OF DIDELPHYS VIRGINIANA. I43 



outer, radial, "long" flexor, and an inner, ulnar, "short" flexor ; one bending one of the 

 two parallel hones of the second segment of the arm, the other the other ; they are the 

 " biceps" (flexor cubiti radialis) and the "brachialis anticus" (flexor cubiti ulnaris). Such 

 appears to be the fundamental morphological condition of the cubital flexors, stripped 

 of teleological modification ; which latter, however, may be varied in degree if not in kind. 

 In the opossum, for instance, the long flexor is bipedal instead of bicipital ; its "long head" 

 is deficient, or rather absorbed in the short or coi-acoid head, and one of its feet is im- 

 planted upon the ulna ; but such conditions as these are of obviously little import in a 

 morphological point of view. Now in the system of crural flexors upon the back of the 

 thigh, the same general conditions are met with. There is an outer, fibular, "long" flexor, 

 and an inner, tibial, set of flexors; the latter, however, subjected to a high degree of 

 modification as compared with the corresponding flexor of the arm. Regarding the "bi- 

 ceps" — the flexor cruris fibularis — there is no difiiculty; it is recognizable as the homo- 

 logue of the muscle of the same name in the arm. Its femoral head, when existing, as in 

 the case of man, etc., appears to be, so to speak, a teleological interpolation ; in the opos- 

 sum the muscle exists in what appears to be its normal condition ; arising by a single head 

 from the hoemapophysis to proceed to the outer side of the second segment, without at- 

 tachment in its course to the first segment, of which it is still, as in other cases, the indi- 

 rect short extensor. Some very suggestive corroborative evidence of its relationships to 

 the cubital flexor has been already noticed at p. 127. The case is diflerent with the in- 

 ternal or tibial flexors ; their homology is not so evident. Aside from the fact that there 

 are three such muscles instead of one (which of itself, however, would not constitute a 

 valid objection to their reference to the brachialis anticus), they are not "short" but "long" 

 flexors ; they ai'ise from the pelvic arch and not from the first segment of its appendage ; 

 and there is, moreover, in other animals, if not in the opossum, a muscle (the poplitaeus) 

 which is precisely the internal direct short flexor of the leg, arising from the femur and in- 

 serted into the tibia, and therefore in a teleological sense at least, the exact representative 

 of the brachialis anticus, which Prof Wilder considers it to be. But if the gracilis, semi- 

 tendiuosus and semi-membranosus arose together or individually from the femur instead of 

 the pelvis, I presume that there would be no hesitation on the part of any in referring them 

 collectively to the brachialis anticus ; so that their origin may fairly be held as the only 

 valid objection to such a view of their homology. The observed range of variation of 

 oi'igin of homologous muscles, however, does not seem to indicate that this is an extreme 

 case, and other tests, as of position, relation, etc., tend to confirm the view that these 

 muscles are really homologues of the brachialis anticus ; more especially their unquestion- 

 able relations to the biceps cruris, the homology of which is so indisputable. If this view 

 be not correct, it is certain tliat there is no muscle of the arm to which these important 

 ones of the leg can be referred. I am of opinion, therefore, that the gracilis, semitendino- 

 sus and semimembranosus are the "short" direct tibial flexors, and consequently the homo- 

 logues of the brachialis anticus, notwithstanding that the poplitseus appears to be such 

 homologue. 



The poplitteus, when present, could then only be regarded as an accessory flexor. Its 

 reference to the pronator radii teres, to which it bears some superficial resemblance, could 

 only be made upon the supposition of the homology of the radius with the tibia — a view 

 which, it is scarcely necessary to repeat, I can not endorse. It is most probable that no 



