RECTUS ABDOMIMS 403 



Insertion. — (1) The anterior surface of the tip of the fifth rib; (2) the front 

 of the costal cartihiges of the fifth, sixth, and seventh ril)s; sometimes also (3) the 

 anterior surface of the base of the ensiform cartilage near its outer border. 



Structure. — The inner head arises tendinous from the other side of the middle 

 line, decussating with its fellow; the outer and stronger head arises by a shorter 

 tendon, and is soon joined l)y the inner head. About an inch (2"5 cm.) above the 

 l")ubes, a fleshy mass is formed, which expands as it ascends into a broad sheet, 

 which l)elow the umbilicus lies close to its fellow of the opposite side. Above, the 

 two muscles arc separated by an interval of at least a cparter of an inch (6 mm.). 

 The insertion, Avhich is by short tendinous fibres, is three or four times the width 

 of the origin. The muscle is also curved considerably forwards to correspond with 

 the convexity of the front wall of the abdomen. At certain intervals, transverse 

 bands of filjrous tissue extend in an irregular zigzag manner across the muscle, 

 especially upon its anterior surface. These are called the lineae transversse, and 

 the transverse depressions which they produce are usually to be seen and felt 

 through the skin. They are generally three or four in number on either side. 

 One is situated opposite the umbilicus; the second opposite the i\\) of the ensiform 

 cartilage; the third half way Ijetween these points; and a fourth is sometimes 

 present which extends incompletely across the muscle at some distance below the 

 uml)ilicus. They are firmly connected with the anterior layer of the strong sheath 

 of the muscle, which will afterwards be described. They do not extend through 

 the whole thickness of the muscle, being deficient behind. 



Nerve-supply. — From the terminal filaments of the anterior branches of the 

 six lower thoracic nerves which enter the muscle on its posterior surface near the 

 outer l)order; and from the ilio-hypogastric branch of the lumbar plexus. 



Action. — (1) By the tendency of the curved bands of the muscle to become 

 straight during contraction, all the viscera contained in its concavity are compressed. 

 It will, therefore, help in defecation, micturition, and parturition; also in expira- 

 tion, and especially in strong expiratory efforts, such as coughing and sneezing. 

 (2) By drawing down the ensiform cartilage and the anterior extremities of the 

 middle ribs, it flexes the thorax upon the pelvis, and at the same time acts as a 

 flexor of the thoracic and lumbar portions of the spine. Acting less strongly, it 

 fixes the sternum, so that the sterno-mastoids by their contraction may flex the 

 head, e.g. in rising from the recumbent position. (3) Taking its fixed point 

 from above, it will draw upwards the pul)ic portion of the pelvis, and so flex the 

 pelvis upon the thorax, as when the lower part of the Ijody is drawn up towards 

 the chest in climbing. 



The linese transversae, Avhich are the remnants of the septa which divide the 

 muscular structure at intervals in the lower vertebrates, and which in the crocodile 

 form the abdominal riljs, have had various uses assigned to them. In the first 

 place, they will tend to keep the muscular fibres in their proper place, and prevent 

 them from being separated so as to allow of ventral hernia. Secondly, they will 

 enaljle the muscle to act not only upon the points of bone which form its direct 

 attachment, but, by means of their connection with the sheath of the muscle and 

 the aponeuroses of which it is formed, they will in some measure dift'use the action 

 of the muscle over the lower ribs and the crest of the ilium. Thirdly, they will 

 ena]»le one part of the muscle to act independently, as for example when the lower 

 part exercises some pressure upon the bladder in micturition. Fourthly, they 

 prevent extensive separation when the muscle is injured. On account of the severe 

 strain to which the muscle is exposed, it is sometimes ruptured. If the muscular 

 fibres extended without interruption from the pubes to the ensiform cartilage, 

 such a rupture would occasion a much wider separation, and conso(|uently much 

 greater disablement than is now found to be the case. 



Relations. — Superficially, the front layer of its sheath above, and below the 

 pyramidahs; deeply, it is separated from the transversalis fascia and peritoneum in 

 the greater part of its course by the posterior layer of its sheath; in its lower 

 fourth it is in contact with the transversalis fascia; and above, it lies on the carti- 

 lages of the fifth to the ninth ribs, and covers the intercostal nnisclcs which lie 

 between them. The deep epigastric artery ascends behind tli<' muscle to join with 

 the superior epigastric branch of the internal mammary. 



