OF DIDELPHYS VIRGINIANA. 107 



muscle upon the testicle. There is scarcely more deviation from a right line than would 

 be produced by the passage of the muscle over the brim of the pelvis alone. 



Cremaster (of the female : Ilio-marsupialis). — The same muscle in the female, instead of 

 forming a "round ligament of the uterus," goes to the pouch, and has essentially the same 

 relations to the mammary glands that the male muscle bears to the testicle. " The 

 compressor muscle [of the marsupial mammas in general] arises from the ilium between 

 or near to the lower attachment of the internal oblique and transversalis abdominis ; it 

 passes out of the abdominal ring, bends round the marsupial bone, expands as it turns 

 upward and inward behind the pouch to suri-ound partly by carneous, partly by sclerous 

 fibres, the mammary glands, dividing into as many insertions as there are glands of its 

 own side. This muscle ( ' ilio-marsupialis ' of Cuvier) is the homotype of the cremaster 

 of the male ; and the chief function of the ossification of the internal pillar of the ab- 

 dominal ring (mai'supial bone) is to add the power of the pulley to the compressor of the 

 mammary gland, and effect the requisite change in the course of the contractile fibres." 

 (OwEK, Comp. Anat. iii, p. 769.) The change in direction is much greater in this case 

 than in the instance of the male cremaster, and undoubtedly may have the effect ascribed. 

 The strong powers of suction that the infant opposum has been shown to have, however, 

 might be supposed to render such office unnecessary, or at least of only secondary impor- 

 tance to the function of protruding the nipples from the folds of the integument in which 

 they are invaginated before coming into use ; but both these actions may be fairly attribu- 

 table in some measure to this muscle. 



3fuscles of the pouch. — The formation of the pouch is essentially the same as that of 

 the scrotum; the two are evidently homologous parts. Each consists of a duplication or 

 bagging of loose integument of the abdomen ; in the one case, a loosely pendulous closed 

 sac is formed by eversion and protrusion of a tegumentary fold from within ; in the other, 

 a more closely appressod open pouch results from inversion and invagination of a duplication 

 of skin from without ; in both, the receptacle so formed lodges the most important sexual 

 glands. Contraction of the scrotum and its contents is effected by the cremaster and the 

 dartos ; corresponding actions upon the pouch and mammary glands by the ilio-marsupialis, 

 as has just been seen; but further contractibility than dartoid tissue could afford being 

 evidently required, special muscles make their appearance. The question of the homology 

 of these muscles with the contractile scrotal tissue is one on which I am not prepared to 

 speak ; nor can I give so full an account of the muscles themselves as would be desirable. 

 My opportunities were very unfavorable, all my female subjects having been virgin, with 

 the pouch consequently not fully developed. In my dissections I only succeeded in demon- 

 strating transverse and lengthwise planes of muscular fibre between the layers of integu- 

 ment, without being able to define them with precision. According to Owen's brief descrip- 

 tion, (the only one accessible to me at the time of writing), the marsupium " is composed 

 of a duplicature of integument, of which the external fold is supported by longitudinal 

 fasciculi of the panniculus carnosus, converging below to be implanted in the s^'mphysis 

 pubis. The mouth of the sac is closed by a strong cutaneous sphincter muscle." (Op. 

 cit., p. 770.) This "sphincter" is undoubtedly the transverse fibres I saw, the longi- 

 tudinal ones just mentioned being those of the general cutaneous muscle. 



