The visible movements of breathing vary according to circumstances. In infants the action of the diaphragm is marked, and the movements of the abdomen are especially obvious. This is called abdominal breathing. In women the action of the ribs as they rise and fall, is emphasized more than in men, and this we call costal breathing. In young persons and in men, the respiration not usually being impeded by tight clothing, the breathing is normal, being deep and abdominal. Disease has a marked effect upon the mode of breathing. Thus, when children suffer from some serious chest disease, the increased movements of the abdominal walls seem distressing. So in fracture of the ribs, the surgeon envelops the overlying part of the chest with long strips of firm adhesive plaster to restrain the motions of chest respiration, that they may not disturb the jagged ends of the broken bones. Again, in painful diseases of the abdomen, the sufferer instinctively suspends the abdominal action and relies upon the chest breathing. These deviations from the natural movements of respiration are useful to the physician in ascertaining the seat of disease. 212. The Nervous Control of Respiration. It is a matter of common experience that one’s breath may be held for a short time, but the need of fresh air speedily gets the mastery, and a long, deep breath is drawn. Hence the efforts of criminals to commit suicide by persistent restraint of their breathing, are always a failure. At the very worst, unconsciousness ensues, and then respiration is automatically resumed. Thus a wise Providence defeats the purpose of crime. The movements of breathing go on without our attention. In sleep the regularity of respiration is even greater than when awake. There is a particular part of the nervous system that presides over the breathing function. It is situated in that part of the brain called the medulla oblongata, and is fancifully called the “vital knot” (sec. 270). It is injury to this respiratory center which proves fatal in cases of broken neck. From this nerve center there is sent out to the nerves that supply the diaphragm and other muscles of breathing, a force which stimulates them to regular contraction. This breathing center is affected by the condition of the blood. It is stimulated by an excess of carbon dioxid in the blood, and is quieted by the presence of oxygen. Experiment 108. _To locate the lungs_. Mark out the boundaries of the lungs by “sounding” them; that is, by _percussion_, as it is called. This means to put the forefinger of the left hand across the chest or back, and to give it a quick, sharp rap with two or three fingers. Note where it sounds hollow, resonant. This experiment can be done by the student with only imperfect success, until practice brings some skill.