THE SUPPLY OF AIR TO THE LUNG To accomplish these ends a ventilation through the respiratory system of about 7 litres of air per minute must be maintained during rest, which may be in creased up to something like ioo litres per minute during violent exercise. The primitive method of ventilation is quite inadequate for the needs of warm blooded animals. The frog carries out the following routine : (I) It fills its mouth with air; (2) closes its nostrils; (3) forces the air in the mouth into the lungs, which become distended; (4) opens the nostrils and lets out the air so that the lung partially collapses. Both the bird and the mammal have invoked the muscles of the body for the purpose of evolving special and efficient mechanisms by which to ventilate the lungs. Their mechanisms are, however, very different, not to say contrary. The bird, like the mammal, possesses a trachea which branches into bronchi, but whereas in the vertebrate each bronchus supplies one lung and that alone, in the birds each bronchus leads not only to a lung but to a series of air sacs which ramify over a great part of the body, even pene trating the bones (see Diverticula in fig. 3), which gives an idea of the size and situation of the air sacs. The function of the air sacs appears to the present author to be incompletely under stood. The following quotation expresses the state of knowledge on the subject:— "In the bird the chest does not exist as a separate chamber. Expiration is effected by the thoracic and abdominal muscles, which compress the thorax and abdomen, driving the air from the air sacs, through the lungs and trachea. Inspiration is effected by the elastic expansion of the thorax and abdomen on relaxation of the muscles; this expansion causes an inrush of air along the trachea and lungs into the air sacs, the lungs being thus filled with fresh air." The above description, given by Marshall and Hurst, refers to the bird at rest; when flying the movements of the wings probably have an important effect on inflating and deflating the chest.
By means of the diaphragm the portion of the body cavity which contains the lungs, the heart and the great vessels is shut off from the rest. The thorax is practically a closed box entirely filled by
the lungs, heart and other structures contained within it. If we were to freeze a dead body until all its tissues were rigid, and then were to remove a portion of the chest wall, we should observe that every corner of the thorax is accurately filled by some por tion or other of its contents. If we were to perform the same operation of removing a part of the chest wall in a body not first frozen we should find, on the other hand, that the contents of the thorax are not by any means in such circumstances bulky enough to fill up the space provided for them. If we were to measure the organs carefully we should find that those which are hollow and whose cavities communicate with the regions outside the thorax are all larger in the frozen corpse than in that which was not frozen. In other words the organs in the thorax are distended somewhat in order that they may completely fill the chest cavity; and the nature of this curious and important condition may best be illustrated by the simple diagrams figs. 4 and 5 (from Her mann's Physiologie des illenschen) where t is the trachea, 1 the lung, v the auricle of the heart, k the ventricle, i an intercostal space with its flexible membranous covering. When the interior of the vessel is rendered vacuous by exhaustion through the tube o, the walls of the lungs and heart are expanded until the limits of the containing vessel are accurately filled, while all flexible portions of the walls of the vessel (corresponding to the intercostal mem branes of the diaphragm of the thorax) are sucked inwards.
From this description it follows that the lungs, even when the thorax is most contracted, are constantly over-distended, and that when the cause of this over-distension is removed, the lungs, being elastic, collapse. It further follows that if the thorax is dilated, the flexible hollow organs it contains must perforce be still more distended—a distention which, in the case of the lungs, is followed by an indrawing of air through the trachea in all cases where the trachea is open. Thus, as the act of respiration is pri marily a dilatation of the thorax, the part played by the lungs is, as Galen knew, a purely passive one.