392 TEXT-BOOK OF PHYSIOLOGY. 



sisted upon also that while the recoil of the elastic tissues is effective in the 

 early stages of an expiration, it is ineffective in the later stages. Hence there 

 arises a necessity for muscle assistance. 



The action of the internal intercostals is less clearly understood than that 

 of the external intercostals. If, however, we consider the direction of these 

 muscles as indicated in Fig. 191, diagram, A by the dotted line ii, ii, it would 

 seem that their action would be the opposite of that of the external inter- 

 costals that is, it would be to depress the ribs. By the shortening of the 

 muscles, the two forces, indicated by the direction of the arrows, are equal 

 and opposite, but as the component acting on the long arm of the lever pre- 

 ponderates over that acting on the short arm of the lever, the ribs are 

 depressed. If this is the case these muscles must therefore be expiratory in 

 function. The action of the band is supposed to disclose and illustrate the 

 action of the muscle. 



The triangularis sterni muscle, judging from its anatomic relations, in all 

 probability assists in expiration by depressing the cartilages to which it is 

 attached and as a further result depressing the anterior extremities of 

 the ribs. 



Forced Expiration. 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 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 the upwardly-displaced abdominal organs, at once restore the 

 thoracic walls to the position they had attained at the end of passive expira- 

 tion. Of the muscles active in forced expiration in addition to the inter- 

 cos tales interni and the triangularis sterni, the following may be mentioned, 

 viz.: the abdominales, the serratus posticus inferior, and the quadratus 

 lumborum. 



The conjoint action of these muscles is to diminish the convexity of the 

 abdominal walls and to exert a pressure on the abdominal organs. These, 

 taking the line of least resistance, are forced upward against the inferior 

 surface of the diaphragm, which in consequence becomes more strongly 

 curved and ascends higher into the thorax. The vertical diameter of the 

 thorax is thus diminished. Acting from the pelvis as a fixed point, these 

 muscles will also draw downward and inward the lower end of the ster- 

 num and the lower ribs and diminish the antero-posterior and transverse 

 diameters. 



Movements of the Lungs. As the thorax is enlarging in all its diam- 

 eters during inspiration, through muscle activity, the lungs are correspondingly 

 enlarging in all their diameters, by virtue of their distensibility, through the 

 pressure of the air within them. The lungs must therefore move downward, 

 outward and forward. That this is the case is made evident both by an 

 examination of the lungs through an intercostal space after removal of the 

 skin and intercostal muscles, and by the methods of percussion. The inferior 

 border of each lung descends from the lower border of the sixth to the 



