596 PHYSIOLOGY OF RESPIRATION. 



in size of the thorax may also be produced in two ways: First, 

 by forcing the diaphragm farther into the thoracic cavity. This 

 result is obtained, not by any direct action of the diaphragm, but 

 by contracting the muscular walls of the abdomen, the external and 

 internal oblique, the rectus, and the transversus. The contraction 

 of these muscles, which form what has been called the abdominal 

 press, raises the pressure in the abdomen and this, acting upon the 

 under surface of the diaphragm, forces it up into the thorax, pro- 

 vided the glottis is open. If the glottis is kept closed firmly the 

 increased abdominal pressure is felt mainly upon the pelvic organs, 

 and this effect is observed in micturition, defecation, and parturition. 

 Second, by depressing the ribs. The muscles which may be sup- 

 posed to exert this action are as follows: M. inter costales interni. 

 The expiratory action of these muscles, so far as the interosseous 

 portion is concerned, was first definitely shown by Martin, who 

 proved that when they contract they act alternately with the dia- 

 phragm. * M . triangularis sterni or the m. transversus thoracis is found 

 on the interior of the thorax on the anterior wall. Its fibers pass 

 from the sternum, running upward and outward, to be inserted into 

 the third to sixth rib. The expiratory action of this muscle was 

 demonstrated by Hough according to the method of Martin. M. 

 iliocostalis lumborum. The anatomical attachments of this muscle 

 are such as would enable it to depress the ribs; but its functional 

 activity in expiration has not been demonstrated. The m. serratus 

 posticus inferior and m. quadratus lumborum are both placed 

 anatomically, especially the former, so that their contractions 

 serve to depress the ribs. It has been suggested, however, that- 

 they may act in forced inspirations so as to antagonize the ten- 

 dency of the diaphragm to pull the lower ribs inward. Whether 

 they really act with the diaphragm or alternately with it can only be 

 determined by actual experiment. 



Quiet and Forced Respiratory Movements; Eupnea and 

 Dyspnea. Our respiratory movements vary much in amplitude, 

 and the muscles actually involved differ naturally with the extent 

 of the movement. In general, we distinguish two different forms of 

 breathing movements. The ordinary quiet respirations, made 

 without obvious effort, form a condition of respiration designated 

 as eupnea. Difficult or labored breathing is known as dyspnea. 

 It is impossible to draw a sharp line between the two. There are 

 many degrees of dyspnea, and doubtless in quiet breathing the 

 amplitude of the movements may vary considerably before they 

 become distinctly dyspneic. In all conditions of eupnea the chief 

 point to bear in mind is that the expiration is entirely passive. 

 * Martin and Hartwell, " Journal of Physiology, " 2, 24, 1879. 



