to the different sets of muscles with which they act in association. They never act alone, and it is therefore idle to try to ascribe to them any constant specific inspiratory or expiratory action. Generally speaking, the intercostals approximate the ribs, and by this action they stiffen the thoracic wall and help to elevate the thorax when its upper part is fixed, or, when its lower part is fixed, to depress it.

Now, if both the upper and lower margins of the thorax be held firmly by strong muscles, as really occurs in inspirationfrom the action of the quadratus and scaleni-the intercostals cannot approximate the ribs. Under these circumstances the results which follow their contraction will be twofold, viz.: (1) the sternum will be pushed forward, and the antero-posterior diameter of the thorax thus increased; and (2) the spaces between the ribs, which are widened by the other muscles, are kept rigid and prevented from sinking inward when the intrathoracic pressure falls. When acting with the elevators of the ribs both intercostal layers of muscle have an inspiratory effect. But when the elevators of the ribs are passive the intercostals, acting with the anterior abdominal muscles, draw down the ribs, and act as muscles of expiration.

Extraordinary Muscles of Inspiration. For forced breathing a great number of muscles are called into play during the inspiratory effort, as may be seen during pathological occlusion of the air passages, where all the thoracic, cervical, facial, abdominal muscles, and even the muscles of the extremities, are one after another thrown into a recurring spasm before suffocation ends the patient's life.

Among the muscles which lend their aid when more energetic inspiratory movements are required, may be mentioned the sterno-mastoid, which helps the scaleni to elevate the front of the thoracic wall; the pectoral muscles and the great serratus, which assist when the arms are fixed.

The deep muscles of the back, which straighten the spine, must thereby act upon the ribs so as to elevate them and widen the intervals between them. This straightening of the dorsal curve probably helps even in quiet breathing, and no doubt has an

important inspiratory influence in forced respiration. Owing to the ribs being fixed to the sternum in front, they can only be separated laterally when the dorsal curve is lessened, and this tends to approximate the sternum and the vertebræ, thus narrowing the antero-posterior diameter of the thorax. It is in preventing this flattening of the chest that the intercostals are particularly useful; by holding the ribs together they push forward the sternum, when the dorsal curve is extended.

EXPIRATION.

FIG. 153.

During quiet breathing expiration requires no muscular effort, the expulsion of the air from the chest being accomplished by the elasticity of the parts.

A powerful expiratory force is the elasticity of the lungs, which are on the stretch even after a forced expiration, and when distended by inspiration are capable of exerting considerable traction. on the thoracic wall.

The ordinary shape of the elastic walls of the thorax when the muscles are not acting, corresponds with the position at the end of gentle expiration; therefore the resiliency of the muscles, costal cartilages, and other elastic tissues which are stretched during inspiration, tends to restore the ribs to the position of expiration.

The weight of the thorax itself, and the elastic gases in the intestinal tract, which have been com

Shows the position of the Ribs and the

Spinal Column in normal form of the thorax, i. e., that assumed in expiration.

pressed by the diaphragm, may also help in expiration.

After death, when the elasticity of the expiratory muscles is lost, the traction exerted by the lungs on the thorax reduces it

below the size its own elastic equilibrium would tend to assume; when, therefore, air is admitted to the pleural cavity by puncture, the thorax expands slightly as the lungs shrink, and the pressure on the pleural surface becomes equal to that within the bronchi.

In forced expiration, or when the air is used during expiration for any purpose, such as the production of voice, or any blowing movement, a number of muscles are called into action. The only muscles that could be called exclusively special muscles of expiration are the weak triangularis sterni, serratus posticus inferior, and parts of the intercostals; but in all violent and forcible expiratory efforts these are aided by the abdominal muscles forming the anterior wall of the abdomen, which, associated with the intercostals and quadratus lumborum, are the most powerful agents in drawing down the thoracic walls and expelling the air.

FUNCTION OF THE PLEURA.

