400 



THE MUSCLES 



The central tendon, or a]-»oneurosis, forms the suiiniiit of the dome, and is of a 

 similar outhne to the (haphragm, hein<f kidney-shaped, -with the concavity hehind. 

 It approaches nearer the back than the front of the thorax. There is a sHght 

 notching of its outUne in front, which divides it into three parts, so that it has 

 somewhat of a trefoil shape, the right leaflet being the largest, and the left the 

 smallest. Its fibres run in many directions. 



The diaphragm contains three large foramina, for the passage of the vena cava 

 inferior, a?sophagus, and aorta. 



Foramina. — Close to the posterior border of the central tendon at the junction 

 of the right and middle leaflet is a quadrilateral opening Avith rounded angles, the 

 foramen for the vena cava inferior, the outer coat of which vessel is blended with its 

 fibrous edges. The (esophageal opening is oval, with the long diameter directed 

 forwards. It is surrounded by fleshy fil^res, and lies to the left of the middle line, 

 opposite the bod}' of the tenth thoracic vertebra. Through it passes the oesophagus, 

 v,-ith the left vagus nerve in front, and the right behind; also a few small oesophageal 

 Vjranches from the thoracic aorta, on their way to join some small cesojihageal 



Fig. 302. — Diaphragm. 



Opening for vena 

 cava inferior 



Right division of 

 tendon 



Psoas parvus 



Psoas 

 Transversalis 



Quadratus 

 lumborum 



Sternal origin 



Middle division 

 of tendon 



CEsophagus 



Left division of 

 tendon 



Costal origin 



Ligamentum 



areuatum 



internum 

 Left erus 

 Ligamentum 



areuatum 



externum 

 Transverse 



process of 



second lumbar 



vertebra 

 Fourth lumbar 

 vertebra 



branches from the gastric artery. The aortic opening, formed by the union of the 

 crura, is fibrous, and of oval shape. It is situated in front of the twelfth thoracic 

 vertebra, and is completed behind by the anterior common ligament. Through it 

 pass the aorta, the vena azygos major, and the thoracic duct. In the crura on 

 either side are small openings which allow of the passage of the great splanchnic 

 nerves, and the left crus is usually perforated also by the vena azygos minor. 



Nerve -supply. — The two phrenic nerves, chiefly derived from the cervical plexus 

 of each side tlirough the fourth cervical nerves, break up close to the diajihragm 

 into many filaments, which penetrate the muscular structure near the anterior 

 border of the central tendon, and are distriljuted to the under surface of the 

 muscle. Sympathetic filaments are also given to it from the i)lexuses which 

 accompany the phrenic arteries. 



Action. — To deepen the chest from al)ove downwards, and so produce a move- 

 ment of inspiration. The central tendon, especially the middle part of it, which 

 is closely connected by means of the pericardium with the deej) fascia of the neck, 

 is l;)ut little depressed; but the arched fleshy fibres all around flatten, and so 



