OF THE RESPIRATORY MOVEMENTS. 511 



of the air-vesicles, the tissue of the lung itself would be almost certain to 

 give way. This has actually happened in numerous instances; and it consti- 

 tutes a very forcible objection to the use of any apparatus for artificial respira- 

 tion, whose action is that of " insufflation." The complete dependence of the 

 expansion of the Lungs upon the enlargement of the cavity of the chest is well 

 shown by the effect of admission of air into the pleural cavity. When an aper- 

 ture is made on either side, so that the air rushes in at each inspiratory move- 

 ment, the expansion of the lung on that side is diminished, or entirely prevented, 

 in proportion to the size of the aperture. If air can enter through it more rea- 

 dily than through the trachea, an entire collapse of the lung takes place; and 

 by making such an aperture on each side, complete asphyxia is produced. But 

 if it be too small to admit the very ready passage of air, the vacuum produced 

 by the inspiratory movement is more easily filled by the distension of the lungs, 

 than by the rush of air into the pleural cavity; so that a sufficient amount of 

 change takes place for the maintenance of life. This is frequently observed in 

 the case of penetrating wounds of the thorax, in the surgical treatment of which, 

 it is of great importance to close the aperture as completely as possible; when 

 this has been accomplished, the air that had found its way into the cavity is 

 soon absorbed, and the lung resumes its full play. Where one lung is ob- 

 structed by tubercular deposit, or is prevented in any other way from rightly 

 discharging its function, an opening that freely admits air into the pleural cavity 

 of the other side, is necessarily attended with an immediately fatal result ; and 

 in this manner it not unfrequently happens that chronic pulmonary diseases 

 suddenly terminate in Asphyxia, a communication being opened by ulceration 

 between a bronchial tube and the cavity of the thorax. 



544. Of the Respiratory Movements. The dilatation of the Pleural cavity 

 during Inspiration is chiefly accomplished by the contraction of the Diaphragm, 

 which, from the high arch that it previously formed, becomes nearly plane ; in 

 this change of figure, it presses on the abdominal viscera, so as to cause them to 

 protrude, which they are enabled to do by the relaxation of the abdominal mus- 

 cles. In ordinary tranquil breathing (especially in children), the action of the 

 diaphragm is alone nearly sufficient to produce the necessary exchange of air ; 

 but, when a full inspiration is required, the cavity of the chest is dilated late- 

 rally and antero-posteriorly, as well as inferiorly. The enlargement of the chest 

 in both these directions is effected by the elevation of the ribs ; for whilst, in 

 the undilated state of the thorax, the ribs form an angle with their cartilages, 

 which becomes less and less obtuse as we pass from the first rib downwards, the 

 elevation of the ribs tends to bring them and their cartilages more nearly into 

 a line, and thus separates them more widely from the median plane, and at the 

 same time causes them to push forwards the sternum. Owing to the greater 

 length of the lower true ribs, and the greater obliquity of their junction with 

 their cartilages, both these changes are more considerable in the lower part of 

 the thorax than in the upper ; and this is especially the case in adult men, whose 

 respiration has been designated as " inferior costal," whilst in females the mo- 

 bility of the first rib and of the whole of the upper part of the thorax is greater, 

 so that their respiration may be designated as " superior costal." The thoracic 

 muscles whose contraction participates in the ordinary movements of Inspiration 

 are (according to Dr. Hutchinson, Op. cit., p. 1055) the external intercostal, 

 with those portions of the internal intercostals which pass between the cartilages, 

 the levatores costarum, and a portion of the triangularis sterni, all of which 

 have the same action, that of elevating the ribs. On the other hand, the tho- 

 racic Expiratory muscles are the proper costal portion of the znfcrwa^ intercostals, 

 with the infracostales, and a part of the triangularis sterni. The expiratory 

 movement will be assisted also by the abdominal muscles, which antagonize the 

 diaphragm by pressing back the abdominal viscera, and thus causing its ascent 



