404 THE MUXCLES 



Variations. — Tlie rectus maybe inserted asliighas the i'uurtli or even the third rilt. Ahiteral 

 rectus is sometimes found between the t-xternal and internal oblique muscles, extending from the 

 tenth rib to the iliac crest. 



Transverse and Oblique Muscles 



This group consists of three niiiscles — the ohliquus externus, the obliquiis inter- 

 niis, and the transversalis — which lie iu successive strata in the abdominal wall. 



1. OBLIQUUS EXTERNUS 



The obliquus externus abdominis — named from its position and direction — 

 is a broad curved sheet, partly muscle and partly aponeurosis, of an irregularly 

 Cjuadrilateral shape. • 



Origin. — The outer surface of the eight lower ribs about their middle by a series 

 of nearly horizontal lines which, after crossing each rib obliquely downwards and 

 backwards, extend for a short distance along their lower borders. 



Insertion. — (1) By a strong aponeurosis along the whole of the linea alba; (2) 

 the front of the os pubis close to the symphysis; (3) the spine of the pubes and 

 the adjacent part of the ilio-pectineal line; (4) the deej? fascia of the thigh in a 

 thickened band which stretches from the spine of the pubes to the anterior superior 

 spine of the ilium: (5) the anterior half of the outer lip of the crest of the ilium. 



Structure. — At their origin the muscular fibres form a series of teeth which 

 interdigitate in the upper part of the muscle with the serratus magnus, and in the 

 lower with the latissimus dorsi. The general direction of the origin is an oblique 

 line somewhat curved upon itself so as be convex upwards and backwards. Above 

 and below, the origin is nearer to the anterior extremities of the ribs. From this 

 origin the fleshy fibres pass downwards and forwards, and at the same time diverge 

 fanwise, at first lying upon the ribs and their cartilages, and then without any bony 

 support as part of the muscular wall of the abdomen. The change from fleshy to 

 tendinous iibres takes place at some distance from the outer border of the rectus 

 muscle, in such a w^ay that the fleshy mass terminates rather al)ruptly in a right 

 angle situated in the iliac region of the abdomen. This angle is fcjrmed in front 

 by a vertical line, which passes downwards from the tip of the ninth costal carti- 

 lage; and below by a horizontal line passing forwards from a point upon the crest 

 of the ilium an inch or two (2 '5 to 5 cm.) behind the anterior superior spine. 

 This abrupt limitation of the muscular fibres gives rise to a projection wdiich is 

 distinctly visible through the skin in a muscular sul)ject. The aponeurosis thus 

 formed blends in nearly the whole of its extent with that of the subjacent muscle, 

 the obliquus internus, and in the middle line it unites at the linea alba with that of 

 the opposite side. Above, it extends U])wards as high as the insertion of the rectus 

 muscle, of which it forms part of the sheath, and in this locality it gives origin to 

 a part of the pectoralis major. 



The insertion into the pubic bone of the same side is interrupted l)y an interval 

 corresponding to the crest of the pubes, and forms the external abdominal ring. 

 From the spine of the pubes to the anterior superior spine of the ilium, the 

 aponeurosis forms a thickened band slightly convex downwards, Avhich blends with 

 the fascia lata, and is called Poupart's ligament. The rest of the insertion into 

 the outer lip of tlie crest of the ilium is l)y short tendinous fi])res. 



The insertion of the lower edge of the aponeurosis is also carried backw'ards and 

 outwards from the spine of the pulses along the inner jiart of the ilio-pectineal line 

 by a horizontal triangle of fascia called Gimbernat's ligament. This ligament is 

 attached to the lower end of Poupart's ligament in front, and it i)resents a concave 

 surface upwards, U]ion which lie the structures Avhich emerge through the external 

 al)dominal ring. The fibres of Gimbernat's ligament are continued upwards and 

 inwards beyond the ilio-pectineal line to the front of the rectus and the linea all)a 

 by a layer called the triangular fascia, which lies behind the inguinal canal and 

 the external abdominal ring. 



