PARIETES OF THE ABDOMEN. 16t 



muscle upon the sternum, which appears, physiologically 

 at least, to be a continuation of this muscle ; it is called 

 the rectus sternalis, and is described as being occasionally 

 connected with the sterno-mastoid, instances being re- 

 corded where the sterno-mastoid and rectns were one con- 

 tinuous muscle. This occasional slip, and the high inser- 

 tion of the rectus, is looked upon as corresponding with 

 the long rectus of the lower orders of animals. 



The PYRAMIDALIS MUSCLE, small and triangular in shape, 

 lies upon and inclosed within the sheath of the rectus, and 

 is, as it were, accessory to that muscle, which becomes 

 nlarged at its lower part when it is absent ; it arises from 

 e crest of the os pubis, and, tapering as it goes upward, 

 inserted into the linea alba from two to four inches above 

 e s} r mph3 T sis pubes. This muscle is very commonly 

 ting on one or both sides. 



The rectus muscle should be divided at the umbilicus ; the two 

 ends will be easily dissected up from their posterior cellular attach- 

 ments, and it will then be seen how it is encased by the aponeurosis 

 of the abdominal muscles. 



In front of the upper half of the rectus muscle expands 

 the aponeurotic tendon of the external oblique, and the 

 anterior layer of that of the internal oblique ; behind the 

 upper half are the posterior layer of the tendon of the 

 internal oblique, and the whole of that of the transversalis. 

 Below the umbilicus all the tendons pass in front, while 

 behind, the rectus is separated from the viscera only by the 

 transversalis fascia and the peritoneum. The lower edge 

 of the tendons forming the posterior sheath of the upper 

 half of the rectus muscle assumes a crescentic shape, and 

 is called the semilunar fold of Douglass; the space below 

 this fold, between the peritoneum and the muscle, Retzius 

 has described as a " pre-peritoneal cavity," having proper 

 walls, formed by the transversalis fascia, and destined to 

 accommodate the bladder in its changes of volume, and 

 enabling the recti muscles to act more directly upon that 

 viscus in emptying it of its contents. Below the umbilicus 

 the linea alba becomes more tendinous than above that 

 point, and the peritoneum is manifestly less closely con- 

 nected with the abdominal walls. The epigastric artery, a 

 branch of the external iliac, will be seen penetrating the 

 lower half Of the rectus, and lying loose between it and its 

 sheath for a portion of its course; if well injected, it may 



