538 ABDOMEN 



Centrum Tendineum (Central Tendon). The central tendon 

 is exceedingly strong. It is composed of tendinous bundles 

 running in different directions, and closely woven together so 

 as to give it a plaited appearance. This is well seen by an 

 inspection of its abdominal surface. In shape the central 

 tendon resembles a trefoil leaf, of which the right lobe is the 

 largest, and the left lobe the smallest. Upon all sides it is 

 surrounded by muscular fibres. Those which spring from 

 the sternum are much the shortest. 



Foramina of the Diaphragm. The continuity of the 

 diaphragm is broken by three large openings, and by some 

 smaller apertures or fissures for the passage of the splanchnic 

 nerves, and the vena hemiazygos. The three main open- 

 ings receive the names of the most important objects which 

 they transmit. They are 



1. The aortic. 



2. The vena caval. 



3. The oesophageal. 



Hiatus Aorticus (the aortic opening] is in the medial plane in 

 front of the first lumbar vertebra, and between the crura of 

 the diaphragm. It is bounded in front by the fibrous band 

 which arches across the middle line and connects the 

 tendinous portions of the two crura. The structures which 

 pass through the aortic opening are (i) the aorta, (2) the 

 thoracic duct, and (3) the vena azygos major in this order 

 from left to right. 



Foramen Vena Cavcz (the vena caval opening) is at a higher 

 level, being situated opposite the disc between the eighth and 

 the ninth dorsal vertebrae, in front and slightly to the right of 

 the aortic opening. It is placed in the back part of the 

 central tendon at the junction between its middle and right 

 lobes. Its form is somewhat quadrangular, and its margins 

 are prolonged upon the walls of the vena cava as it passes 

 through it. The contraction of the muscular fibres of the 

 diaphragm will therefore tend to increase the size of this 

 opening and the calibre of the vein which it holds. 



In addition to the vena cava, one or two minute twigs 

 from the right phrenic nerve may be transmitted through the 

 vena caval opening. 



Hiatus (Esophageus (the cesophageal opening] is an oval or 

 elliptical foramen in the muscular part of the diaphragm. 

 It lies in front and slightly to the left of the aortic aperture, 



