ABDOMINAL CAVITY 535 



of the abdomen proper, the student should, in the next place, direct his 

 attention to the diaphragm the great muscle which constitutes a mov- 

 able partition between the thoracic and abdominal cavities. Strip the 

 peritoneum from its lower concave surface, clean the muscular fibres and 

 the central tendinous expansion towards which they ascend. In making 

 this dissection be careful to preserve the inferior phrenic arteries which 

 ramify upon this aspect of the diaphragm and also the nerves which accom- 

 pany them. 



Diaphragma (Diaphragm). The diaphragm, after the heart, 

 is the most important muscle in the body. It forms the dome- 

 shaped roof of the abdomen, and the highly arched and convex 

 floor of the thorax. It is the chief muscle of respiration. 

 Each respiratory act is accompanied by its descent and ascent, 

 and in this way the capacity of the thoracic cavity is alternately 

 increased and decreased in the vertical direction. The vault 

 or cupola of the diaphragm is higher on the right side than 

 upon the left side of the body. In forced expiration it rises 

 on the right side as high as the upper margin of the fourth 

 rib, close to the sternum ; whereas, on the left side, it only 

 reaches the upper border of the fifth rib. 



The central portion of the diaphragm is tendinous. From 

 this the fleshy fibres will be observed to radiate, and, at the 

 same time, to arch downwards, so as to obtain attachment to 

 the circumference of the lower aperture or outlet of the 

 thorax. In front, it takes origin from the back of the lowest 

 segment of the sternum ; behind, it springs by two powerful 

 partly fleshy and partly tendinous processes, called the crura, 

 from the bodies of the upper three lumbar vertebrae, and upon 

 each side of these from two ligamentous .arches, termed the 

 lumbo-costal arches (O.T. ligamenta arcuata) ; laterally, it arises 

 from the lower six costal arches. 



Anterior Attachment The sternal origin consists of two 

 slips which spring from the back of the xiphoid process. 

 These are separated from each other by a narrow linear 

 interval filled by areolar tissue, and comparable with the wider 

 interval, in the medial plane behind, which separates the two 

 crura of the diaphragm. 



Lateral Attachments. The costal origin consists of six 

 pointed and fleshy slips which spring from the deep surfaces 

 of the lower six costal cartilages on each side. These inter- 

 digitate with the digitations of the transversus abdominis. 

 The sternal and costal origins of the diaphragm, on each side, 

 are separated by a small triangular interval, in which the 



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