RESPIRATION. 



35 1 



above, lies nearer the sternum, nearer the fulcrum, than the point of 

 attachment below. Hence the same action is attributed to them as 

 to the external intercostals : vjz., that they elevate, the cartilages and 

 the anterior extremities of the ribs. 



Expiratory Forces and Muscles. Expiration, as previously 

 stated, is a passive process brought about by the recoil ofj the elastic 

 tissues of the thoracic and abdominal walls, and of the lungs, all of 

 which are stretched and made tense during inspiration. With the 

 cessation of the inspiratory effort the elastic forces, assisted by the 

 weight of the ribs, sternum, and soft tissues, return the thorax to its 

 former condition. It is somewhat uncertain if a normal expiratory 

 movement necessitates active muscle contraction. But after the 

 elastic forces have ceased to act and the normal expiratory movement 

 has been brought to a close, the thorax can be, to a considerable 



v v 



A B 



FIG. 171. DIAGRAM ILLUSTRATING THE ACTION or A, THE EXTERNAL INTERCOSTAL, 

 AND B, THE INTERNAL INTERCOSTAL MUSCLES. V, V. Vertical support. R, R'. 

 Parallel bars. S. Vertical strip representing respectively the vertebral column, 

 two ribs, and sternum. 



extent, still further diminished in all its diameters by the contraction, 

 through volitional effort, of abdominal and thoracic muscles. To 

 this decrease in the capacity of the thorax, as a result of which a much 

 larger volume of air is expelled from the lungs than during passive 

 expiration, the term forced expiration has been given. With the 

 cessation of muscle activity the elastic forces of the now-compressed 

 thoracic walls, aided by the return of upward displaced abdominal 

 organs, at once restore the thoracic walls to the position they had 

 attained at the end of passive expiration. WThe muscles active in 

 forced expiration are the abdominales, the inter -co stales interni, the 

 triangularis sterni, the serratus posticus inferior, and the quadratus 



The externus abdominis arises by a series of muscle slips from the 

 outer surface of the lower eight ribs. After pursuing an oblique 



