ultimate bronchial tubes. The whole extent of respiratory surface in both lungs has been calculated by Lieberkühn' at fourteen hundred square feet. It is plainly impossible to make a precisely accurate calculation of this extent; but there is every reason to believe that the estimate adopted by Lieberkühn, regarded as approximative, is not by any means an exaggerated one. The great multiplication of the minute pulmonary vesicles, and of the partitions between them, must evidently increase to an extraordinary degree the extent of surface over which the blood, spread out in a thin layer, is exposed to the action of the air. These anatomical conditions are, therefore, the most favorable to its rapid and complete arterialization.

RESPIRATORY MOVEMENTS OF THE CHEST.-The air which is contained in the pulmonary lobules and vesicles becomes rapidly vitiated in the process of respiration, and requires therefore to be expelled and replaced by a fresh supply. This exchange or renovation of the air is effected by alternate movements of the chest, of expansion and collapse, which are termed the "respiratory movements of the chest." The expansion of the chest is effected by two sets of muscles, viz., first, the diaphragm, and, second, the intercostals. While the diaphragm is in a state of relaxation, it has the form of a vaulted partition between the thorax and abdomen, the edges of which are attached to the inferior extremity of the sternum, the inferior costal cartilages, the borders of the lower ribs and the bodies of the lumbar vertebra, while its convexity rises high into the cavity of the chest, as far as the level of the fifth rib. When the fibres of the diaphragm contract, their curvature is necessarily diminished; and they approximate a straight line, exactly in proportion to the extent of their contraction. Consequently, the entire convexity of the diaphragm is diminished in the same proportion; and it descends toward the abdomen, enlarging the cavity of the chest from above downward. (Fig. 71.) At the same time the intercostal muscles enlarge it in a lateral direction. For the ribs, articulated behind with the bodies of the vertebræ, and joined in front to the sternum by the flexible and elastic costal cartilages, are su arranged that, in a position of rest, their convexities look obliquely outward and downward. When the movement of inspiration is about to commence, the first rib is fixed by the contraction of the

' In Simon's Chemistry of Man, Philada. ed., 1846, p. 109.

Fig. 71.

scaleni muscles, and the intercostal muscles then contracting simultaneously, the ribs are drawn upward. In this movement, as each rib rotates upon its articulation with the spinal column at one extremity, and with the sternum at the other, its convexity is necessarily carried outward at the same time that it is drawn upward, and the pa rietes of the chest are, therefore, expanded laterally. The sternum itself rises slightly with the same movement, and enlarges to some extent the antero-posterior diameter of the thorax. By the simultaneous action, therefore, of the diaphragm which descends, and of the intercostal muscles which lift the ribs and the sternum, the cavity of the chest is expanded in every direction, and the air passes inward, through the trachea and bronchial tubes, by the simple force of aspiration.

a

DIAGRAM ILLUSTRATING

After the movement of inspiration is accomplished, and the lungs are filled with air, the diaphragm and intercostal muscles relax, and a movement of expiration takes place, by which the chest is partially collapsed, and a portion of the air contained

THE RESPIRATORY MOVE in the pulmonary cavity expelled. The movement of expiration is entirely a passive

MENTS. a. Cavity of the chest.

b. Diaphragm. The dark out

when collapsed; the dotted lines show the same when expanded.

lines show the figure of the chest one, and is accomplished by the action of three different forces. First, the abdominal organs, which have been pushed out of their usual position by the descent of the diaphragm, fall backward by their own weight and carry upward the relaxed diaphragm before them. Secondly, the costal cartilages, which are slightly twisted out of shape when the ribs are drawn upward, resume their natural position as soon as the muscles are relaxed, and, drawing the ribs down again, compress the sides of the chest. Thirdly, the pulmonary tissue, as we have already remarked, is abundantly supplied with yellow elastic fibres, which retract by virtue of their own elasticity, in every part of the lungs, after they have been forcibly distended, and, compressing the pulmonary vesicles, drive out a portion of the air which they contained. By the

recurrence of these alternating movements of inspiration and expiration, fresh portions of air are constantly introduced into and expelled from the chest.

