Experiment 109. Borrow a stethoscope, and listen to the respiration over the chest on the right side. This is known as _auscultation_. Note the difference of the sounds in inspiration and in expiration. Do not confuse the heart sounds with those of respiration. The respiratory murmurs may be heard fairly well by applying the ear flat to the chest, with only one garment interposed. Experiment 110. Get a sheep’s lungs, with the windpipe attached. Ask for the heart and lungs all in one mass. Take pains to examine the specimen first, and accept only a good one. Parts are apt to be hastily snipped or mangled. Examine the windpipe. Note the horseshoe-shaped rings of cartilage in front, which serve to keep it open. Experiment 111. Examine one bronchus, carefully dissecting away the lung tissue with curved scissors. Follow along until small branches of the bronchial tubes are reached. Take time for the dissection, and save the specimen in dilute alcohol. Put pieces of the lung tissue in a basin of water, and note that they float. The labored breathing of suffocation and of lung diseases is due to the excessive stimulation of this center, caused by the excess of carbon dioxid in the blood. Various mental influences from the brain itself, as the emotions of alarm or joy or distress, modify the action of the respiratory center. Again, nerves of sensation on the surface of the body convey influences to this nerve center and lead to its stimulation, resulting in a vigorous breathing movement. Thus a dash of cold water on the face or neck of a fainting person instantly produces a deep, long-drawn breath. Certain drugs, as opium, act to reduce the activity of this nerve center. Hence, in opium poisoning, special attention should be paid to keeping up the respiration. The condition of the lungs themselves is made known to the breathing center, by messages sent along the branches of the great pneumogastric nerve (page 276), leading from the lungs to the medulla oblongata. 213. Effects of Respiration upon the Blood. The blood contains three gases, partly dissolved in it and partly in chemical union with certain of its constituents. These are oxygen, carbon dioxid, and nitrogen. The latter need not be taken into account. The oxygen is the nourishing material which the tissues require to carry on their work. The carbon dioxid is a waste substance which the tissues produce by their activity, and which the blood carries away from them.