The cavity of the chest is enlarged from above downwards by the descent of the dia phragm, but this is not all. Its walls are also movable. The ribs may be raised by the con traction of the intercostal and other muscles, and by their rising they push forward the breast-bone. The chest cavity is by this means enlarged from side to side and from before backwards. Thas the cavity of the chest is increased in three directions, from above down wards, from side to side, and from before back wards, by the descent of the diaphragm and by the elevation of the ribs. As a result the expansion of the lungs is made more vigorous. This is the cause of the breathing in of air, of inspiration, which is seen to be essentially a muscular act—the result, that is to say, of a series of muscular contractions. But as soon as the contraction is over, the ribs tend, by their weight, to fall down into their position of rest, the muscles tend to relax, and the lung tissue, which is elastic and has been stretched by the expansion, tends to return to its original unstretehed condition, as an elastic band which has been stretched recovers itself as soon as the stretching force is removed. The result is, the original size of the chest cavity is restored, and a quantity of air is expelled from the lungs.
So expiration is seen to be essentially the result of an elastic recoil, and not active muscular con traction. While this is the mechanism of orclt nary quiet breathing, other forces are brought into play when breathing is difficult, as when the windpipe or bronchial tubes are obstructed, notably as in asthma, where there is spasmodic closure of the bronchial tubes, and in diseased conditions of the lungs, which create an obstacle to their expansion. In such cases many addi tional muscles are called into play, which con tract vigorously for the purpose of pulling up the ribs more completely and compelling the lungs to expand,producing what is called forced inspiration. Thus asthmatic people, who are awakened out of sleep by a spasm, instinctively raise their arms to grasp something above the level of the head. This is for the purpose of obtaining a fixed point of support from which muscles, passing from the arms and shoulders to the chest, may act in forcibly raising the ribs. Again, when the output of air is hindered, forced expiration results. Breathing out ceases to be the result of a mere elastic recoil, and muscles are called into play, such as those pass ' ing up from behind to the lower ribs, whose contraction pulls down the ribs, and those of the abdominal walls, whose contraction presses on the contents of the belly and forces them up against the lower surface of the diaphragm, causing it to ascend.
• Varieties of Breathing. — The two main agents in the production of breathing being the descent of the diaphragm and the elevation of the ribs, the movements visible from the out , side vary according to the predominance of one or the other.
In men and in children the action of the dia phragm is marked. Its descent diminishes the space in the cavity of the belly, presses on the organs therein contained, and causes an out ward movement of the belly walls, so that the movements of its walls are specially noticeable. This is called abdominal or diaphragmatic breathing.
In women the action of the ribs is more strongly marked, and the chest rises and falls more than in man. This is costal (costa, a rib) breathing.
Abdominal breathing is well seen in infants. It ought, therefore, to be plain how the com mon practice of binding children tightly round the belly is extremely injurious, since it inter feres with the natural performance of the res piratory act.
These natural varieties of breathing are altered in various ways by disease. A painful disease of the belly will compel a man to pre vent movement of the abdominal walls as much as possible, and then the costal variety will take its place, while some disease of the chest will often cause a woman to suspend her natural method of breathing, and to adopt the ab dominal form. In this way one is sometimes able to form an idea of the seat of a disease simply by watching the movements of respira tion, and observing whether they are of the kind that would be expected in ordinary states of health.
Facial breathing is the term applied to the movements of the nostrils, seen particularly well when breathing is laboured. It consists in an expansion of the nostrils with each inspira tion, and a return to their previous condition in expiration. In children with bronchitis, or in flammation of the lungs, it is marked. Indeed whenever it is specially noticeable, unless after considerable exertion, it should call attention to the condition of the lungs.
The result of the movements of respira tion is alternately to introduce and to expel from the lungs a certain quantity of air. When a man breathes quietly, 30 cubic inches of air enter the lungs with each inspiration, and 30 cubic inches pass out with each expiration. After taking an ordinary breath a man may, by forced inspiration, by taking the deepest possible breath, introduce into his lungs an ad ditional 100 cubic inches. Again, after breath ing out an ordinary breath (30 cubic inches) one may by an effort expel 100 cubic inches more. But even after the utmost expulsive effort 100 cubic inches remain in the lungs which cannot be expelled. The 30 cubic inches that pass in and out in quiet breathing form what is gilled the tidal air. The 100 extra cubic inches one may draw in with effort are the complemental air, and the 100 extra cubic inches one may expel with effort after breathing out the ordinary amount form the supplemental air. The 100 cubic inches which remain and cannot be expelled form the re sidual air. Thus the lungs of an adult man are capable of containing altogether 330 cubic inches of air, thus composed:— If, then, a person, after taking the deepest possible breath, proceeds to breathe out as much as he can, he expels Making a total of 230 cubic inches, forming what is termed the vital capacity of the chest, after which there remain in the lungs the 100 cubic inches of residual air. The vital capacity is then determined by the amount of air a person can expel with the utmost effort after taking the deepest possible breath, and ought in a healthy adult man of average height and weight to equal 230 cubic inches. In women it is less than in men. There are various forms of instruments for measuring the amount. They are called spirometers, or measurers of breath ing, and are some form of what is familiar to everyone under the name gasometer. The vital capacity varies with height and weight, increas ing with increasing height above the average, and slightly decreasing with increased weight above the average.