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.

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.