291. As a "living body has no power of forming elements, or of converting one elementary substance into another, it therefore follows that the elements of which the body of an animal is composed must be in the food." (Chap. III.) Of the essential constituents of the human body, carbon, hydrogen, oxygen, and nitrogen are the most important, because they compose the principal part of the animal body; while the other elements are found in very small proportions, and many of them only in a few organs of the system. (Appendix G.) _Observation._ Nitrogen renders food more stimulating, particularly if combined with a large quantity of carbon, as beef. Those articles that contain the greatest amount of the constituent elements of the system are most nutritious. As milk and eggs contain all the essential elements of the human system, so they are adapted to almost universal use, and are highly nutritious. -=-=-=-=-=-=-=-=-=-=-=-= 290. In what proportion are substances nutritious? Digestible? Why does beef stimulate the system? What is said of milk and eggs? -=-=-=-=-=-=-=-=-=-=-=-= 292. The following table, by Pereira, in his treatise on Food and Diet may aid the student in approximating to correct conclusions of the quantity of nutriment in different kinds of food, and its adaptation to the wants of the system. TABLE, SHOWING THE AVERAGE QUANTITY OF DRY, OR SOLID MATTER, CARBON, NITROGEN, AND MOISTURE, IN DIFFERENT ARTICLES OF DIET. -------------------+---------+---------+-----------+------------ One hundred Parts. | Dry | Carbon. | Nitrogen. | Water | Matter. | | | -------------------+---------+---------+-----------+------------ Arrowroot, | 81.8 | 36.4 | | 18.2 Beans, | 85.89 | 38.24 | | 14.11 Beef, fresh, | 25 | 12.957 | 3.752 | 75 Bread, rye, | 67.79 | 30.674 | | 32.21 Butter, | 100 | 65.6 | | Cabbage, | 7.7 | | 0.28 | 92.3 Carrot, | 12.4 | | 0.30 | 87.6 Cherries, | 25.15 | | | 74.85 Chickens, | 22.7 | | | 77.3 Codfish, | 20 | | | 80 Cucumbers, | 2.86 | | | 97.14 Eggs, whites, | 20 | | | 80 ----, yolk, | 46.23 | | | 53.77 Lard, hog's, | 100 | 79.098 | | Milk, cow's, | 12.98 | | | 87.02 Oats, | 79.2 | 40.154 | 1.742 | 20.8 Oatmeal, | 93.4 | | | 6.6 Olive-oil, | 100 | 77.50 | | Oysters, | 12.6 | | | 87.4 Peaches, | 19.76 | | | 80.24 Pears, | 16.12 | | | 83.88