280 me. f. g. paesons on the [Mar. 20, 



In Layostomvs, Hygtrix, and Sphingurus the muscle, on careful 

 dissection, was found not to be continued forwards to the first rib 

 with the rectus. In Aretomys it went not only to the first rib, 

 but also to the junctions of the 2nd, 3rd, and 4th with the 

 sternum. 



Internal Oblique and Transversalis. — These muscles are closely 

 blended, requiring careful examination to make out their separate 

 attachments. 



The internal oblique rises from the lumbar fascia, the crest of 

 the ilium, and a large part of Poupart's ligament. The fibres run 

 forwards and inwards to a few of the posterior ribs and to the 

 linea alba. At the abdominal ring a muscular pouch representing 

 the cremaster supports the testis. There is usually more aponeu- 

 rosis ventrally than in the external oblique. The transversalis 

 rises from the inner surfaces of a large number of the posterior 

 ribs, from the lumbar fascia, from the iliac crest, and from the 

 outer part of Poupart's ligament, and is inserted into the linea 

 alba, passing deep to the rectus in its whole extent. 



In Aulacodv.s, Oetodoa, Dasyprocta, Cavia cobaya, and Sciurus 

 the muscles are practically inseparable, but in Dasyprocta a white 

 line was noticed running downwards and backwards from the 

 cartilage of the last false rib to the outer edge of the rectus, which 

 appeared to mark the place where the internal oblique became 

 aponeurotic. In Layostomus the internal oblique becomes apo- 

 neurotic near the edge of the rectus, forming a linea semilunaris. 

 It passes superficial to the rectus, while the transversalis remains 

 fleshy and passes deep to it. 



In Hy stria; , Sphingurus, Cceloyenys, and Aretomys the muscles 

 are more separable, and the internal oblique rises from the outer 

 three-quarters of Poupart's ligament as well as its other origins, 

 and is inserted into the posterior ribs — into three in Hystrix, five 

 in Sphingurus^ and six in Cceloyenys. 



Rectus Abdominis. — The rectus arises by one bead from the 

 ventral surface of the symphysis pubis and runs forwards between 

 the internal oblique and transversalis, with which it is closely 

 blended, to the ventral surface of the first rib near its junction 

 with the sternum ; it is also inserted into the succeeding four or 

 five costal cartilages at their sternal ends by small slips. It has 

 already been noticed that the external oblique is usually continued 

 forwards with it. In Sphingurus it only reaches as far forwards as 

 the second rib. The lineae transversa are very feebly marked ; 

 they are usually five or six in number, but in Aretomys only three 

 could be made out. 



The Octodontidae are remarkable for having a well-marked 

 decussation of the two recti close to their origin ; this has been 

 pointed out by Owen l and Dobson 2 in Capromys foumieri 

 and C. melanurus, as well as by Martin in Myopotamus 3 and 

 Octodon *. 



1 P. Z. S. 1832, p. 68. a P. Z. S. 1835, p. 176. 



2 P. Z. S. 1884, p. 234. * P. Z. S. 1836, p. 72. 



