54 U.S. NATIONAL MUSEUM BULLETIN 2 73 



The description of this muscle by Sirena (1871) agrees in all details 

 with mine. Nevertheless, his explanation of the contribution of this 

 muacle to the rectus sheath is somehow obscured by wordiness. He 

 indicates that a portion of the muscle corresponds to Gimbernat'js 

 ligament (lig. lacunare). In my opinion, the fibers so identified are 

 those which in the specimens I studied make the two crura continuous 

 below the superficial inguinal ring. They are inserted on the pectineal 

 line, but do not show the arrangement which in the human is at- 

 tributed to this ligament. Hill's account (1962) of the broad abdominal 

 muscles in the howling monkey seems to follow that of Sirena (1871). 



Nerve supply: The muscle is innervated by twigs from the corre- 

 sponding lateral cutaneous branches of the intercostal, subcostal, and 

 iliohypogastric nerves. 



M. ohliquus internus abdominis: It is also a partly muscular and 

 aponeurotic sheet extended between the ribs and coxal bone in one 

 direction, the linea alba and the thoracolumbar fascia in the other. It 

 arises (1) caudally by short tendinous fibers from the outer half of the 

 free border of the external oblique aponeurosis where this is fused to 

 the fascia iliaca, (2) by fleshy fascicles from the intermediate lip of the 

 acetabular border between the origin of m. sartorius and the pyramidal 

 process, and (3) the origin of the muscle is extended by a fibrous 

 aponeurosis to the side of the thoracolumbar fascia (fig. 11). All 

 muscular elements pass ventrally in a cephalomedial direction, except 

 those from the caudal fifth which arching toward the pubis contribute 

 to form the roof of the inguinal canal. The fleshy fibers end (1) on the 

 distal border of the caudal eight or seven ribs and (2) along a semilunar 

 line at some distance from the rectus abdominis. The aponeurotic 

 lamina here formed contributes to the rectus sheath in various man- 

 ners. In one male (fig. 18a) it passes entirely behind the rectus ab- 

 dominis but sends also a weak subdivision in front of that muscle in 

 the lower third of the abdominal wall. In another male (fig. 18b) the 

 aponeurosis divides in the upper three-fourths of the abdomen into 

 two parts, one strong sheet going behind the rectus and the other, a 

 very feeble one, in front. In the lower fourth the whole lamina passes 

 in front. The aponeurosis of insertion of the internal oblique reaches 

 the linea alba behind the rectus abdominis throughout the abdomen in 

 the female and the remaining male (fig. 18c). This extensive lamina is 

 fixed to the last costal cartilage and xiphoid process at one end, to 

 the pubic crest at the other. 



M. transversus abdominis: It is weaker than the overlying oblique 

 from which it is not easy to separate. Its caudal origin is by means of 

 short tendinous fibers from the outer half of the free border of the 

 external oblique aponeurosis. Fleshy bundles arise in addition from the 

 inner lip of the entire acetabular border, from the thoracolumbar fascia 



