402 THE MUSCLES 



lateral half may be looked U])on as a four-sided figure of Avhich the upper boundary 

 slopes backwards and downwards; the lower, backwards and U]iwards; Avhile the 

 long anterior boundary and short posterior l)oundary are vertical and parallel. 

 The muscles contained in the parietes may be divided into vertical and transverse. 

 The former, three in number, are situated two in front, and one behind; while the 

 latter, also three in numl»er, pass transversely, or with some oljliquity, l)etween 

 the anterior and jiosterior 1;)oundaries of the space. 



The two lateral halves unite in front in a strong filirous Ijand called the linea 

 alba, which stretches from the tip of the ensiform cartilage to the upper part of 

 the symphysis pubis. It is partly formed by vertical fibres stretching Ijetween 

 these two points, but chiefly by the interlacement of the transverse and oblique 

 bands of fibrous tissue which pass between the aponeuroses of the muscles upon 

 either side. In its lower two-fifths it is not more than one-eighth of an inch 

 (3 mm.) broad; in its upper three-fifths it is broader, usually not less than a 

 quarter of an inch (6 mm. ) in width, but in some bodies it may be stretched to a 

 much greater extent. At the junction <^f tlie loAver twd-fiftlis and upper three- 

 fifths is the small fibrous ring of the umbilicus through which pass the remnants 

 of the fcetal vessels. 



Anterior Vertical Muscles 



These are two in numl)er — the ])vramidalis and rectus abdominis. 



1. PYRAMIDALIS 



The pyramidalis (fig. 266) — named somewhat fancifully from its triangular 

 shape — is a fan-shaped sheet of muscular fibre forming a right-angled triangle, of 

 which the shortest side corresponds to the origin, and the other side containing the 

 right angle to the linea alba. 



Origin. — (1) The front of the pubic crest; and (2) the fil)rous structures 

 which cover the front of the body of the os pubis and its symphysis. 



Insertion. — The linea alba at a point about half-way between the pubes and 

 the umbilicus. 



Structure. — Arising by a short tendinous sheet, the fleshy fibres converge as 

 they pass upwards. Those nearer to the middle line ascend vertically, while those 

 which arise near the pubic spine pass obliquely U]iwards and inwards to the 

 tendinous insertion of the muscle into the linea allia three or four inches (8 to 10 

 cm. ) above the symphysis pubis. 



Nerve-supply. — From the eleventh and twelfth thoracic nerves and from the 

 ilio-hypogastric branch of the lumbar plexus, through their terminal filaments 

 which enter the deep surface of the muscle. 



Action. — By its contraction, it pulls upon the linea alba and so upon the lower 

 end of the ensiform cartilage. It will therefore assist the rectus in flexion of the 

 thorax upon the pelvis, or of the pelvis upon the thorax. It can also help feel^ly 

 to compress the abdominal viscera. 



Relations. — Su])erficially, the aponeuroses of the ^ra».srer.sc' al)dominal muscles; 

 deejily, the rectus abdominis, from Avhich it is separated by a thin fibrous lamella. 



Variations. — The height to which this muscle extends is variable. It is often absent on one 

 or both sides ; or it may be double. 



2. RECTUS ABDOMINIS 



The rectus abdominis (fig. 206) — named from its straight direction — is a 

 strong ril>bon-shai)t'(l muscle running vertically on either side of the linea alba 

 from the ])ubes to the ensiform and adjacent costal cartilages. 



Origin. — By two tendons: (1) the outer head from the whole of the crest of 

 the ])ub(_'s; ( 2) the inner head crosses tlu' niiihllc lint' of the l)ody, and arises from 

 the filn'ous structures lying in front of the symphysis. 



