[1] The form adopted by Bichat, in this work, has been much blamed by some, and extravagantly praised by others. The blame and the praise appear to me to be equally misplaced. His object was to exhibit the various phenomena of life; the order in which this was to be done was a matter of indifference. If Bichat gave a preference to this form, it was because it was conformable to the nature of his mind; and he accomplished his task in a very happy manner. The division that he has adopted is not new, it may be found, with slight modifications, in writers of different periods, and even in Aristotle. Besides, it is not necessary in the sciences to attach a very great importance to classification. All these contrivances have been invented only to aid the memory; and the functions of living bodies are not so numerous, as to render it necessary in studying them to lean upon systematic divisions.
[2] The word life has been employed by physiologists in two different senses. With some, it means an imaginary being, the sole principle of all the functions which living bodies exhibit; with others, it means only the assemblage of these functions. It is in this last sense that Bichat employs it. This is what he means to say in the following sentence. Life is the assemblage of the functions which resist death. He is wrong only in allowing the idea of death to enter into it; for this idea necessarily supposes that of life. There is then really a bad circle in this definition; but in putting aside what is defective in the expression, it may be seen that Bichat considers life as a result, not as a cause.
Before and since the time of Bichat, a great number of definitions of life has been given, which are either false or incomplete. It should not be required of a definition, that it should give all the properties of the thing which it is designed to make known, this would be a description; but we have a right to expect that it should assign to this thing certain characters which belong to it alone, and thus distinguish it from every thing else.
Let us examine by this principle the definition adopted in a modern work. Life, it is said, is the assemblage of the phenomena which succeed each other, for a limited time, in an organized being. This is no doubt true of life; but, if it can also be applied to another state, it ceases to be a definition. An animal has just died; its organs from that moment are subject to the action of chemical affinities only; decomposition takes place, gases are disengaged, fluids flow out and new solid aggregates are formed. After a time every molecular motion ceases; there remains only a certain number of binary, ternary combinations, &c. Here then is an assemblage of phenomena taking place for a limited time in an organized body, and yet it is not life.
[3] This distinction of the two lives is bad, inasmuch as it tends to separate phenomena which have a very intimate connexion, which relate to a common object, and which are often produced by means in every respect similar. Why should I rank among the organs of animal life the muscular apparatus which carries the alimentary mass from the mouth into the oesophagus, and among those of the other life, that which takes it from the cardiac orifice to the anus? Is not the action of the first apparatus in relation with nutrition as well as the action of the last, and does not the muscular apparatus of the oesophagus act upon a body which is foreign to us, as well as that of the tongue and the pharynx? Do the motions of mastication differ in their object from those of which we have just spoken, and as to the means of execution, does not the muscular action still perform the principal part?
We might in the same way bring near each other the motions by means of which we seize our food. The action itself of the senses, which directs these motions, is, with nutrition, in a relation more remote, but not less necessary, and we see in the various classes of animals that their apparatus is modified according to the different kinds of nourishment. If the distinction of the two lives be wanting in justice, as to the object of the functions it separates, we shall soon see that the characters attached to the organs of one and the other do not establish this division in a more striking manner.
[4] This division between vegetables and animals is far from being so striking as is here supposed; these two classes of beings, so different when we examine them in the individuals endowed with a very complicated organization, approximate each other in a remarkable degree, when we descend to those species whose structure is most simple; it is even remarkable that the most constant character which distinguishes one from the other, is not found in the organs of animal life, but in those of vegetable or organic life. The senses are one after the other found wanting; for in an individual in whom we can discover no nervous system, there is no more reason to suppose the existence of the sense of touch as a sensation, than to suppose it in the sensitive plant, the dionæa muscipula, and other similar plants; we see only action and reaction. The motions of the arms of certain polypi no more suppose volition than the motion of the root which follows a wet sponge, or that of the branches which turn towards the light; the only very constant character is the absence or presence of a digestive cavity. To speak of an animal as a vegetable clothed with an external apparatus of organs of relation, is a more brilliant than profound view of the subject. Buisson, who, in his division of the physiological phenomena, avoids this inaccuracy, has himself fallen into error; he pretends that respiration belongs exclusively to animals; and that thus the division of Bichat was not only unfounded but also incomplete, since this function, which is neither of vegetation nor of relation, could be ranked under neither life. Buisson was not well informed; no doubt the respiration of vegetables does not exhibit the most apparent phenomena of the respiration of the mammalia, but every thing, which essentially constitutes the function, is found in the one as well as in the other; absorption of the atmospheric air, and the formation and exhalation of a new gas; the rest is only accidental and is not an appendage but in certain classes of animals. In some reptiles, though we find a particular organ for respiration, this organ is not indispensable; it may be removed, and the skin becomes the only respiratory organ; and when finally we come to consider animals with tracheæ, we see that the conformity becomes more and more evident.
[5] Bichat seems here to adopt the generally received opinion that it is the chyle which furnishes to the mammary gland the materials of which the milk is composed. We know not whence this opinion arises, if it be not from the gross resemblance which the chyle and milk often exhibit. This resemblance, if it were very great, would be a poor reason for admitting, without anatomical proof, so singular a fact; but it is very far from being perfect. The chyle in fact does not exhibit the milky appearance and the white opaque colour, only when the animal from whom it is taken, has fed upon substances containing fat; in all other cases, it is almost transparent; its odour and taste, under all circumstances, differ entirely from those of milk; if these two fluids are left to themselves, the milk remains a long time without coagulating, but the chyle almost immediately coagulates, and then separates into three parts. The solid portion soon exhibits cells, and an appearance of organization; nothing similar is seen in the coagulum of milk; the serum of the milk remains colourless when exposed to the simple contact of the air, that of the chyle assumes a rosy tint, often very vivid. Finally, if we examine the chemical composition of these two fluids, we shall find in them differences still more striking. (See for farther details, my Elements of Physiology, Vol. 2d.)
[CHAPTER II.]
GENERAL DIFFERENCES OF THE TWO LIVES WITH REGARD TO THE OUTWARD FORM OF THEIR RESPECTIVE ORGANS.
The organs of the animal life are symmetrical, those of the organic life irregular in their conformation; in this circumstance consists the most essential of their differences. Such character, however, to some animals, and among the fish, to the sole and turbot especially, is not applicable; but in man it is exactly traced, as well as in all the genera which are nearest to him in perfection. In them alone am I about to examine it.