CHAP, ii.] RESPIRATION. 323 



ing a function of the lower false ribs, not very noticeable in easy 

 breathing, comes into play. They are depressed, retracted, and 

 fixed, thereby giving increased support to the diaphragm, and 

 directing the whole energies of that muscle to the vertical enlarge- 

 ment 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 quadratus lumborum and lower portions of the sacro- 

 lumbalis may have a similar function. 



All these muscles may come into action even in breathing 

 which, 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 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 hurnerus 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. 



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 is, in the main, simple elastic reaction ; but 

 there is probably some, though possibly in most cases, a very 



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