470 RESPIRATION 



entire respiratory cycle and keeps the lungs in the fullest possible 

 state of expansion. Moreover, their action as tensors immediately 

 assumes a much greater functional importance if the respiratory 

 motions become forced or if the intrathoracic pressures are momen- 

 tarily greatly increased or decreased. Conditions of this kind arise, 

 for example, during the acts of coughing and sneezing and during 

 sudden inspiratory efforts, such as are required during speaking and 

 singing. At this time the contracting intercostal muscles actually 

 protect the thorax and its contents against injury, just because they 

 prevent the outward and inward bellying of its intercostal septa. 



The Expiratory Movement. Expiration, as has been stated above, 

 is largely a passive process in which three factors play a part, namely 

 gravity, the recoil of the stretched tissues, and muscular activity. In- 

 asmuch as the thorax is raised during inspiration, there must be pres- 

 ent a tendency on the part of the ribs, sternum and soft structures to 

 resume their former position on account of their weight. This factor, 

 however, cannot make itself felt until the muscular force acting upon 

 them during inspiration has been made to cease. This is also true of 

 the elastic recoil of the soft and hard parts constituting the thoracic 

 wall, and naturally, this factor makes itself felt in two ways. On the 

 one hand, we have the recoil of the lungs upon which the preceding 

 inspiratory movement has forced a condition of hyperexpansion, 

 and, on the other, the recoil of the cartilaginous and bony constitu- 

 ents of the thorax which by the same means have been placed under 

 a considerable elastic tension. These conditions must necessarily 

 augment one another, because the direction of their action is toward 

 the center of the thorax. The only muscle which participates in the 

 expiratory movement is the internal intercostal, but since quiet 

 breathing is effected very largely by the diaphragm alone, it is doubt- 

 ful whether much importance can be attached to its action, At best, 

 solely the lowermost rows of this muscle would be called into play. 

 Various conditions, however, may arise in which the elastic forces 

 must be promptly and efficiently assisted by this muscle and possibly 

 also by the triangularis sterni. The latter, in all probability, depresses 

 the cartilages and anterior extremities of the ribs. As far as the capac- 

 ity of the thorax is concerned, the three factors just enumerated unite 

 to decrease it along the anteroposterior and transverse planes. 



Essentially the same factors take part in decreasing the vertical 

 diameter of the chest. To begin with, the descent of the diaphragm 

 places the abdominal viscera under pressure with the result that the 

 anterior and lateral walls of the abdomen are forced outward and are 

 put on the stretch. Below, the diaphragm meets with the resistance of 

 the pelvic floor, and posteriorly with that of the vertebral column. 

 Thus, it is commonly noted that this outward movement is also 

 participated in by the floating ribs and the lowermost true ribs, but 

 a pronounced outward bulging of the thoracic-abdominal junction 

 cannot result unless the downward progression of the diaphragm is 



