CHAI-. ii.] RESPIRATION. 437 



strongly contracted, so as distinctly to raise or at least gi\ 

 very fixed support to the first and second ribs. In the same 

 way the serratus postiens superior, which descends from the li\-.l 

 spine in the lower cervical and upper dorsal regions to the second, 

 third, fourth, and fifth ribs, by its contract ion> rai>-s those ribs. 

 In laboured breathing a function of the lower false ribs, not very 

 noticeable in easy breathing, comes into play. They are !- 

 pressed, retracted, and fixed, thereby giving increased sup|>ort 

 to the diaphragm, and directing the whole energies of that mus- 

 cle to the vertical enlargement of the chest. In this way the 

 serratus posticus inferior, which passes upward from the lumbar 

 aponeurosis to the last four ribs, by depressing and fixing those 

 ribs becomes an adjuvant inspiratory muscle. The quadrat us 

 lumborum and lower portions of the sacro-lumbalis may have a 

 similar function. 



All these muscles may come into action even in breathing 

 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 magnus passing from 

 the scapula to the middle of the first eight or nine ribs, the pec- 

 toralis 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 mus- 

 cles 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 

 scapula; and rhomboidei by fixing the si -apula. may. in the in- 

 spiratory efforts which accompany dyspncea, be brought into 

 play. 



266. Expiration. In normal easy breathing, expiration is 

 in the main a simple effect of elastic reaction. Hy the inspira- 

 tory 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, t In- 

 elasticity of the lungs comes into play and drives out a portion 

 of the air contained in them. Similarly the clastic 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 sternum and costal cartilages 

 causes them to return to their previous position, thus depressing 

 the ribs, and diminishing the dimensions of the chest. When 



