220 MUSCLES OF THE ABDOMEN. 



portions of the muscle ; the right being the larger and more rounded, 

 and the left smaller and lengthened in its form. 



Between the sides of the ensiform cartilage and the cartilages of the 

 adjoining ribs, is a small triangular space where the muscular fibres of 

 the diaphragm are deficient. This space is closed only by peritoneum 

 on the side of the abdomen, and by pleura within the chest. It is 

 therefore a weak point, and a portion of the contents of the abdomen 

 might, by violent exertion, be forced through it, producing phrenic, or 

 diaphragmatic hernia. 



The lesser muscle of the diaphragm takes its origin from the bodies 

 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 vertebrae ; 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 pncumogastric 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, 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 peri- 

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

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

 renal capsules, the duodenum, and the solar plexus. By its circum- 

 ference 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 abdo- 

 men 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 medium of the lineae 

 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 muscles, and the chief agents of 



