308 THE MUSCLES. 



layer— the fascia transversalis. This fascia is not distinctly limited upwards or outwards, 

 below, it is fixed to the crural arch, in the vicinity of the inguinal canal, and seada a layer to 

 tlie surface of the cord spermatic. 



Diaphragmatic Region. 



This is composed of a single muscle, the diaphragm. 



DiAPHEAaM. 



Preparation. Place the subject in the first position; open the abdomen and remove the 



viscera it contains, as well as the large vascular trunks lying upon the sublumbar region ; 

 detach the peritoneum from the fleshy portion of the muscle, in order tu show the digitatians 

 of the latter more distinctly, taking care not to allow the air to enter the thoracic cavity, as it 

 would destroy the tense and concave form of the diaphragm. 



Situation — Direction. — The diaphragm is a vast musculo-aponeurotic partition, 

 separating the thoracic from the abdominal cavity, between which it is placed in 

 an oblique direction downwards and forwards. 



Form. — It is flattened before and behind, elliptical, wider above than below, 

 ooncave posteriorly, and convex anteriorly. 



Structure. — This muscle comprises : 1. A central aponeurotic portion desig- 

 nated the phrenic centre, which is incompletely divided into two leaflets by the 

 pillars or crura — fleshy columns which descend from the sublumbar region. 2. 

 A peripheral or circumferential portion, forming a wide muscular band around 

 the phrenic centre. 



Th^ phrenic centre (also named the speculum ffelmontii, or mirror of Helmont) 

 is composed of white, glistening, radiating fibres which, originating from the 

 pillars, extend in every direction to join the muscular fibres of the peripheral 

 portion. It is pierced, in its right leaflet, by a large opening for the posterior 

 vena cava {foramen dextrum). 



The crura or pillars are two in number — a right and left. The right crus or 

 pillar, the most considerable, is a very thick, fleshy fasciculus which commences 

 under the loins by a strong tendon, united to the inferior common vertebral 

 ligament. It descends to the phrenic centre, to which it gives a heart-shaped 

 appearance. Near its inferior extremity, it presents an opening for the passage 

 of the oesophagus and pneumogastric nerves, in the abdominal cavity (foramen 

 sinistrum). The left pillar is a small triangular fasciculus, partly separated from 

 the preceding by an orifice {hiatus aorticvs) for the transmission of the posterior 

 aorta, vena azygos, and thoracic duct. It also arises from the sublumbar 

 region by a tendon, which is confounded with that of its congener. 



The peripheral muscular portion is continuous, by its concentric border, with 

 the central aponeurosis. Its eccentric border is divided into dentations. Above, 

 and on the left side, it nearly always joins the left pillar ; but on the right side it 

 stops at a certain distance from the corresponding pillar, so that towards this 

 point the phrenic centre is not enveloped by the peripheral portion, and is in 

 contact with the sublumbar region. 



Attachments. — 1. To the bodies of the lumbar vertebrae by the tendons of its 

 two pillars, which tendons are confounded with the inferior common vertebral 

 ligament, 2. By the external contour of its muscular portion, to the superior 

 face of the xiphoid cartilage ^ and the inner face of the last twelve ribs, near 



' This sternal fasciculus sometimes shows a depression, in which the serous membranes of 

 the abdomen and thorax cnme into contact. If this fosette enlarjies, it is converted into an 

 opening through whic^h hernia of tlu- intestine into thu cavity of the chest may take place. 