From what has been already said, it is obvious that by far the greatest amount of enlargement takes place in the lower part of the thorax, while the capacity of the apex changes but little. The increase of capacity in the chest during inspiration takes place practically between the costal wall and the diaphragm (compare Figs. 148, 149). If the lungs and the walls of the thorax were fused together, without the interposition of serous membranes, the different parts of the lungs would have to follow the movements of that part of the thorax to which they are attached. Thus the lower parts of the lung would be much distended during inspiration, and the apices would receive but little addition to their contained air. This condition is often found when disease of the pleura leads to adhesion of the visceral and parietal layers. When such cases live for some time after the pleurisy and the adhesions persist, the air cells of the lower margins of the lungs are commonly found to be distended and bloodless (i. e., local emphysema from habitual over distention); while, on the other hand, the apices become abnormally dense, and the alveoli are contracted and airless.

The surface of the soft elastic lung tissue is normally quite

free, being encased in a serous membrane, the smooth surface of which can slide uninterruptedly and freely over the similar lining of the costal wall. That this motion of the lung actually occurs may be seen from watching the lung through the exposed parietal pleura, or recognized by studying the sounds produced by a roughness of the pleura, such as occurs in inflammation, when a "friction sound" can be detected by the ear.

The lungs move in a definite direction. From the least movable points of the thorax, namely, the apex and vertebral margin, they pass toward the more movable inferior costal and sternal regions. In short, the anterior part of the lungs passes downward and forward to fill up the gap made by the descent of the diaphragm and by the passing of the costal wall upward and forward.

The position of the inferior margin of the lung may be easily recognized by percussion over the liver, and may thus be shown to move up and down with the expiration and inspiration respectively. By percussion we also find that the space between the two lungs in front is increased during expiration and diminished during inspiration, so that the heart is more or less covered by lung, and the præcordial dullness is altered every time we draw a breath.

By means of this free movement of the lungs in the cavities lined by serous membrane the air exerts equal force on the walls of all the air cells whether they are situated in the apex or base of the lung, and the alveoli are all equally filled with air.

If the pleural cavity be brought into contact with the air, either by puncture of the thoracic walls or by rupture of the visceral pleura, the lung, owing to the great elasticity of its tissue, shrinks to very small dimensions, and the pleural cavity becomes filled with air (pneumothorax).

If air be admitted to both pleural cavities so as to produce double pneumothorax, death must ensue, for if the opening remain free the motions of the thorax only alter the quantity of air in the pleural cavity, and cannot ventilate the lungs. This demonstrates the important fact that it is the atmospheric pressure which, having access to them only through the trachea,

maintains the distention of the elastic lungs, and keeps them pressed against the wall of the thorax.

The power with which the lungs can contract when the atmospheric pressure is admitted to the pleura, has been found after death, without inflation, to be six millimetres of mercury, which is probably below the pressure exerted during life, when the smooth muscle of the bronchi is acting and the tubes are free from mucus, for this rapidly collects in the minute air tubes at death, and impedes the outflow of air.

When the lungs are inflated before the pleura is opened, the pressure can easily be made to rise to nearly 1 inches (30 mm. mercury).

From this it would appear probable that when the lungs are stretched by inspiration they exert a negative pressure equal to 30 mm., and when the lungs are in a position of expiration they still tend to contract with a force of 6 mm. mercury.

PRESSURE DIFFERENCES IN THE AIR.

The immediate effect of the increase in capacity of the chest is that a pressure difference is established between the interior of the thoracic cavity and the atmosphere.

The reduction in pressure produced in the lungs and air passages by inspiratory movements, or the increase of pressure accompanying expiration, is very slight during ordinary quiet breathing with free air passages. But the least impediment to the entrance or to the exit of the air at once makes the difference very notable.

It is difficult to obtain an accurate experimental estimate of the variations in the pressure in different parts of the air passages during quiet breathing, because even the most careful attempt to measure the pressure causes an increase which is still further magnified by the sensitive muscular mechanism of the air passages.

The variations in pressure occurring in the pulmonary air are greatest in the alveoli, and gradually diminish toward the larger air tubes, so that they disappear at the nasal orifice, where, if no impediment be placed to the course of the air, the pressure will