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 æsthetics 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., is 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 dextrins 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 ferment or enzyme known as ptyalin.