THE DIAPHEAGM. 311 



abdomen are separated only by a small quantity of loose connective 

 tissue. The anterior fibres of the diaphragm are much the shortest. 

 The fibres of the sternal and costal slips, after being united, rise in an 

 arched and converging manner to be inserted into the anterior and 

 external margins of the central tendon. 



il. The central tendon — trefoil or cordiform tendon, is a strong aponeu- 

 rosis, forming the central and highest part of the diaphragm. It is 

 elongated from side to side, and consists of three lohes or cdcE; partlv 

 separated by indentations. The right lobe is the largest ; and the 

 left, which is elongated and narrow, is the smallest of the three. The 

 central tendon is surrounded on every side by the muscular portion of 

 the diaphragm, the fibres of which are directly continuous with those of 

 the tendon. The tendinous fibres cross one another, and are inter- 

 woven in various directions. 



Foramina. — There are in the diaphragm three large perforations for 

 the passage respectively of the aorta, the (Dcsophagus, and the vena 

 cava, besides some smaller holes or fissures which are less regular. — 

 a. The foramen for the aorta {hiatus aorticus), placed in front of the 

 vertebrae, is bounded by tendinous fibres of the crura as ah'eady de- 

 scribed. Besides the aorta, this opening transmits the thoracic duct, 

 and generally also the vena azygos. h. The foramen for the oeso- 

 phagus, higher and farther forward than the preceding, as well as a 

 little to its left, is separated from that opening by the decussating 

 fibres of the crura. It is oval in form, and is generally entirely sur- 

 rounded by muscular fibres ; in some rare cases, however, a small part, 

 the anterior margin, is found to be tendinous, being formed by the 

 margin of the central tendon, c. The opening for the vena cava 

 (foramen quadratum) is placed in the highest part of the diaphragm, in 

 the tendinous centre at the junction of the right and middle alte, poste- 

 riorly. Its form is somewhat quadrangular ; and it is bounded by 

 fasciculi of tendinous fibres running parallel with its sides. Besides 

 the foregoing large foramina there are small perforations through the 

 crura for the sympathetic and splanchnic nerves on both sides, and for 

 the vena azygos minor on the left side. Moreover, the larger azygos 

 vein often takes its course through the right crus. 



JRdatiom.— The upper or thoracic surface of the diaphragm is highly arched. 

 Its posterior and lateral fibres, ascending from their connection with the lower 

 margin of the thorax, are for a considerable extent placed close to the ribs, the 

 lungs, especially in their collapsed condition, not descending so far as their 

 attachments. The vault of the diaphragm rises higher on the right than on the 

 left side. On the right side in the dead body it rises to the level of the fifth 

 rib at the sternum, and on the left side only as high as the sixth. This difference 

 has relation to the great size and firmness of the liver on the right side. It 

 is covered superiorly by the pleura and the pericardium ; the fibrous layer of 

 the latter membrane blending with the tendinous centre, as well as with the 

 fascia covering its muscular substance. The lower sui-face, of a deeply concave 

 form, is lined by the peritoneum, and has in apposition with it the liver, the 

 stomach, the pancreas and spleen, and the kidneys. 



Nerves.— The intercostals, subcostals, levatores costai-um and triangularis 

 stemi are supplied by the intercostal nerves. The diaphragm is supplied by the 

 phrenic nerves from the fourth and fifth cervical nerves, and likewise by sjanpa- 

 thetic filaments from the plexuses round the phrenic arteries. 



Actions.— Movements of Respiration.— The mechanical act of respii-ation 



