Fat, when absorbed from the digestive tract, is in the form of fatty acids and glycerin, but immediately recombines into its original form after it has passed through the intestinal walls. This fat then enters the lacteals, which unite to form the thoracic duct. This duct or tube empties its contents into one of the large veins near the heart, whence it is distributed throughout the body. The fat of the blood is not regulated to a definite amount, like the sugar content. After a meal, very heavy in fat, the blood for a time is whitish in appearance, due to the numerous minute globules of fat taken into the circulation. [Sidenote: Body-fat may be absorbed directly from food] The fat of the body may be deposited directly from food-fat. This can be verified if an animal that has been starved until its own body has been greatly reduced, be fed upon some particular form of fat. The fat immediately deposited will then have the peculiar characteristics of the fat taken with the food. Thus a starved dog that has been given a heavy diet of tallow will deposit fat which will contain a large quantity of stearin and palmitin, and consequently have a higher melting point than normal dog fat. Ordinary animal fat, as has been shown in Lesson IV, is composed of various fats, each of which is a distinct chemical compound. [Sidenote: Human fat not identical with food-fat] The distinction between tallow, lard, olive-oil, and human fat, is chiefly due to the various portions of stearin, and olein, which composes the mixed fat. In normal cases, where fat is deposited at the usual rate, the body-fat is of uniform composition regardless of the food-fats. The reason human fat is not identical with food-fat is because the body has selective power in depositing these fats. Thus, if the sole source of fat which a man takes in his food is tallow, the fat-depositing cells in the human body would refuse a certain proportion of the stearin, depositing a larger percentage of olein, thus giving a softer or more liquid fat than that which was supplied in the food. The excess of stearin would be consumed in the production of heat and muscular energy. [Sidenote: Why exercise reduces obesity] When the consumption of glucose in the muscles becomes greater than the supply available in the blood, and from the glycogen of the liver, body-fat must be consumed. This explains why exercise reduces obesity. The method of preventing, or of curing, obesity, is a double process: 1 The diet is selected and proportioned so as to reduce the amount of ingested fat 2 Exercises are prescribed to consume the fat that has accumulated [Sidenote: Fat, the chief source of energy] Of all food materials, fat is the least changed by digestion, and has no particular function in the life-processes except the storing of energy. More body-energy can be stored in a pound of body-fat than in any other form.