CHAI>. ii.] RESPIRATION. 547 



which, though deeper than usual, can hardly perhaps be called 

 laboured. When, however, the need for greater inspiratory efforts 

 becomes urgent, all the muscles which can, from any fixed point, 

 act in enlarging the chest, come into play. Thus the arms and 

 shoulder being fixed, the serratus inagnus passing from the scapula 

 to the middle of the first eight or nine ribs, the pectoralis minor 

 passing from the coracoid to the front parts of the third, fourth, 

 and fifth ribs, the pectoralis major passing from the humerus to 

 the costal cartilages, from the second to the sixth, and that portion 

 of the latissimus dorsi which passes from the humerus to the last 

 three ribs, all serve to elevate the ribs and thus to enlarge the 

 chest. The sterno-mastoid and other muscles passing from the 

 neck to the sternum, are also called into action. In fact, every 

 muscle which by its contraction can either elevate the ribs or 

 contribute to the fixed support of muscles which do elevate the 

 ribs, such as the trapezius, levator anguli scapulae and rhomboidei 

 by fixing the scapula, may, in the inspiratory efforts which 

 accompany dyspnoea, be brought into play. 



335. Expiration. In normal easy breathing, expiration is 

 in the main a simple effect of elastic reaction. By the inspiratory 

 effort the elastic tissue of the lungs is put on the stretch ; so long 

 as the inspiratory muscles continue contracting, the tissue remains 

 stretched, but directly those muscles relax, the elasticity of the 

 lungs comes into play and drives out a portion of the air contained 

 in them. Similarly the elastic sternum and costal cartilages are by 

 the elevation of the ribs put on the stretch : they are driven into a 

 position which is unnatural to them. When the intercostal and 

 other elevator muscles cease to contract, the elasticity of the ster- 

 num and costal cartilages causes them to return to their previous 

 position, thus depressing the ribs, and diminishing the dimensions 

 of the chest. When the diaphragm descends, in pushing down the 

 abdominal viscera, it puts the abdominal walls on the stretch : and 

 hence, when at the end of inspiration the diaphragm relaxes, the 

 abdominal walls return to their place, and by pressing on the ab- 

 dominal viscera, push the diaphragm up again into its position of 

 rest. ^-"Expiration then during easy breathing is, in the main? 

 simple elastic reaction; but there is probably some, though 

 possibly in most cases, a very slight, expenditure of muscular 

 energy to bring the chest more rapidly to its former condition. 

 This is, as we have seen, supposed by many to be afforded by the 

 internal intercostals acting as depressors of the ribs. If these do 

 not act in this way, we may suppose that the elastic return of the 

 abdominal walls is accompanied and assisted by a contraction of 

 the abdominal muscles. The triangularis sterni, the effect of 

 whose contraction is to pull down the costal cartilages, may also be 

 regarded as an expiratory muscle. 



When expiration becomes laboured, the abdominal muscles 

 become important expiratory agents. By pressing on the contents 



352 



