The application of these facts to our subject is perfectly obvious, since they suggest at once how the production or secretion of an important digestive fluid - upon which the utilization of a given class of foodstuffs may be quite dependent - is controlled and modified through the nervous system by a variety of circumstances. We might reason that the appearance, odor, and palatability of food are factors of prime importance in its utilization by the body; that the aesthetics of eating are not to be ignored, since they have an important influence upon the flow of the digestive secretions. A peaceful mind, pleasurable anticipation, freedom from care and anxiety, cheerful companionship, all form desirable table accessories which play the part of true psychical stimuli in accelerating the flow of the digestive juices and thus pave the way for easy and thorough digestion. Further, it is easy to see how thorough mastication of food may prolong mechanical stimulation of the salivary glands and thus increase the flow of the secretion, while the longer stay of sapid substances in the mouth cavity increases the duration of the chemical stimulation of the sensory fibres of the lingual and glossopharyngeal nerves. In this connection, we may cite the view recently advanced by Pawlow that the individual salivary glands respond normally to different stimuli. Thus, there are three pairs of salivary glands concerned in the production of saliva, - the submaxillary, parotid, and sublingual, all of which pour their secretions through separate ducts into the mouth cavity. By experiment, Pawlow has found that in the dog the submaxillary gland yields a copious flow of saliva when stimulated by acids, the chewing of meats, the sight of food, etc.. while the parotid gland fails to respond. On the other hand, the latter gland responds with an abundant secretion when dry food, such as dry powdered meat, dried bread, etc., ia placed in the mouth. With this gland, the inference is that dryness is the active stimulus.

As a digestive secretion, saliva serves several important purposes. By moistening the food it renders mastication and deglutition possible; its natural alkalinity tends to neutralize somewhat such acidity as may be present in the food; it dissolves various solid substances, thus making a solution capable of stimulating the taste nerves; lastly, and most important, it has a marked digestive and solvent action on starchy foods. A large proportion of the non-nitrogenous food consumed by man - in most countries - is composed of some form of starch, and this the body cannot use until it has undergone conversion into soluble forms, such as dex-trins and sugar. This it is the function of saliva to accomplish, and it owes its activity in this direction to the presence of a soluble fermeut or enzyme known as ptyatin.

Enzymes, which play so important a part in all digestive processes, are a peculiar class of substances produced by the living cells which constitute the various secreting glands. They are of unknown composition, and are peculiar in that the chemical changes they induce are the result of what is termed catalysis, i. v., contact. That is, the enzyme or catalyzer does-not-enter into the reaction, it is not destroyed. or used up, but by its mere presence sets in motion or accelerates a reaction between two other substances. The ordinary illustration from the inorganic world is spongy platinum, which, if placed in contact with a mixture of oxygen and hydrogen, causes the two gases to unite with formation of water, although the two gases alone at ordinary temperature will not so combine. In this reaction the platinum is not altered, neither does it apparently enter into the reaction; it is a simple catalyzer. The chemical nature of the change which most digestive enzymes produce is usually denned as hydrolytic, in which the substance undergoing transformation is made to combine with water, thus becoming hydrolyzed, this reaction generally being accompanied by a cleavage or splitting of the molecule into simpler substances. It. is to be noted further that enzymes are specific in their action. An enzyme that acts upon starch, for example, cannot act on proteids or fats. Some digestive fluids have the power of producing changes in different classes of foodstuffs, but such diversity of action is always assumed to he due to the presence in the same fluid of different enzymes. Eroil Fischer1 has advanced the theory that the specificity of aa enzyme is related to the geometrical structure of the substance undergoing change; i. e., that each enzyme is capable of acting upon or attaching itself only to such molecules as have a definite structure with which the enzyme is in harmony. Or, the enzyme may be considered as a key which will fit only into the lock (structure) of the molecule it acta upon. One characteristic feature of enzymes is the incomplete. nees of their action. Thus, the enzyme of saliva transforms starch by a series of progressive changes into soluble starch, two or more dextrins, and the sugar maltose as the chief end-product. A mixture of starch paste and saliva under ordinary conditions, however, never results in the formation of a hundred per cent of maltose, but there always remains a variable amount of dextrin which appears to resist further change. This is apparently due to what is known as the reversible action of enzymes. Thus, the chemical reactions involved here are reversible actions, i. e., they take place in opposite directions. The catalyzer not only accelerates or incites a reaction in the direction of breaking down the substance acted upon, but it also aids in the recomposition of the products bo formed into the original or kindred substance. With reversible reactions of this sort the opposite changes sooner or later strike an equilibrium, which remains constant until some alteration in the conditions brings about an inequality and the reactions proceed until a new equilibrium is established. In the body, however, where the circulating blood and lymph provide facilities for the speedy removal by absorption of the soluble products formed, the reaction may proceed until the original substance undergoing change is completely transformed into the characteristic end-product. This reversible action of enyzmes is an important feature, and helps explain certain nutritional changes to be referred to later. Whether all enzymes behave in this way is not as yet determined

1 Emil Fisher - Bedeutung der Steroociiemie fur die Pbyiioiogie. Zcitechr. fur physlogicbe Chemie, Band 36, p. 60.