THE ABDOMEN 659 



sometimes partially inserted into the side of the ensifoim process 

 of the sternum. 



Nerve-supply. — ^The lower five intercostal nerves and the sub- 

 costal nerve. 



Action. — ^The action is similar to that of the two oblique and 

 transversalis muscles, with the exception that it does not produce 

 lateral rotation of the thorax. 



The muscle is long, flat, and strap-like, its fibres being directed 

 vertically upwards by the side of the linea alba. It is narrow 

 below and broad above, its greatest breadth being about 3 inches. 

 Below the umibilcus the two muscles are very near each other, the 

 interval between them being not more than about | inch. Above 

 the umbilicus, however, the muscles are separated by an interval 

 of about \ inch. Each muscle is marked by tendinous intersections 

 or inscriptions, which cross it in a somewhat irregular manner at 

 certain intervals. They are usually three in number, and are situated 

 as follows : one at the level of the umbilicus, a second opposite the tip 

 of the ensiform process, and a third about midway between these 

 two. Sometimes there is a fourth inscription, which crosses the 

 muscle a little below the umbilicus, but this one is faint, and does 

 not usually extend the whole width. These inscriptions, which do 

 not usually penetrate to the posterior surface, map out the upper 

 part of the muscle into two quadrangular areas, and are firmly 

 boimd to the anterior wall of its sheath. Above the level of the 

 umbiUcus, therefore, the interior of the sheath, anterior to the muscle, 

 is divided into two distinct compeurtments. 



The tendinous inscriptions are the remains of the septa which separated 

 the primitive myotomes or muscle-segments, and by some they are regarded 

 as indicating the abdominal ribs of cert:ain animals, such as the crocodile. 



Pyramidalis Abdominis — Origin. — ^The front of the pubic crest. 



Insertion. — ^The linea alba for 2 inches or more above the sjnm- 

 physis pubis. 



Nerve-supply. — The subcostal nerve. 



Action. — To render tense the linea alba. 



The muscle is sometimes present only on one side ; sometimes it 

 is absent on both sides ; and sometimes it is double on one or 

 both sides. In man it is a small muscle, and vestigial like the 

 plantaris ; but it attains a large size in marsupials, and especially 

 in omithorhynchus. 



Sheath of Rectus Abdominis. — Over about the upper three-fourths 

 of the muscle the anterior wall is formed by the anterior lamina of 

 the internal oblique aponeurosis and the external oblique aponeu- 

 rosis, the two being closely connected ; and the posterior wall is 

 formed by the posterior lamina of the internal oblique aponeurosis 

 and the anterior aponeurosis of the transversalis abdominis, thpse 

 two being likewise closely connected. Over about the lower 

 fourth the anterior wall is formed by the external oblique 

 aponeurosis, the undivided internal oblique aponeurosis, and the 



