RESPIRATION 405 



to disclose and illustrate the action of the muscle. If this is the case the 

 internal intercostal muscles must therefore be expiratory in function. 



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- 

 costales 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 diame- 

 ters 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 

 eleventh rib, inserting itself into the space developed between the thorax 

 and diaphragm as the latter contracts and is drawn away from the former. 

 In consequence of the lateral expansion the anterior border of each lung 

 advances toward the middle line until the heart is almost covered. With 

 the beginning and continuance of expiration the lungs exhibit a reverse 

 movement which continues until they reach their original position. At all 

 times, however, the movements of the lungs are entirely passive and deter- 

 mined by the movements of the thorax. 



The Changes in the Relation of the Thoracic Organs, and in the In- 

 tra-pulmonic and Intra-thoracic Pressures. In the dynamic condition, as 

 previously stated, the relations of the thoracic organs undergo a change as 

 well as the intra-pulm^nic and intra-thoracic pressures. Thus during 

 inspiration the diaphragm descends, the ribs ascend and outwardly rotate 