The whole of the air, however, is not exchanged at each movement of respiration. On the contrary, a very considerable quantity remains in the pulmonary cavity after the most complete expiration; and even after the lungs have been removed from the chest, they still contain a large quantity of air which cannot be entirely displaced by any violence short of disintegrating and disorganizing the pulmonary tissue. It is evident, therefore, that only a comparatively small portion of the air in the lungs passes in and out with each respiratory movement; and it will require several successive respirations before all the air in the chest can be entirely changed. It has not been possible to ascertain with certainty the exact proportion in volume which exists between the air which is alternately inspired and expired, or "tidal" air, and that which remains constantly in the chest, or "residual" air, as it is called. It has been estimated, however, by Dr. Carpenter,' from the reports of various observers, that the volume of inspired and expired air varies from 10 to 13 per cent. of the entire quantity contained in the chest. If this estimate be correct, it will require from eight to ten respirations to change the whole quantity of air in the cavity of the chest.

It is evident, however, from the foregoing, that the inspiratory and expiratory movements of the chest cannot be sufficient to change the air at all in the pulmonary lobules and vesicles. The air which is drawn in with each inspiration penetrates only into the trachea and bronchial tubes, until it occupies the place of that which was driven out by the last expiration. By the ordinary respiratory movements, therefore, only that small portion of the air lying nearest the exterior, in the trachea and large bronchi would fluctuate backward and forward, without ever penetrating into the deeper parts of the lung, were there no other means provided for its renovation. There are, however, two other forces in play for this purpose. The first of these is the diffusive power of the gases themselves. The air remaining in the deeper parts of the chest is richer in carbonic acid and poorer in oxygen than that which has been recently inspired; and by the laws of gaseous dif fusion there must be a constant interchange of these gases between

man Physiology, Philada. ed., 1855, p. 300.

the pulmonary vesicles and the trachea, tending to mix them equally in all parts of the lung. This mutual diffusion and intermixture of the gases will therefore tend to renovate, partially at least, the air in the pulmonary lobules and vesicles. Secondly the trachea and bronchial tubes, down to those even of the smallest size, are lined with a mucous membrane which is covered with a ciliated epithelium. The movement of these cilia is found to be directed always from below upward; and, like ciliary motion wherever it occurs, has the effect of producing a current in the same direction, in the fluids covering the mucous membrane. The

Fig. 72.

air in the tubes must participate, to a certain extent, in this current, and a double stream of air therefore is established in each bronchial tube; one current passing from within outward along the walls of the tube, and a return current

SMALL BRONCHIAL TUBE, showing outward passing from without inward,

along the central part of its cavity. (Fig. 72.) By this

means a kind of aerial circulation is constantly maintained in the interior of the bronchial tubes; which, combined with the mutual diffusion of the gases and the alternate expansion and collapse of the chest, effectually accomplish the renovation of the air contained in all parts of the pulmonary cavity.

RESPIRATORY MOVEMENTS OF THE GLOTTIS.-Beside the movements of expansion and collapse already described, belonging to the chest, there are similar respiratory movements which take place in the larynx. If the respiratory passages be examined after death, in the state of collapse in which they are usually found, it will be noticed that the opening of the glottis is very much smaller than the cavity of the trachea below. The glottis itself presents the appearance of a narrow chink, while the passage for the inspired air widens in the lower part of the larynx, and in the trachea constitutes a spacious tube, nearly cylindrical in shape, and over half an inch in diameter. We have found, for instance, that in the human subject the space included between the vocal chords has an area of only 0.15 to 0.17 square inch; while the calibre of the trachea in the middle of its length is 0.45 square inch.

This disproportion, however, which is so evident after death, does not exist during life. While respiration is going on, there is a constant and regular movement of the vocal chords, synchronous with the inspiratory and expiratory movements of the chest, by

Fig. 75.

which the size of the glottis is alternately enlarged and diminished. At every inspiration, the glottis opens and allows the air to pass freely into the trachea; at every expiration it collapses, and the air is driven out through it from below. These movements are called the "respiratory movements of the glottis." They correspond in every respect with those of the chest, and are excited or retarded by similar causes. Whenever the general movements of respira tion are hurried and labored, those of the glottis become accelerated and increased in intensity at the same time; and when the movements of the chest are slower or fainter than usual, owing to debility, coma, or the like, those of the glottis are diminished in the same proportion.

In the respiratory motions of the glottis, as in those of the chest, the movement of inspiration is an active

HUMAN

LARYNX, POSTERIOR

VIEW. a. Thyroid cartilage. b. Epi

glottis. cc. Arytenoid cartilages. d.

Cricoid cartilage. ee. Posterior crico

arytenoid muscles. f. Trachea.