This section is from the book "The Epicurean", by Charles Ranhofer. Also available from Amazon: The Epicurean, a Complete Treatise of Analytical and Practical Studies on the Culinary Art.
Fibrine is insoluble; it forms the base of the muscles or flesh. After meat has been very much cooked, after it has boiled a long time, the remainder of it is almost pure fibrine. fibrine is not very nutritious, and when it has thoroughly exhausted its soluble qualities, it becomes difficult to digest. Pure fibrine has no savor, it is insipid or flavorless and becomes yellow and brittle after drying.
Gelatin is soluble in very hot or boiling water, in tepid water it swells and dissolves only partially, and in cold water it softens without dissolving. It is colorless, insipid, inodorous and is susceptible to pass rapidly into a state of acetic fermentation. There is very little nutrition in gelatin; when in sufficient quantity it gives the broth the peculiar quality of forming into a jelly when cold. Gelatin exists in all parts of the meat, but more profusely in gristle and bone. In a pure state it is insipid.
Soluble even when cold, this is a part of the flesh of the beef, of the brain and of certain mushrooms. It is osmazome which gives to the broth its savor, its aroma and its sapidity. It is supplied with an exceedingly stimulating property, exciting the appetite and helping considerably to facilitate digestion; it seems to exist only in the flesh and blood, and more abundantly in old cattle and in dark meats, than in young animals and white meat The properties of osmazome are more perceptible when the meats are broiled or roasted; then the sapidity is stronger and the aroma more exhilarating. Poultry gives very little sapidity to broths unless they be old and very fat, for their grease has a more pronounced flavor than that found in quadrupeds. Gristle, fat and bones are entirely free of osmazome; in broths there is one part osmazome to seven parts gelatin.