An argument employed by many physiolo gists for the existence of a distinct set of vital affinities, is founded upon the evident truth, that the tissues and fluids which maintain a certain composition when possessed of vitality, speedily resolve themselves into new combina tions when this has become extinct. hence it is inferred that the affinities which hold toge ther the elements during life, are of a different nature from those which operate in producing their subsequent separation. Now, it may be objected to this inference, that no solid or fluid compounds which have a disposition to spon taneous decay after death, can continue to exist without change during life ; that the activity of the processes of interstitial absorption and re position seems to bear a pretty constant ratio, in every case, with the natural tendency to de composition; and that the maintenance of the original combination is not so much owing to anything peculiar in the affinities which hold together its elements, as to the constant removal of particles in a state of incipient decay, and their replacement by others freshly united. Thus, we find that all the most permanent parts of the animal frame, such as the massive skele tons of the polypifera, the calcareous tegument of the molluscs, or the bony scales of fishes, to the possible duration of which geologists scarcely dare to assign a limit, are extravascular in the living animal, undergoing scarcely any interstitial change when once formed. Next to these in order of durability are the osseous structures of animals, and the woody fibre of plants, whose connection with the circulating system appears rather adapted to meet the exi gencies of growth, injury, or disease, than to maintain a constant change required by the ten dency to decomposition. When we examine the softer tissues, on the other hand, we find that the rapidity of interstitial change full compensates for the increased tendency to d cay ; and that the perfect exercise of their respective functions imperatively demands the constant maintenance of their normal constitu tion. Moreover, there are many organic com pounds which are as permanent as those formed in the laboratory of the chemist ; of this kind are gum, sugar, and many other proximate prin ciples, which simply require for their preserva tion such external conditions as are necessary to prevent the spontaneous decomposition of many inorganic bodies. The degree in which these are subject to ordinary chemical opera tions will be presently mentioned. appear then, to be an inference better founded on fact than that first mentioned, that the preservation of the normal constitution of organic com pounds in the living body, is dependent on the continuance of the vital actions of the eco nomy, rather than due to its mere possession of the property of vitality. In fact, that may be reasonably maintained as an inference from these phenomena, which we have already at tempted to prove on other grounds;—that the vitality of each tissue, that is to say, its posses sion of vital properties, is dependent on the perfect condition of its organisation, and that, so far from preserving the organism from decay, it merely remains until decay has commenced. These inferences are, we think, fully borne out by the two following facts. When life is being extinguished by starvation, the whole body ex hales a putrid odour even before the occurrence of death, and rapidlypasses into putrefaction afterwards : here it would seem that the process of spontaneous decomposition, which we have represented as constantly occurring in the tis sues, has been unbalanced by the reposition of nutrient materials ; and that it has therefore manifested itself in the body even during life. Again, when spontaneous gangrene occurs from obstruction to the circulation, decomposi tion slowly supervenes in the part from which the supply of nutrient fluid is cut off; and coincident with its progress is the extinction of the vital properties, constituting molecular death. (See vol. i. p. 791.) Corresponding changes may result in the whole body when the nutritive functions are interrupted, not by obstruction to the motion of the circulating fluid, but by depravation of its character ; and we then perceive the vital properties of each tissue impaired in a degree correspondent to the dependence of the integrity of its structure upon the constant renewal of its elements.
The presumed impossibility of forming, by the chemical combination of their elements, any of the class of organic compounds or prox imate principles, is regarded by many physiolo gists as in itself a sufficient ground for the as sumption that the affinities which act in the living body are different from those which we recognize in the inorganic world. The fact, however, which we have already noticed re garding the artificial production of area is one which powerfully opposes such an assump tion.* This is slurred over by Muller, with the remark that it can scarcely be considered as organic matter, being rather an excretion than a component of the body—a distinction which does not remove it from the pale of the operation of the supposed laws of vital affi nity. Seeing the vast progress which organic chemistry has made during the last few years, and the rapid increase of our knowledge re garding not merely the composition but the mutual relations of the class of bodies under consideration, we cannot but think it premature to assert that other compounds may not be pro duced in a similar manner. Be it observed, however, that the doctrine for which we are now arguing only concerns the production of those compounds which are destined either to be thrown off from the system, or to undergo subsequent organisation ; and cannot apply to those in which the process of organisation, and the consequent development of vital properties have already commenced. This distinction is a very important one, and may, we think, by being kept steadily in view, save much un successful because misdirected labour. If, for example, our view be correct, it may be pos sible for the chemist to produce the gum or sugar which he finds in the ascending sap of plants; but he can never hope to imitate the latex or elaborated sap, which already shows traces of organisation and of the possession of vital properties. In like manner the formation of albumen may be a worthy object of his endeavours, whilst these would be totally fruit less if directed to the production of fibrin, which differs from it but little if at all in che mical constitution, but which is endowed in its fluid state with properties that nothing but the influence of a living system can generate.
But quitting these speculations, we shall in quire what positive evidence may be produced of the operation of chemical affinities in the changes of composition that form so important a part of vital action. Many facts might be collected which favour such a belief; but the following must here suffice. In the progress of vegetation we have frequent occasion to observe the conversion of gum and of fecula, which consists of gum enclosed in vesicles, into sugar. This takes place in germination, in the budding of the potato and other fleshy stems, in flowering, in the ripening of fruit, as well as in many other instances ; in all these in which fecula is the subject of the change, it would. seem that product, having been stored away out of the current of the circu lation against the time of need, is to be again brought into use, and to supply the pabulum of young or rapidly-growing parts by conver sion into sugar. These changes are effected in various modes. Where gum is the subject of the conversion, we commonly find an acid employed to produce it, as in the ripening of fruits, where lignin as well as gum seems to undergo this change. The chemist can pro duce the same effect by digesting gum or lignin with an acid at a certain temperature. Again, where the conversion of fecula into sugar takes place as one of the ordinary processes of the vegetable economy, it is effected by the pro duction of a secretion termed diastase, which occasions both the rupture of the starch-vesicles and the change of their contained gum into sugar. This diastase, which is abundantly stored up in the neighbourhood of the eyes or buds of the potato, may be separately obtained by the chemist ; and it acts as effectually in his laboratory as in that of the vegetable organism. Further, lie can imitate its effects by other che mical agents; for, by the joint operation of heat and acid, he can produce the same trans formation.