ONE KIND OF FOOD IS INSUFFICIENT.--A person fed on starch alone, would die. It would be a clear case of nitrogen starvation. On the other hand, as nitrogenous food contains carbon, the elements of water, and various mineral matters, life could be supported on that alone. But such a prodigious quantity of lean meat, for example, would be required to furnish the other elements, that not only would it be very expensive, but it is likely that after a time the labor of digestion would be too onerous, and the system would give up the task in despair. The need of a diet containing both nitrogenous and carbonaceous elements is shown in the fact that even in the tropical regions oil is relished as a dressing upon salad. Instinct everywhere suggests the blending. Butter is used with bread; rice is boiled with milk; cheese is eaten with macaroni, and beans are baked with pork. FIG. 45. [Illustration: _The Stomach and Intestines._ 1, _stomach;_ 2, _duodenum;_ 3, _small intestine;_ 4, _termination of the ileum;_ 5, _cœcum;_ 6, _vermiforn appendix;_ 7, _ascending colon;_ 8, _transverse colon;_ 9, _descending colon;_ 10, _sigmoid flexure of the colon;_ 11, _rectum;_ 12, _spleen--a gland whose action is not understood._--LEIDY'S _Anatomy._] THE OBJECT OF DIGESTION.--If our food were cast directly into the blood, it could not be used. For example, although the chemist can not see wherein the albumen of the egg differs from the albumen of the blood, yet if it be injected into the veins it is unavailable for the purposes required, and is thrown out again. In the course of digestion the food is modified in various ways whereby it is fitted for the use of the body, into which it is finally incorporated. We call this change of food into flesh _assimilation_, a name for a work done solely by the vital organs, and so mysterious in its nature that the wisest physiologist gets only glimpses here and there of its operations.