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 14 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

remain very nearly equal to that of the atmosphere. By connecting one nostril with a manometer and breathing through the nose with the mouth shut, it can be shown that inspiration causes a negative pressure of about 1 mm. mercury, and expiration a positive pressure of 2 to 3 mm. ; these results must be divided by two, since by plugging one nostril they shut off half the normal inlet. Forced inspiration and expiration give respectively— 57 and 87 mm.

This great difference depends on the elastic forces against which the inspiratory muscles act in distending the thorax, all of which assist in expiration.

THE VOLUME OF AIR.

During ordinary respiration the volume of the inspiratory and expiratory stream of air is surprisingly small when compared with the volume of air sojourning in the lungs.

After an ordinary expiratory act we can force out a great quantity of air by a voluntary effort; but even after this is got rid of the lungs are still well filled. Some of this residual air, which never leaves the chest during the life of the animal, is pressed out by the elasticity of the lungs when the pleura is opened. But a certain amount of air cannot be removed in any way from the alveoli. Even when the lung is cut out of the chest and divided into pieces, enough air is retained in the air cells to render it buoyant. This fact is relied on by medical jurists as an evidence that an infant has been born alive and distended the lungs with air, for except breathing has been well established, the tolerably fresh lung of an infant will sink in

water.

In order to have a clear idea of the volume of air at rest and in motion during pulmonary ventilation, it is convenient to follow the classification from which the nomenclature in common use has been borrowed.

Tidal air is the current of air which passes in and out of the air passages in quiet natural breathing. It amounts to about 500 cc. (30 cubic inches).

Reserve air is that volume which can be voluntarily emitted

after the end of a normal tidal expiration, and which, therefore, during ordinary respiration remains in the lungs; it is estimated at about 1500 cc. (or nearly 100 cubic inches).

Complemental air is that which can be voluntarily taken in after an ordinary inspiration by a forced inspiration; it also amounts to about 1500 cc., but is not used during ordinary breathing.

Residual air is the volume which remains in the lungs after a forced expiration, that is to say, which no voluntary effort can remove from the lungs; it includes the air which leaves the lungs when the pleura is opened after death and the air which persistently remains in them after they have collapsed. This amounts to about 2000 cc. (or about 120 cubic inches).

(4) The work

Vital capacity is a term meaning the greatest amount of air that can be emitted by a forced expiration immediately following a forced inspiration, so that it equals the sum of the tidal, reserve and complemental air. The vital capacity is estimated by spirometers of different kinds, and gives an approximate measurement of (1) The capacity of the chest. (2) The power of the respiratory muscles. (3) The resistance offered by the elasticity or rigidity of the walls of the thorax. ing capacity of the lungs, i. e., their extensibility or freedom from disease. It, therefore, varies greatly according to the age, sex, position of the body, the occupation, weight, height, the fullness of the hollow viscera of the abdomen, and the pathological condition of the lungs. It can be much increased by practice, and this fact, apart from the injury forced respirations may produce in a morbid state of the lung, renders it inapplicable as a gauge of pulmonary disease.

DIFFUSION.

From the foregoing it appears that the volume of air habitually sojourning in the lungs during natural respiration, or stationary air, is about 3500 cc. (nearly 220 cubic inches), while the fresh air introduced by each inspiration is only a little over 500 cc. (30 cubic inches), or, in other words, about one-seventh of the air in the lungs is changed at each breath. Indeed, the 500 cc. of air

is only just sufficient to fill the trachea and larger bronchial passages, so that the fresh air does not reach the pulmonary alveoli, or directly replace any of the air they contain. The tidal stream is, however, brought into immediate relation with the stationary air, and the thoracic movements cause them to mix mechanically, so that rapid diffusion takes place in the minute bronchi. Diffusion is also constantly occurring between the air of the small tubes and the terminal sacs, and it alone suffices to maintain the necessary standard of purity in the air of the alveoli. If the harmless gas, hydrogen, be inhaled during one inspiration, it requires 6 to 10 respirations to get rid of the impurity from the expired air. From this it has been inferred that this number of respiratory acts would be necessary to render the air in the alveoli quite pure even if no fresh impurities were allowed to enter from the blood.

RESPIRATORY SOUNDS.

As the streams of air enter the air passages and lungs they produce sounds which are of the greatest importance to the physician, owing to the manner in which they become altered by disease.

A sound called "bronchial breathing" is produced in the large bronchi and trachea, and is like the noise of air blowing through a tube. This can normally be heard over the trachea, or at the back between the shoulder blades over the entrance of the large bronchi into the root of the lung.

Another sound called "vesicular" can be heard all over the chest, being most distinct where the lung is most superficial, and where other sounds are absent, as in the subaxillary region. It is a gentle rustling sound caused by the air passing into the infundibuli. It varies much with the force of respiration and many other circumstances. In children up to ten or twelve years of age it is remarkably sharp and loud, and is called "puerile breathing."

NERVOUS MECHANISM OF RESPIRATION.

The movements of respiration go on rhythmically without any voluntary effort, and even when we are quite awake they occur almost without our being conscious of them, and repeated varia