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. 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 proteid 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.