246 



which are distributed near the external surface of the external oblique 

 muscle, while the main lateral muscle-branches of the 9th, 10th, 1 1th, 



and 12th intercostal (and 



S^|v the hypogastric) nerves are 



.^\'~J distributed nearer the in- 



>c^ / ternal surface of the muscle. 



~^"\ SH The course of these main 



«p: w->T branches is subject to much 



individual variation, both 

 in the chief plane of the 

 muscle and with respect 

 to its two surfaces. The 

 number of nerves supplying 

 the muscle is also subject 

 to variation 1 ). 



From the larger nerve- 

 branches an extensive in- 

 tramuscular plexus arises, 

 similar to that found near 

 the margins of the muscle 

 in rodents. 



There is, as a rule, no 

 trace of the primary seg- 

 mentation characteristic of 

 rodents. On the other hand, 

 the secondary segmentation 

 is fairly well marked. 



Fig. 3. M. obliquus ab- 

 dominis externus of an adult 

 male negro. VI sixth intercostal 

 \ ;„ ■ nerve. XII twelfth intercostal 



-'"" nerve. Two-fifths natural size. 



The bundles of muscle -fibres composing the secondary segments 

 usually anastomose in such a way that one can distinguish no definite 

 compound "units" composed of muscle-fibres grouped together 2 ). In 

 unusually well developed muscles, however, one may find, especially 



1) Bardben, A statistical study of the abdominal and border-nerves 

 in man. American Journ. of Anatomy, Vol. 1, 1902, p. 203—228. 



2) Frohse (Anat. Anzeiger, 1893, p. 321) describes as „primary 

 muscles" quadrilateral units entering into the structure of many complex 

 muscles. 



