THE MOVEMENTS OF BREATHING.
If a healthy person be observed it will be noticed that the act of respiration or breathing consists of a regular series of movements. With the taking in of breath the chest rises and the belly is slightly pushed outwards. That is immediately followed by the falling of the chest, the return of the belly to its former size, and the output of air.. Succeeding this is a pause, and the same set of movements is thereafter repeated. The first act is called Inspiration (Latin, in, and Spiro, I breathe), and the second is expiration (Latin, ex, out, and spiro, I breathe). What is, therefore, called the rhythm of respiration consists of three parts, inspiration, expiration, pause, one after the other in regular order, the three parts forming one respiration. In an adult healthy man the number of respirations should be about 16 per minute, but the number varies with age, that of a newly-born child being 44 (see p. 41). During the time of one respiration there should be about 4 beats of the heart. Besides age, other things affect the number of respirations per minute. Exercise increases the number, while rest diminishes it. The number is smallest during sleep. Mental emotion and excitement quicken the rate; the mere paying attention to one's own breathing affects the rate. Of greatest consequence is the effect of'disease. In fevers the rate is increased and the rapidity of the pulse is increased in proportion. In diseases specially affecting the lAngs, such as bronchitis, pleurisy, consump tion, &c., the rate of breathing is very marked, and usually where the lungs specially are involved, the increased rate of the breath ing is out of all proportion to the increased rapidity of the heart. In such diseases there may be as many as 60 or 70 respirations per minute.
The cause of respiratory movements may be made plain by a simple experiment. Let a glass bell-jar with a neck and open mouth be taken (Fig. 153). Let two small india-rubber bags (b b) be connected to the end of a glass tube (cc), and let them be intro duced into the jar, the glass tube passing through a tightly-fitting india-rubber cork. Then let a leather floor (e) be fixed to the jar, shaped as shown in Fig. 153, so that it may be pushed up into the jar into the position marked e, or pulled down into the position marked d. The floor must also be air-tight.
Now here we have a chamber (ch) entirely shut off from the outside air, and hanging in it are two bags, whose cavities have no con nection with that of eh, but communicate with the outside air by the tube cc. The air in the chamber and the air in the bags are both under the same pressure, that of the atmos phere, a little more than 14 lbs. to the square inch. Now suppose the leather floor to be in the position of e; let it be caught by the hand and pulled down to the posi tion of d. By this move ment the cavity of the chamber is »laved in size, and, as it his no communi cation with the outside, no air can enter to occupy the increased space, and conse quently the air already present expands to fill the larger cavity, that is, it be comes rarefied. The result
is that the pressure of air in the interior of the chamber is less than it was before.
But the pressure of air in the inside of the elastic bags (b b) is undiminished, so that the pressure within them is greater than the pressure outside of them, that is in ch. In consequence the bags yield to the pressure of air within them, they expand, air entering by the tube cc, and they continue to expand until their increased size (shown by the dotted lines f f) makes up for the addition to the size of ch., until they occupy the increased space. Next let the leather floor be pushed up ; the size of ch is diminished, the bags are pressed on from within, air is expelled from them, so that they become smaller, and the original state of affairs is restored. Now this is precisely what takes place in the chest and lungs. Let us compare the two things step by step. The chest is a chamber, formed of bony walls, the ribs, connected in front with the breast-bone and behind with the back-bone (see p. 61). The spaces between the ribs are occupied by muscles—the intercostal muscles (Latin, inter, between, and costa, a rib), while large masses of muscle clothe the chest in front and behind, layers of fat and connective tissue covering them, and the skin being outside of all. The chest has for its movable floor the diaphragm already described. It is an air-tight chamber comparable in every respect to ch of Fig. 153, with this additional advantage, that its walls are movable as well as its floor. It has suspended in it the lungs, whose air-cells communicate with the outside by means of the bronchial tubes and windpipe, but have no connection with the general chest cavity, and which are represented on the figure by the elastic bags (b b). This much, however, must be noticed, that the lungs and other contents of the chest completely fill its cavity, and do not simply occupy a small space in it. The contraction of the muscular fibres of the dia phragm causes it to descend, and thus increases the size of the chest cavity. The pressure of air in the lungs is thus made to exceed that in the chest, a quantity of air, consequently, enters the lungs, which expand to fill up the in creased space, just as pulling down the leather bottom of oh causes the elastic bags to distend. As soon as the contraction of the diaphragm is over, it relaxes, returns to its original position, reduces the size of the chest cavity, and air is expelled from the lungs to permit them to diminish in size. After a short pause the dia phragm again descends by contraction of its muscular fibres, and the round of operations is repeated.