INFRACOSTALES— DIAPHRAGM 399 



5. INFRACOSTALES 



Theinfracostales, orsubcostales — named from their position beneath the ri1)S 

 — form a thin n)usculo-meml)ranous sheet lining the back of the thorax external to 

 the tnljercles of the ribs; l)roader and better developed below, becoming narrower 

 and thinner above. Frequently it consists of only a few bundles of fibres whicli 

 can be distinguished from the internal intercostals by the fact that they are not 

 confined to one intercostal space. 



Origin. — The lower part of the inner surface of the ribs near their angles. 



Insertion. — The upper part of the inner surface of the ribs, each bundle of 

 filires usually passing over one rib to be inserted upon the next higher. 



Structure. — The filjres arise tendinous, run upwards and outwards, and have 

 tendinous insertions. The higher fibres riin more vertically. The lower approach 

 nearer to the vertebral column, arising from the ribs just external to their tubercles. 



Nerve-supply. — The intercostal nerves, which enter their outer surface. 



Action. — To depress the ribs, and assist in expiration. 



Relations. — Externally, the external and internal intercostal muscles; intern- 

 ally, the parietal layer of the pleura, which is separated from them by a thin 

 aponeurosis sometimes called the endothoracic fascia. 



6. THE DIAPHRAGM 



The diaphragm — named from its function as the (^idcpayij-a^ or partition wall 

 betw^een the thorax and abdomen — is a dome-shaped musculo-membranous sheet of 

 a kidney-shaped outline when seen from above, and consists of a pair of muscles 

 with a lateral origin and a central aponeurotic insertion, resembling the two 

 transversales abdominis, which unite in the linea alba so as also to form a single 

 dome-shaped biventral muscle. 



Origin. — By three portions: — 



1. Anterior or sternal portion. — The lower border and back of the ensiform 

 cartilage, and the adjacent part of the back of the anterior aponeurosis of the 

 transversalis abdominis. 



2. Lateral or costal portion. — The lower borders and inner surfaces of the 

 cartilages of the six lower ribs, and sometimes also from the adjacent part of 

 the ribs. 



3. Posterior or vertebral portion. — (1) The ligamentum arcuatum exter- 

 num, a fi])rous thickening of the anterior layer of the lumbar fascia, uhieh stretches 

 from the tip of the transverse process of the second lumbar vertel)ra to the tip of the 

 last rib; (2) the ligamentum arcuatum internum — a fibrous thickening of the 

 iliac fascia, which arches over the upper part of the psoas from the side of the l)ody 

 of the second lumbar vertebra to the tip of its transverse process; (3) the crus of 

 the diaphragm — a strong vertical band, fleshy externally, tendinous internally — 

 arising on the right side from the front of the bodies of the first to the third or 

 fourth lumbar vertebrae, from the intervening vertebral discs, and the anterior 

 common ligament; on the left side, from the bodies of the first to the second or 

 third vertebrte only, as well as the discs and anterior common ligament. 



Insertion. — The front, sides, and back, of the central tendon. 



Structure. — The fibres, arising fleshy from their extensive origin, pass at first 

 vertically upwards, and then arch inwards to be attached to the borders of the 

 central tendon. The sternal filires are the shortest, and they are often separated 

 from the costal portion by a small triangular interval filled with areolar tissue, and 

 giving passage to the superior epigastric vessels. The costal origin forms a series 

 of teeth Avhich do not correspond accurately Avith the number of ribs, some rib 

 cartilages having two teeth attached to them. They interdigitate with the serrations 

 of the transversalis abdominis (fig. 301). The aponeurotic fibres which form the 

 inner portion of the two crura, after arching in front of the abdominal aorta, are 

 continued 1>y fleshy fibres which decussate, and so changing sides form a loop round 

 the oesophagus before joining the central aponeurosis. 



