70. If there be one law in the animal economy which, above every other is irresistibly forced upon our attention, and which must command our unqualified assent, it is this, that the adjustment of supplies should Always Bear A Direct Relation To The Waste And growth of the individual - and yet it would be difficult to point out a principle so little regarded, and so generally abused. Do we not daily see the adult indulging in an excess adapted only to the demands of a rapid growth? - the sedentary and indolent feeding like the active and laborious? Do we not witness the greatest alterations and changes in the habits of men - the active countryman becoming the sedentary artist, - the sportsman, inhaling the invigorating breezes of the morning, converted into the legislator, doomed to breathe during the greater portion of the twenty-four hours, smoke, carbonic acid, and animal effluvia? Do we not, I repeat, see these, and equally violent changes in the habits of men; but when, let me ask, do we see alterations in the diet in accordance with such changes?

71. If chemical change be defined that change of composition in which the elements of bodies are differently arranged, with regard to their proportions and modes of combination, the conversion of aliment into blood strictly falls under that description; but the definition generally includes the operation of certain known laws, by which such changes are produced. In this latter sense the analogy fails us; for the forces which determine the decomposition of food, and its recombination into chyle, are undoubtedly not to be measured or appreciated by the laws which govern the transmutations of inanimate matter: we may, nevertheless, conventionally retain the term, in order to distinguish such actions from those which are more strictly mechanical; and although, in the progress of such discussions, we may lapse into the common language of chemistry, the reader will, from this explanation, readily understand the latitude with which it is to be received.

72. In every change which the aliment undergoes, from its introduction into the mouth to the exclusion of its refuse, we shall discover the combined operation of chemical and mechanical agents. When, for instance, the food enters the mouth, it is at once submitted to the mechanical process of division by the teeth; and, during its mastication, it becomes intimately mixed and combined with a chemical agent, which prepares it for the process which it has shortly to undergo in the stomach.

73. But there exists, moreover, a vital action which no natural philosophy can elucidate - the lights of chemistry, feeble as they are in the stomach and first intestine, fail us altogether after the change produced by the admixture of bile with the chyme in the duodenum. At this period of the process it is probable that certain organic molecules result, and are developed and modified by a new class of actions, wholly different from what we understand by chemical transmutation; in short, the organization, and not the composition, of the products is to be regarded, and which during their passage through the different glands, are doubtless elaborated by a series of ascending changes into forms of increasing complication.

74. The quantity of the salivary secretion appears to be augmented by the pressure occasioned upon the glands by the act of mastication; but its flow, although perhaps less in quantity, equally takes place without the aid of such pressure, as is proved by the phenomena observed during the repast of the criminal already alluded to (29). The glands appointed to secrete this fluid seem to act in sympathy with those of the stomach, both of which are simultaneously excited by the stimulus of the food, or even by the contemplation of a favourite meal. Macbride considered the saliva as a fermentl.

1 This opinion has been revived by M. Raspail.

The ground of this opinion arose from his having made experiments, in which pieces of meat and water were mixed together alone in one vessel, and in another the same substances were mixed with saliva: in the former case no bubbles of air were perceptible, but in the latter a copious evolution of them took place. This, however, was a fallacy, depending upon the viscidity which the saliva imparted to the water, by which the escape of air was prevented until it became sensible. Dr. Fordyce, on the other hand, contends, that the saliva answers no other purpose than to lubricate the passages through which the food is to pass, because he cannot discover in the composition of that fluid any ingredients which are likely to act as powerful solvents. But the processes of nature are more refined than those of art; and where chemistry refuses its aid, we may often derive information from simple experience. This happens in the question before us; the introduction of saliva into the stomach is obviously essential to a healthy digestion.

That a dry state of the fauces should be attended with loss of appetite, may, perhaps, be reconciled, on the supposition that the salivary glands sympathize with those of the stomach, and that, therefore such a condition of the fauces is merely indicative of a deranged state of the gastric secretion; but this explanation will not apply to those cases of anorexia, in which the saliva is duly secreted, but is, from some mechanical cause, not swallowed. Ruysch knew a man who was wholly deprived of his appetite by a fistula in one of the salivary ducts; and it is well known to the physician who has attended maniacal patients, that the constant spitting in which such persons occasionally indulge, is invariably attended with loss of appetite, dyspepsia, and emaciation. Insaliva-tion, therefore, is as essential as mastication; and although it will not supersede the necessity of this latter operation, as we find that persons who do not chew their food have often, on that account, a laborious digestion, yet it may, to a certain degree, compensate for it; and it is probable that the abundance of saliva in children may render mastication less necessary; in like manner, we find that, in old age, the loss of teeth is followed by an increased salivary discharge.

The change which the savour and odour of food undergo in the mouth sufficiently testifies some chemical action; but it must, at the same time, be admitted, that the deglutition is assisted by the moisture and lubrication which the saliva affords.