354 THE MUSCLES AND FASCIA 



in it. It is an osseo-aponeurotic aperture, formed by a tendinous arch thrown 

 across the front of the bodies of the vertebra, from the crus on one side to that 

 on the other, and transmits the aorta, vena azygos major, and thoracic duct. 

 Sometimes the vena azygos major is transmitted upward through the right crus. 

 Occasionally some tendinous fibres are prolonged across the bodies of the ver- 

 tebra from the inner part of the lower end of the crura, passing behind the aorta, 

 and thus converting the opening into a fibrous ring. 



The cesophageal opening is situated at the level of the tenth dorsal vertebra ; 

 it is elliptical in form, muscular in structure, and, formed by the decussating fibres 

 of the two crura, is placed above, and, at the same time, anterior, and a little to 

 the left of the preceding. It transmits the oesophagus and pneumogastric nerves 

 and some small oesophageal arteries. The anterior margin of this aperture is 

 occasionally tendinous, being formed by the margin of the central tendon. 



The opening for the vena cava (foramen quadratum) is the highest, about on 

 the level of the disc between the eighth and ninth dorsal vertebrae ; it is quadri- 

 lateral in form, tendinous in structure, and placed at the junction of the right 

 and middle leaflets of the central tendon, its margins being adherent to the wall 

 of the inferior vena cava. 



The right crus transmits the greater and lesser splanchnic nerves of the right 

 side ; the left crus transmits the greater and lesser splanchnic nerves of the left 

 side, and the vena azygos minor. The gangliated cords of the sympathetic 

 usually enter the abdominal cavity by passing behind the internal arcuate 

 ligaments. 



The Serous Membranes in relation with the Diaphragm are four in number: 

 three lining its upper or thoracic surface ; one, its abdominal. The three serous 

 membranes on its upper surface are the pleura on either side arid the serous layer 

 of the pericardium, which covers the middle portion of the tendinous centre. The 

 serous membrane covering its under surface is a portion of the general peritoneal 

 membrane of the abdominal cavity. 



The Diaphragm is arched, being convex toward the chest and concave to the 

 abdomen. The right portion forms a complete arch from before backward, being 

 accurately moulded over the convex surface of the liver, and having resting upon 

 it the concave base of the right lung. The left portion is arched from before back- 

 ward in a similar manner ; but the arch is narrower in front, being encroached upon 

 by the pericardium, and lower than the right, at its summit, by about three-quarters 

 of an inch. It supports the base of the left lung, and covers the great end of the 

 stomach, the spleen, and left kidney. At its circumference the Diaphragm is 

 higher in the mesial line of the body than at either side ; but in the middle of the 

 thorax the central portion, which supports the heart, is on a lower level than the 

 two lateral portions. 



Nerves. The Diaphragm is supplied by the phrenic nerves and lower inter- 

 costal nerves and phrenic plexus of the sympathetic. 



Actions. The Intercostals are the chief agents in the movement of the ribs 

 in ordinary respiration. When the first rib is elevated and fixed by the Scaleni, the 

 External intercostals raise the other ribs, especially their fore part, and so increase 

 the capacity of the chest from before backward ; at the same time they evert their 

 lower borders, and so enlarge the thoracic cavity transversely. The Internal 

 intercostals, at the side of the thorax, depress the ribs and invert their lower 

 borders, and so diminish the thoracic cavity ; but at the fore part of the chest these 

 muscles assist the External intercostals in raising the cartilages. 1 The Levatores 



1 The view of the action of the Intercostal muscles given in the text is that which is taught by 

 Hutchinson (Oycl. of Anat. and Phys., art. " Thorax "), and is usually adopted in our schools. It is, 

 however, much disputed. Hamberger believed that the External intercostals act as elevators of the 

 ribs, or muscles of inspiration, while the Internal act in expiration. Haller taught that both sets of 

 muscles act in common viz. as muscles of inspiration and this view is adopted by many of the best 

 anatomists of the Continent, and appears supported by many observations made on the human subject 

 under various conditions of disease, and on living animals after the muscles have been exposed under 

 chloroform. The reader may consult an interesting paper by Dr. Cleland in the Journal of Anat. and 



