1867.] MYOLOGY OF IGUANA TUBERCULATA. 771 



The lateral superficial margins of the muscle are indistinguishably 

 blended with the external oblique, which sends oblique tendinous 

 fibres right across the sviperficial surface of the rectus. 



External oblique. This muscle seems to consist of three parts * : — 



(1) The most superficial portion arises by tendinous interdigita- 

 tions with the dorsal extensor muscles, from the last cervical and all 

 the thoracic ribs. It is a very delicate layer, and ends below by 

 blending with the upper margin of the rectus, with which it becomes 

 inseparably united. It is also inserted (by a tongue of muscular 

 fibres) just above the very strong tendon of insertion of the third 

 portion. It has five tendinous intersections. 



(2) The second part arises, beneath the first, by less marked di- 

 gitations from all the thoracic ribs. Passing downwards and back- 

 wards, its most anterior portion is strongly inserted into the fifth 

 thoracic rib ; elsewhere it ends in a delicate aponeurosis, which is 

 connected with the sixth thoracic rib, and blends with the inner 

 surface of the first or most superficial layer of the muscle. 



(3) The third part of the external oblique (fig. 1, E. O., and 

 fig. 13, Ex.0.) arises from the posterior surface of the last thoracic 

 rib and from the lumbar fascia. It is inserted, by a very strong 

 tendon, into the spine of the pubis. 



This muscle appears to continue backwards the upper, or ex- 

 ternal, intercostals. 



The internal oblique is of great extent, lining the whole of the 

 thorax. It arises from the inner surfaces of all the thoracic ribs, 

 beginning by a tendinous aponeurosis which is attached to them 

 along a line nearly corresponding to the outer edge of the dorsal 

 extensor muscular mass. It also takes origin from the lumbar fascia 

 just behind the third part of the external oblique. It is inserted by 

 muscular digitations into the inner surfaces of the sternal ribs and 

 (in the abdomen) into the margin of the rectus. 



This muscle is evidently not continuous with the internal (or 

 sternal) intercostals, as these are superficial to the fleshy insertions 

 of the internal oblique, the fibres of which, moreover, run more 

 parallel to the sternal ribs themselves than do the fibres of the in- 

 ternal intercostals. 



Transversa lis. This muscle is also very extensive, arising by a fascia 

 extending from the pelvis to the last but one cervical rib. Indeed 

 the fascia is continued on into the neck, where it passes beneath the 

 nerves of the brachial plexus, which are external to it, like the ab- 

 dominal nerves which pass between it and the internal oblique. It 

 is inserted into the border of the rectus and the inner sternal part of 

 the thorax. 



Internal intercostals. These extend between the sternal thoracic 

 ribs, reaching vertically to the vertebral ribs, and underlying for 

 some distance the external intercostals. 



External intercostals. The external ones extend between the cer- 

 vical ribs, as well as those of the trunk. In the thorax they only 



* Stannius considers that the external oblique and internal oblique each con- 

 sist of two layers (/oc. cit, p. 104). 



