236 



THE ACTION OF THE DIAPHRAGM. 



It represents an arched double cupola or dome-shaped partition, directed towards 

 the chest ; in the larger concavity on the right side lies the liver, while the smaller 

 arch on the left side is occupied by the spleen and stomach. During the passive 

 condition, these viscera are pressed against the under surface of the diaphragm, by 

 the elasticity of the abdominal walls and by the intra- abdominal pressure, so that 

 the arch of the diaphragm is pressed upwards into the chest. The elastic traction 

 of the lungs also aids in producing this result. The greater part of the upper surface 

 of the central tendon of the diaphragm is united to the pericardium. The part on 

 which the heart rests, and which is perforated by the inferior vena cava (foramen 

 quadrilaterum) is the deepest part of the middle portion of the diaphragm during 

 the passive condition. 



Action of the Diaphragm. When the diaphragm contracts, both 

 arched portions become natter, and the chest is thereby elongated from 

 above downwards. In this act, the lateral muscular parts of the 

 diaphragm pass from an arched condition into a flatter form (Fig. 105), 



and during a forced inspiration, 

 the lowest lateral portions, which 

 during rest are in contact with 

 the chest-wall, become separated 

 from it. The middle of the cen- 

 tral tendon where the heart rests 

 (fixed by means of the pericardium 

 and inferior vena cava) takes no 

 share in this movement ; hence, 

 this part is highest in the thorax 

 during a forced inspiration. 



Undoubtedly, the diaphragm is 

 the most powerful agent in in- 

 creasing the cavity of the chest. 

 Britcke, in fact, believes that in 

 addition to increasing the length 

 of the thoracic cavity from above 

 downwards, it also increases the 

 Sagittal section through the second rib transverse diameter of the lower 

 on the right side. This figure shows part O f t h e c l ies t. It presses upon 



that when the arched muscular part , -, i -i i c i 



,,, ,. , the abdominal viscera from above, 



ot the diaphragm contracts, a wedge- 

 shaped space, with its apex clown- and strives to press these out- 

 wards, is formed around the circum- wards, thus tending to push out 

 ference of the lower part of the chest, the ad j oining thoracic wall, 

 so that the chest is enlarged from 



above downwards. If tne contents of the abdomen are 



removed from a living animal, every 



time the diaphragm contracts, the ribs are drawn inwards (Haller). This, of 

 course, hinders the chest from becoming wider below, hence the presence of the 

 abdominal viscera seems to be necessary for the normal activity of the diaphragm. 

 The immense importance of the diaphragm as the great inspiratory muscle is 

 proved by the fact that, after both phrenic nerves (third and fourth cervica nerves) 



Fig. 105. 



