RESPIRATION 401 



diminished by the recoil of the tense abdominal walls, the replacement of the 

 abdominal organs and the consequent ascent of the diaphragm to its former 

 position. The transverse and antero -posterior diameters are diminished by 

 the descent of the ribs, sternum, and lungs. Coincident with the return of the 

 thoracic walls to their former condition there is a recoil of the elastic tissue 

 of the lungs, in consequence of which there is a compression of the intra- 

 pulmonic air. With its compression there is a rise of pressure above atmos- 

 pheric and at once there is an outflow of intra-pulmonic air until atmospheric 

 pressure is again established at the end of expiration. 



It is somewhat uncertain if a normal expiratory movement necessitates 

 active muscle contraction. If, however, there is any impairment of the 

 elasticity of the lungs or ribs, or any interference with the free exit of the 

 intra-pulmonic air, it is highly probable that the elastic forces are assisted 

 by the internal intercostal and triangularis sterni muscles. It has been in- 

 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, the direction of these muscles as 

 indicated in Fig. 190, diagram A, by the dotted line ii, ii, be considered it 

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

 intercostals that is, it would be to depress the ribs. Employing the Ham- 

 berger model to elucidate the functions of these muscles it is apparent 

 that if the elastic band, ii, ii, recoils, the elastic force of the two halves, though 

 equal, will act in opposite directions, but as the component acting on the 

 long arm of the level preponderates over that acting on the short arm of 

 the lever, the ribs will be depressed. The action of the band is supposed 

 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 H 

 attached and as a further result depressing the anterior extremities c 



the ribs. 



Forced Expiration. After the elastic forces have ceased to act am 

 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 t 

 contraction, through volitional effort, of abdominal and thoracic muse 

 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 cessati^o! musdc 

 activity the elastic forces of the now-compressed thoracic walls, aided by 

 return of the upwardly displaced abdominal organs, at once : 

 thoracic walls to the position they had attained at the end of P"* 

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

 costales interni and the triangularis sterni, the following may be menUo 



.: the abdominales, the serratus posticus inferior, and 



vz. 



" njoint action of these muscles is to diminish the convexity of the 

 abdominal walls and to exert a pressure on the aM0^)^ns. These, 

 taking the line of least resistance, are forced upward against 



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