IQQ THE OSTEOLOGY AND MYOLOGY 



fleshy fibres cease, and thence downward the muscle is inserted into the linea alba by an 

 aponeurosis as broad as the rectus is wide, completing, with its fellow of the opposite side, 

 an oval space. This foscial layer represents the conjoined aponeuroses of transversalis and 

 internal oblique ; it lies wholly inside the rectus. 



Eedus. {''Rectus internus.")— The straight muscle is noteworthy for its extensive thor- 

 acic prolongation, absence of "lineoe transversa;," and situation wholly outside the two 

 preceding. It is a continuous broad flat strip, running the whole length of the trunk from 

 apex of thorax to symphysis pubis. It arises fleshy, about half an inch broad, from the 

 sternal portion of the first rib, and lies along either side of the median line of the body, at 

 first directly upon the ribs and sternum, where it is separated from its fellow by the origin 

 of the pectoralis ; and afterward upon the transversalis and internal oblique aponeurosis. 

 Its thoracic portion is immediately covered, at first by the little " sterno-costalis " (see 

 supra) and then by the pectoralis ; but the greater part of this portion is directly overlaid 

 by the external oblique. In the upper half of its abdominal extent, it is also covered by 

 the same, and intimately adherent thereto ; as well as, mediately, by the third division 

 of the pectoralis. From the termination of the latter to the symphysis, the pyramidalis 

 intervenes between the rectus and the external oblique. The rectus is inserted fleshy 

 into the lower part of the marsupial bone, and the symphysis pubis. Its great size, con- 

 tinuity of contractile tissue, and especially its thoracic prolongation, render it an unusually 

 powerful flexor of the body, in obvious relation to the several indications that have already 

 been pointed out, as requiring fulfilment in the opossum's economy. 



Pyramidalis. {" Bechis externus.'') — Of much the same proportions as in the true 

 Monotremes : similarly developed in both sexes. It is a large triangular plane, wholly 

 muscular, lying between the body of the rectus and the aponeurosis of the external oblique. 

 It arises from the symphysis and whole length of the marsupial bone ; the fibres proceed- 

 ing diagonally upward and inward to be inserted into the linea alba, as high as the origin of 

 the lowest pectoral. This muscle, acting with its fellow, approximates the marsupial bones, 

 and appresses them to the abdomen, which action is furthered by the rectus, and opposed 

 by the three lateral muscles of the abdomen, and also by the pectineus.^ 



Cremaster (of the male). — The spermatic cord, coming from the abdominal scrotum, 

 passes over the base of the marsupial bone, enters the ring at the lower part, and at once 

 divides in three parts. The vas deferens, abruptly changing its course, wdnds around the 

 bone just named and passes inward to the base of the bladder, soon joined by the ureter. 

 The artery and vein take a middle course together, running, generally upon a bed of fat, 

 only about halfway up to the renal vessels, to empty into the aorta and cava (not renal) 

 vein, on either side. The cremaster, only slightly changing its course, runs outward, par- 

 allel with the lower border of the transversalis, and is apparently given off from the latter ; 

 but it is, however, usually traceable in its integrity to the top of the ilium, where it really 

 takes origin. I doubt that the change in the direction of the cremaster, by passing over 

 the foot of the marsupial bone, is sufficient to decidedly augment the traction of the 



1 Tlie lunnaii pyramidalis, small, variable and rudimentary who hazard so absurd a conjecture would do well to look up 



or wantinfr, has been supposed to be present for the purpose their comparative anatomy, and learn that morphology may 



of facilitatiiijr " the expulsion of the last few drops of urine." require presence of parts for its own sake. 

 (See respectable text books of human anatomy.) Writers 



