DIAPHRAGM. 195 



of the lumbar vertebrae by two tendons. The right, larger and longer 

 than the left, arises from the anterior surface of the bodies of the 

 second, third, and fourth vertebras ; and the left from the side of the 

 second and third. The tendons form two large fleshy bellies (crura), 

 which ascend to be inserted into the central tendon. The inner 

 fasciculi of the two crura cross each other in front of the aorta, and 

 again diverge to surround the oesophagus, so as to present the appear- 

 ance of a figure of eight. The' anterior fasciculus of the decussation 

 is formed by the right crus. 



The openings in the diaphragm are three : one, quadrilateral, in 

 the tendinous centre, at the union of the right and middle leaflets, 

 for the passage of the inferior vena cava; a muscular opening of an 

 elliptic shape formed by the two crura, for the transmission of the 

 oesophagus and pneumogastric nerves ; and a third, the aortic, which 

 is formed by a tendinous arch thrown from the tendon of one crus 

 to that of the other, across the vertebral column, beneath which pass 

 the aorta, the right vena azygos, and thoracic duct. The great 

 splanchnic nerves pass through openings in the lesser muscle on 

 each side, and the lesser splanchnic nerves through the fibres which 

 arise from the ligamentum arcuatum internum. 



Relations. By its superior surface with the pleurae, the pericar- 

 dium, the heart, and the lungs. By its inferior surface with the 

 peritoneum ; on the left with the stomach and spleen ; on the right 

 with the convexity of the liver ; and behind with the kidneys, the 

 suprarenal capsules, the duodenum, and the solar plexus. By its 

 circumference with the ribs and intercostal muscles, and with the 

 vertebral column. 



Actions. The external oblique muscle, acting singly, would draw 

 the thorax towards the pelvis, and twist the body to the opposite 

 side. Both muscles, acting together, would flex the thorax directly 

 on the pelvis. The internal oblique of one side draws the chest 

 downwards and outwards : both together bend it directly forwards. 

 Either transversalis muscle, acting singly, will diminish the size of 

 the abdomen on its own side, and both together will constrict the 

 entire cylinder of the cavity. The recti muscles, assisted by the 

 pyramidales, flex the thorax upon the chest, and, through the me- 

 dium of the linae transversae, are enabled to act when their sheath is 

 curved inwards by the action of the transversales. The pyramidales 

 are tensors of the linea alba. The abdominal are expiratory mus- 

 cles, and the chief agents of expulsion ; by their action the foetus is 

 expelled from the uterus, the urine from the bladder, the fasces from 

 the rectum, the bile from the gall-bladder, the ingesta from the sto- 

 mach and bowels in vomiting, and the mucous and irritating sub- 

 stances from the bronchial tubes, trachea, and nasal passages during 

 coughing and sneezing. To produce these efforts they all act 

 together. Their violent and continued action produces hernia ; and, 

 acting spasmodically, they may occasion rupture of the viscera. 

 The quadratus lumborum draws the last rib downwards, and is an 

 expiratory muscle ; it also serves to bend the vertebral column to 



