14. ORGANIC SUBSTANCES.--These substances are derived from the vegetable and animal creations. They comprise all those articles which are commonly spoken of as "food," and which are essential to sustain the body in life and strength. They are divided into three groups, namely: the Albuminoid substances, the Fats, and Sugars. [Sidenote: 15. The Albuminoid class, includes what? These compounds constitute what? The food? Their importance? Their properties?] 15. THE ALBUMINOIDS.--This class includes three important nutritive substances--(1) _Albumen_, which gives it its name; (2) _Fibrin_, including _gluten_; and (3) _Casein_. These compounds constitute a large part of the human body, and the food contains them in proportionally large quantities. Their importance is so great, and the system so promptly suffers from their absence, that they have been styled the "_nutritious_ substances." The properties which they hold in common are, that they do not crystallize, and have a jelly-like form, except when heat is applied to them, when they harden, or _coagulate_. {58} [Sidenote: 16. Decomposition? Effect of cold? Illustrations? Elephants?] 16. They likewise decompose, or _putrefy_, under the influence of warmth and moisture. Hence the decay of all dead animal tissues. Cold arrests this process. It is well known that milk, eggs, and the like, "keep" much longer in winter than at other seasons. The bodies of elephants, caught in the ice many hundred years ago, are occasionally borne by the icebergs to the coast of Siberia, completely frozen, but preserved almost perfectly in form and limb. [Sidenote: 17. In what substances does albumen exist? What further is said of the egg?] 17. ALBUMEN exists in milk, meat, the grains, and the juices of many plants; but the purest form is obtained from the white of egg. When we consider that an egg is composed chiefly of albumen and water--namely, six parts in seven; and when we also consider the numerous, diverse, and complex tissues--the muscles, bones, internal organs, bill, claws, and feathers--with which the chick is equipped on leaving his shell, we are impressed with the importance of these apparently simple constituents of the food and body. [Sidenote: 18. Fibrin, gluten, clotting of the blood?] 18. FIBRIN is derived from meats, and exists in the blood both of man and the lower animals. _Gluten_, or vegetable fibrin, resembles closely true fibrin, and is abundantly furnished in wheat and other grains from which flour is commonly made. Animal fibrin coagulates spontaneously when it is removed from the body, and thus causes the "clotting" of the blood. [Sidenote: 19. Casein? Its coagulation? Effect of rennet? Making of cheese?]