At present, we accept for all purposes of computation the following figures as representing the physiological or available (to the body) fuel value of the three classes of organic foodstuffs:

From these data, it is evident at a glance that 1 gram of fat is isodynamic with 2.27 grams of either carbohydrate or proteid; and since carbohydrate and fat are of use to the body mainly because of their energy value, it is obvious that 50 grams of fat taken as food will be of as much service to the body as 113 grams of starch. In view of the relatively high fuel value of fats, it follows that the physiological heat of combustion of any given food material will correspond largely with the content of fat therein. This is quite apparent from the data given in the table showing chemical composition of food materials, where the fuel value per pound is seen to run more or less closely parallel with the percentage of fat. Experience, as well as direct physiological experiment, teaches us, however, that fat and carbohydrate cannot be interchanged indefinitely, because of the difficulty in utilization of fat when the amount is increased beyond a certain point. Personal experience provides ample evidence of the difference in availability between the two classes of foodstuffs. Carbohydrates are easily utilizable, fats with more difficulty. Palate, as well as stomach, rebels at large quantities of fat; a statement that certainly holds good for most civilized people, though exceptions may be found, as in the Esquimeaux and certain savage races.

1 Stohmann: Ueber den Warmewerth der Bestandtheile der Nahrungsmittel. Zeitschr. f. Biol., Band 31, p. 373.

2 See Rubner: Calorimetrische Untersuchungen. Zeitschr. f. Biol., Band 21, p. 250. Also, Rubner: Die Quelle der thierischen Warme. Ibid., Band 30, p. 78.

In the nourishment of the body, the various factors that aid in the utilization of food are of great moment and must not be overlooked. It is not enough that the body be supplied with the proper proportion of nutrients, with sufficient pro-teid to meet the demand for nitrogen, and with carbohydrate and fat adequate to yield the needed energy; but all those physiological processes which have to do with the preparation of the foodstuffs for absorption into the circulating blood and lymph must be in effective working order. There is an intricacy of detail here which calls for careful oversight, and it is one of the functions of the nervous system to control and regulate both the mechanical and the chemical processes that are concerned in this seemingly automatic progression of foodstuffs from their entry into the mouth cavity to their final discharge from the alimentary tract, after removal of the last vestige of true nutritive material.

Mastication; deglutition; secretion of the various digestive juices, saliva, gastric juice, pancreatic juice, bile, intestinal juice, etc.; peristalsis, or the rhythmical movements of the muscular walls of the gastrointestinal tract; the solvent action of the several digestive fluids on the different types of foodstuffs; the absorption of the products formed as a preliminary step in their transportation to the tissues and organs of the body, where they are to serve their ultimate purpose in nutrition; the interaction of these several processes one on the other; and, finally, the influence of the various nerve fibres and nerve centres concerned in the control of these varied activities, - all must work together in harmony and precision if the full measure of available nitrogen and energy-yielding material is to be extracted and absorbed from the ingested food, without undue expenditure of physiological labor. Further, the various processes of cell and tissue metabolism, by which the absorbed food material is built up into living protoplasm, and the chemical processes of oxidation, hydrolysis, reduction, etc., by which the intra and extra cellular material is broken down progressively into varied katabolic or excretory products, with liberation of energy; all these must move forward harmoniously and with due regard to the preservation of an even balance between intake and outgo, if the nutrition of the body is to be maintained at a proper level, and with that degree of physiological economy which is coincident with good health and high efficiency.

We may well pause here and consider briefly some of these processes which play so prominent a part in the proper utilization of the three classes of organic foodstuffs. The first digestive fluid which the ingested food comes in contact with is the saliva. Sensory nerve fibres, chiefly of the glossopharyngeal and lingual nerves which supply the mouth and tongue, are stimulated by the sapid substances of the food, and likewise by mere contact of the food particles with the mucous membrane lining the mouth cavity as the food is masticated and rolled about prior to deglutition. Impulses communicated in this way to the above sensory nerves are transmitted to certain nerve centres in the medulla oblongata, whence impulses are reflected back through secretory nerves going to the individual salivary glands, thereby calling forth a secretion. The production of saliva is thus a simple reflex act, in which the food consumed serves as a true stimulant or excitant. Pawlow,1 indeed, claims a certain degree of adaptability of the secretion to the character of the food taken into the mouth. Thus, he finds that dry, solid food excites a large flow of saliva, such as would be needed to masticate it properly and bring it into a suitable condition for swallowing. On the other hand, foods containing an abundance of water cause only a scanty flow of saliva. The situation of this secretory centre in the medulla, and the many branchings of nerve cells in this locality would naturally suggest the possibility of salivary secretion being incited by stimuli from a variety of sources. This is indeed the case, and it is worthy of note that a flow of saliva may result from stimulation of the sensory fibres of the vagus nerves as well as of the splanchnic and sciatic, thus indicating how a given secreting gland may be called into activity by impulses or stimuli which come to the centre through very indirect and devious pathways. Further, the secretory centre may be stimulated, and likewise inhibited, by impulses which have their origin in higher nerve centres in the brain. These facts are of great importance in throwing light upon the ways in which a secretion like saliva is called forth and its digestive action thus made possible. The thought and the odor of savory food cause the mouth to water, the flow of saliva so incited being the result of psychical stimulation. Similarly, fear, embarrassment, and anxiety frequently cause a. dry mouth and parched throat through inhibition of the secretory centre by impulses which have their origin in higher centres in the brain.

1 Pawlow: The Work of the Digestive Glands. Translated by Thompson. London, 1902.