Footnote 44: Does not include deaths from general alcoholic paralysis or other organic diseases due to alcohol. Liver cirrhosis due to alcohol conservatively estimated at 75 per cent of total cases. "Alcoholic indulgence stands almost if not altogether in the front rank of the enemies to be combated in the battle for health."--PROFESSOR WILLIAM T. SEDGWICK. XXI. THE BLOOD AND ITS CIRCULATION _Problems.--To discover the composition and uses of the different parts of the blood._ _To find out the means by which the blood is circulated about the body._ LABORATORY SUGGESTIONS _Demonstration._--Structure of blood, fresh frog's blood and human blood. Drawings. _Demonstration._--Clotting of blood. _Demonstration._--Use of models to demonstrate that the heart is a force pump. _Demonstration._--Capillary circulation in web of frog's foot or tadpole's tail. Drawing. _Home or laboratory exercise._--On relation of exercise on rate of heart beat. Function of the Blood.--The chief function of the digestive tract is to change foods to such form that they can be absorbed through the walls of the food tube and become part of the blood.[45] Footnote 45: This change is due to the action of certain enzymes upon the nutrients in various foods. But we also find that peptones are changed back again to proteins when once in the blood. This appears to be due to the _reversible_ action of the enzymes acting upon them. (See page 307.) If we examine under the microscope a drop of blood taken from the frog or man, we find it made up of a fluid called _plasma_ and two kinds of bodies, the so-called _red corpuscles_ and _colorless corpuscles_, floating in this plasma. Composition of Plasma.--The plasma of blood is found to be largely (about 90 per cent) water. It also contains a considerable amount of protein, some sugar, fat, and mineral material. It is, then, the medium which holds the fluid food that has been absorbed from within the intestine. This food is pumped to the body cells where, as work is performed, oxidation takes place and heat is given off as a form of energy. The almost constant temperature of the body is also due to the blood, which brings to the surface of the body much of the heat given off by oxidation of food in the muscles and other tissues. When the blood returns from the tissues where the food is oxidized, the plasma brings back with it to the lungs part of the carbon dioxide liberated where oxidation has taken place. Some waste products, to be spoken of later, are also found in the plasma. [Illustration: Human blood as seen under the high power of the compound microscope; at the extreme right is a colorless corpuscle.]