242 



acid and left there for a day, and then placed in pure glycerine 1 ), it 

 will be seen on spreading the muscle out between two sheets of glass 

 that a double segmentation exists (Fig. 1). There is a primary meta- 



meric segmentation marked 

 by a series of transverse 

 tendons (in scriptiones 

 tendineae) which re- 

 semble those of the M. 

 rectus abdominis; and 

 an incomplete secondary seg- 

 mentation which follows the 

 course of the fibres and di- 

 vides the muscle up into a 

 series of areas each of which 

 extends from a rib to the 

 ventral or to the posterior 

 margin of the muscle. The 

 primary segmentation is less 

 perfect at the anterior, ven- 

 tral, and posterior margins 

 of the muscle than within 

 the central region. Thus, 

 anteriorly, there are no trans- 

 verse tendons corresponding 

 to the sixth or seventh ribs; 

 posteriorly, there is none 

 corresponding to the first 

 lumbar vertebra; and ven- 



tj* 



Fig. 1. The right M. obliquus abdominis extern us of a guinea-pig. 

 5, 18 approximate positions of 5th and 13th ribs. XIII, XXI 13th and 21st spinal 

 nerves. 



trally, few of the transverse tendons reach the tendinous sheath which 

 is continued over the rectus muscle. 



Where the primary segmentation is well marked it will be seen 

 that the nerves are distributed chiefly over the middle two-thirds of 

 each segment. In the figure only those nerves are represented which 



1) Excellent acetic - osmic acid methods for making nerve -muscle 

 preparations have been developed by Mays, Zeitschrift für Biologie, 

 Bd. 20, 1884, p. 249; and by Nussbaüm, Verhandlungen der Anatomi- 

 schen Gesellschaft, 1895. 



