Mechanics of Respiration.— Breathing con sists in the rhythmical expansion and contrac tion of the walls of the chest, by means of which process air passes into the lungs and thus comes in contact with the blood. The lung structure itself possesses no marked ex pansile power, but it is in a constant state of tension, there being negative pressure in the pleural cavity. The muscles attached to the ribs, Inc/tiding the diaphragm (q.v.), expand the cavity of the chest in all three diameters by raising. the ribs and lowering the diaphragm; the air rushes in and inflates the lungs; the muscles then relax, the elasticity of the ribs causes the chest-wall to contract, the lungs themselves contract somewhat and the air Is forced out through the bronchi and larynx. Thus it is evident that lungs themselves are merely passive agents in inspiration, being filled and emptied of air largely in consequence of the movement of the chest-wall. The control of the muscular movements is partly yoluntary and partly involuntary, and the breathing centre so called is in reality a complex one, or rather a series of centres, all co-ordinated. Thus the motor centres for certain muscles attached to the ribs are located in the spinal cord, and their excitation can bring about restricted move ments of expansit -., while their destruction may result in paralysis of certain of the muscles of respiration. Another breathing centre, the most important one, spoken of as the automatic or true respiratory centre, is found in certain ganglion cells in the floor of the fourth, ven tricle of the medulla oblongata (q.v.). This is the vagus respiratory muscles. Their de struction destroys the co-ordination of the muscular movements and brings about death. A third centre is present in some portion of the fore brain, probably in the frontal lobes. It is the centre that enables one to ((hold the breath,' or to make extra deep breaths, etc. The distinction between conscious and uncon scious breathing is very distinct and yet very subtle. Voluntary influences constantly modify the rate of breathing. While one is asleep these conscious influences are usually temporarily ex cluded, and then the so-called normal rhythm is seen. This is usually from 18 to 20 to the minute in the adult, the ratio of length of in spiration to that of expiration being about 10 to 12. Numerous individual variations may be present, however, the rate varying from infancy to old age, and in diseased conditions the rate is nearly always modified, usually quickened in consequence of the increased oxidation that often' accompanies disease. Occasionally res piration is slowed when there is painful breath ing, or some form of poisoning; opium-poison ing is a noted illustration. In this condition the breathing rhythm may sink as low as four to five to the minute. In children of both sexes the movements of breathing are similar, but as adult conditions develop, and changes in dress become pronounced, it happens that women are more apt to breathe more in the chest (costal breathing), while men make more use of the lower ribs and the diaphragm (dia phragmatic or abdominal breathing). The amount of air taken into the lungs varies widely with the size of the individual, the shape of his chest and a number of other factors that make it impossible to give a general average. It is usual to measure that portion of air that passes in and out of the lungs during normal respiration as tidal air; this has been known to be from 150 to 700 cubic centimeters. In forced breathing a certain extra amount of complemental air can be taken into the chest, its average being cubic centimeters. A certain amount of air (residual air) cannot be expelled from the chest even by forced ex piration. This has been found to vary in amount from 400 to 1,500 cubic centimeters. The vital capacity, or the amount of air which can be expelled from the thorax by forced expira tion following a deep inspiration — a spirometer, or the clung-tester * of the circus, being the instrument for measuring the same — varies very widely according to practice, height, weight and age. For the comparative anatomy of respiration, see ANATOMY, COMPARATIVE, Respiratory System.
Artificial Respiration.— When death is im minent, owing to a cessation of the natural res piration movements, it may sometimes be averted by an imitation of them carried on regularly for a while. Such a condition may occur in disease (for example, asthma, epi lepsy), though very rarely; it is most common in suffocation, either by drowning, choking or strangulation, and is sometimes met with also in poisoning by noxious vapors (for example, carbonic acid, carbonic oxide, coal-gas, chloro form, etc.). In order that any method may have a chance of being successful it is of course necessary that the entrance of air into the lungs be not impeded, whether by a piece of food or by water in the windpipe. or by the
tongue falling back and closing the upper open ing. A piece of food may sometimes be re moved through the mouth by the finger; if this fails, the windpipe should be opened. In those apparently drowned the body should first be laid on the face, with the head low, and the thorax and abdomen be pressed on in order to expel fluids which may have been drawn into the trachea and bronchial tubes. The tongue may need to be held forward,• this may be done by an assistant, or an elastic band passed round the tongue and the chin will effect the object. No general consensus of opinion has yet been arrived at as to which is the best method of producing artificial respiration. The methods most employed fall into three divisions: (1) insufflauon, or blowing of air into the lungs, either by the mouth or by means of bel lows; (2) manual methods, in which external manipulations of the chest-walls are made to effect the entrance and exit of air; (3) elec trical stimulation of the respiratory muscles. In all cases where artificial respiration is re quired every movement is of importance. It is doubtful whether life can ever be restored when the heart has ceased to beat for more than a few seconds; and when breathing has stopped failure of the heart's action is always imminent. That method is, therefore, best which can be applied with the least possible loss of time, so that under ordinary circumstances the methods which require bellows or electric batteries are out of the question. Direct insufflation, or blow ing of air into the patient's lungs by the mouth applied to his mouth, is now hardly ever used except in the case of very young children, al though within recent years specially devised pumps have been made that are at times very serviceable. Of the manual methods those most in use are Marshall Hall's (1856), Sil vester's (1857) and Howard's (1877). The second is certainly the most easy to learn, but is more fatiguing to carry out for a length of time than either of the others. In Marshall Hall's method the body is laid upon its face and rolled (in what may be termed cradle fashion' from this position on to one side and a little beyond it (inspiration), and then back on to the face (expiration). In Silvester's method the patient is laid on his back on a plane, inclined a little from the feet upward, and the shoulders are gently raised by a firm cushion placed under them, which also throws the head back. The operator then grasps the patient's arms just above the elbows and raises them till they nearly meet above the head. This action imitates inspiration. The patient's arms are then turned down and firmly pressed for a moment against the sides of the chest.
A deep expiration is thus imitated. In Howard's method the patient is laid on his back with a cushion below the middle. The operator kneels astride his hips, places his hands with fingers spread outward over the lower part of the chest-wall and alternately bends forward, throwing his weight on the chest to imitate expiration, and springs back to allow the elastic recoil of the chest-wall to imitate inspiration.
Whatever method be adopted, the move ments must be gentle, regular and perseveringly carried out, at the rate of from 10 to 15 times in the minute; and when the faintest natural effort at respiration is observed they must at once be timed so as to reinforce and not to oppose it. In some cases life has been restored under artificial respiration when no respiratory movements have occurred for an hour or even several hours. In all cases, but especially in that of persons apparently drowned, artificial respiration should be conducted in a warm at mosphere, 90° F., or even more if possible, and should be supplemented by warmth applied to the body and by vigorous friction. In those apparently drowned recovery is very rare after complete immersion for as much as•five minutes. If stunning or fainting has occurred at the moment of immersion, so that the respiratory movements have been annulled or much dimin ished for the time, less water will have entered the lungs, and the chance of recovery may be greater. In other modes of death by suffoca tion, such as choking or strangulation, the ac tion of the heart may continue longer, and restoration to life be, therefore, possible after a longer deprivation of airs In recent years a variety of instruments, pulmometers, have been constructed to do the work of artificial res piration. These are widely used in industrial asphyxiation from fires, etc. See ANATOMY; ASPHYXIA; DROWNING; HEMOGLOBIN; LUNGS.
Consult Bert, (Lecons sur la I'hysiologie comp. de la (1870) ; Foster, on the History of Physiology' (1901) ; Bayliss, (General Physiology' (2d ed., 1918).