere of activity for motion? But why should the contiguity of the nerve never be sufficient to produce it in the muscles? Why is it not the same of sensation?
Animal contractility. Influence of the nerves upon that of the other parts.
The texture of the nerves is wholly destitute of this contractility. No kind of sensible motion is ever observed in them; they perform however an essential part in this property, considered in relation to the muscles of animal life. We shall see that they are the essential agents which transmit to them the principle of motion; so that animal contractility always supposes the exercise of three successive actions, viz. that of the brain, the nerves and the muscles.
The opinions of physiologists have been singularly divided upon the manner in which the nervous influence is propagated. Some have admitted a kind of vibration, others a fluid pervading the insensible canals of these organs. This last hypothesis is still in much credit. What has not been said upon the albuminous, electric, magnetic nature, &c. of this fluid? The article upon the nerves, in most physiological treatises, is almost wholly devoted to the examination of this question, but I shall say nothing upon it, for we do not know any thing that rests upon experiment. Moreover, are we not able without knowing the mode of the nervous action, to study and analyze the phenomena of the nerves? It is the common fault of all the ancient physiologists, to have wished to begin where they should one day end. Science was in its infancy when all the questions they discussed turned upon the first causes of the vital phenomena. What is the result of it? An immense deal of rubbish, and the necessity of finally coming to the accurate study of these phenomena by abandoning that of their causes, until we have observed enough to establish theories. Thus mankind have disputed for ages, upon the nature of fire, of light, of heat, of cold, &c. until philosophers finally perceived that before reasoning it was necessary to have a foundation upon which their reasoning should rest, they then sought for these foundations and thus created experimental philosophy. Thus interminable disputes have existed in the schools upon the nature of the soul, of judgment, &c. until metaphysicians have perceived the necessity of analyzing the operations of our intellectual faculties before they can know their essence. Each of the natural sciences has almost had two epochs; 1st. that of the last age, in which first causes were the only subject of discussion; an epoch useless to the sciences; 2d. that in which they have begun to be composed of the study of the phenomena only that experience and observation offer. Physiology has still one foot in the first epoch, whilst it has placed the other in the second. Physiologists of the present day should advance it still further.