Effects of Tobacco upon the Circulation.--"The frequent use of cigars or cigarettes by the young seriously affects the quality of the blood. The red blood corpuscles are not fully developed and charged with their normal supply of life-giving oxygen. This causes paleness of the skin, often noticed in the face of the young smoker. Palpitation of the heart is also a common result, followed by permanent weakness, so that the whole system is enfeebled, and mental vigor is impaired as well as physical strength."--MACY, _Physiology_. XXII. RESPIRATION AND EXCRETION _Problems.--A study of respiration to find out:-- (a) What changes in blood and air take place within the lungs. (b) The mechanics of respiration. A study of ventilation to discover:-- (a) The reason for ventilation. (b) The best method of ventilation. A study of the organs of excretion._ LABORATORY SUGGESTIONS _Demonstration._--Comparison of lungs of frog with those of bird or mammal. _Experiment._--The changes of blood within the lungs. _Experiment._--Changes taking place in air in the lungs. _Experiment._--The use of the ribs in respiration. _Demonstration experiment._--What causes the filling of air sacs of the lungs? _Demonstration experiment._--What are the best methods of ventilating a room? _Demonstration._--Best methods of dusting and cleaning. _Demonstration._--Beef or sheep's kidney to show areas. Necessity for Respiration.--We have seen that plants and animals need oxygen in order that the life processes may go on. Food is oxidized to release energy, just as coal is burned to give heat to run an engine. As a draft of air is required to make fire under the boiler, so, in the human body, oxygen must be given so that food in tissues may be oxidized to release energy used in work. This oxidation takes place in the cells of the body, be they part of a muscle, a gland, or the brain. _Blood, in its circulation to all parts of the body, is the medium which conveys the oxygen to that place in the body where it will be used._ [Illustration: Air passages in the human lungs. _a_, larynx; _b_, trachea (or windpipe); _c_, _d_, bronchi; _e_, bronchial tubes; _f_, cluster of air cells.]