THE MUSCULAR SYSTEM 203 



marily of an obliquus internus, an obliquus externus derived 

 from the former, and a rectus db do minis, which runs along 

 each side of the midventral line and is differentiated from 

 fibers belonging to the other two. To these are added during 

 the metamorphosis, an obliquus externus superficialis, formed 

 from the primary externus, and a transver sails, differentiated 

 from the primary internus. These muscles, with the well- 

 known differences in the direction of their fibers, persist in all 

 higher vertebrates. The obliquus externus superficialis of the 

 amphibians is no longer present as an abdominal muscle, but 

 seems to be identical with the two serrati posteriores, superior 

 and inferior, usually treated as muscles of the back although 

 innerved by ventral nerves. They appear in rodents and in 

 some other mammals as a continuous sheet, but in man the 

 two muscular portions (superior and inferior) are separated 

 by a considerable interval, although occasionally connected 

 by a thin tendinous sheet, the rudiment of the intermediate 

 portion. 



The rectus abdominis is the only one that retains the primi- 

 tive longitudinal direction of its fibers, and, undoubtedly cor- 

 related with this, is another primitive character, that of the 

 persistence of some of its original myocommata, the " ten- 

 dinous interscriptions " of human anatomy. Connected with 

 the pubic end of the rectus in mammals is a small muscle, of 

 uncertain occurrence in man, the pyramidalis. This muscle is 

 a rudiment of the pouch muscle of marsupials, in which animal 

 it is extremely well developed, extending from the marsupial 

 bones, upon which it arises, as far anteriorly as the sternum. 

 The cremaster muscle of the mammalian scrotum, noticed more 

 particularly in connection with the decensus testiculorum in 

 Chapter IX, is a derivative of the obliquus internus. The 

 quadratus lumborum belongs also in this place, although its 

 derivation has not yet been worked out. It appears first as a 

 distinct muscle in reptiles and birds, where it is represented 

 by a few fibers associated with the transver satis. In mammals 

 it is large and important. 



The essential elements of the abdominal musculature may be 



