The external oblique (obliquus externus), besides the usual origin by digitations from the ribs, also arises from the fascia lumborutn ; it is inserted fleshy into the summit of the mar supial hone (a), over which its strong inner tendon is spread; the external oblique becomes aponeurotic at a line continued from the mar supial bone outwards, with a gentle curve, to wards the anterior extremity of the ilium ; and in the opposite direction, or inwards, the car neous fibres of the external oblique terminate in an aponeurosis along a line parallel with the oblique outer margin of the pyramidalis; the fascia continued from the latter boundary of the fleshy fibres passes over, or dermad of, the so called pyramidalis, and meets its fellow at the lines alba ; it is strictly analogous to the anterior layer of the sheath of the rectos in ordinary Mam malia It is seen reflected from the pyranridalis, at b, fig. 112. The aponeurosis continued from the external and inferior boundary of the car mons fibres divides as usual into two distinct portions; one, corresponding to the internal or mesial pillar of the abdominal ring, spreads its glistening fibres, as above described, over the dermal surface of the marsupial bone (c), to which it closely adheres: the other column (d) contracts as it descends obliquely inwards, forms, like Poupart's ligament, the upper boun dary of the space through which the psoas and iliacus muscles and femoral vessels and nerves escape from the pelvis, and is finally inserted, thick and strong, into the outer end of the base of the marsupial bone.
This bone is so connected with the pubis that its movements are almost limited to di rections forwards and backwards, or those con cerned with the dilatation and diminution of the abdominal space ; the contraction of the abdominal muscles must draw the bones in wards so as to compress the contents of the abdomen, and so far as the connections of the bone permit, which is to a very trifling degree, the external oblique may draw it outwards to wards the ilium.
In some Marsupials,as the Koala, the triceps adductor lemons sends a slip of fibres to the external angle of the base of the marsupial bone, and would more directly tend to bend that bone outwards.
The upper or anterior fibres of the internal oblique have the usual origin ; the lower ones ( e) arise fleshy from the outer and anterior spine of the ilium, and for an inch along an aponeurotic chord extended from that process to the upper part of the acetabulum these car neous fibres pass inwards, and slightly upwards, and terminate close to the outer margin of the rectus, where they adhere very strongly to the transversalis, but give off a separate sheet of thin aponeurosis which is lost in the cellular sheath of the posterior rectus.
The fleshy fibres of the transversalis abdomi nis (f) are closely connected by dense cellular tissue with those of the internal oblique ; they are arranged in finer fasciculi, and have, as usual, a more transverse direction ; they termi nate along the same line as those of the internal oblique in an aponeurosis (g), which is con tinued along the inner or central surface of the posterior rectus to the median line. The lower
boundaty of the fleshy fibres of the transversalis is parallel with the line extended transversely between the anterior extremities of the ilia ; a fascia, less compact than an aponeurosis, is con tinued downwards from this margin, and enve lopes the cremaster and the constituents of the spermatic chord, as they pass outwards and forwards beneath the lower edge of the internal oblique.
The pyramidalis ( h ) arises from the whole inner or mesial margin of the marsupial bone, from which the fibres diverge, the lower ones passing transversely across the interspace of the bones, and meeting at a very fine raphe, or Linea alba ; while those fibres from the anterior ends of the marsupial bones gradually exchange their transverse direction for one obliquely for wards. The breadth of each pyramidalis oppo site the upper end of the marsupial bone is more than an inch, the thickness of the muscle one line.
The rectus abdominis or posterior rectus (i) comes off from the pubis along the inner part of the strong ligamentous union of the broad base of the marsupial bone, and ex pands as it ascends until it attains the level of the ensiform cartilage, when the rectus dimi nishes as it is inserted into the sternal extre mities of the ribs reaching to the manuhrium sterni and first rib in the Dasyures, as in the cats.
The slight indications of tendinous inter sections which were noticed in this dissection were confined to the posterior or central su per ficies of the muscle; the first extended only half-way across from the outer margin.
The cremastcr (k), in the Phalanger and Opossum, is not a fasciculus of fibres simply detached from the lower margin of the internal oblique or transversalis, but arises by a narrow though strong aponeurosis from the ilium, within and a little above the lower boundary of the internal oblique, with the fibres of which the course of the cremaster is not parallel ; it might be considered as a part of the trans versalis, but it is separated by the fascia above mentioned from the carneous part of that mus cle. Having emerged from beneath the margin of the internal oblique, the cremaster escapes by the large elliptic abdominal ring (1), bends round the marsupial bone near its free ex tremity, and expands upon the tunica vaginalis testis. In the female it has the same origin, course, and size, but spreads over the mam mary glands at the back of the pouch. If the anterior fascicles of the diverging and em bracing fibres be dissected from the posterior ones, the appearance of the cremaster dividing into two layers is produced.