OF DIDELPHYS VIRGINIANA. 145 



at the first (outer or " fifth " of ordinary anatomical language), and the other is carried 

 quite across the foot to the fifth (inner or "first"). But this seems to be a result of special 

 teleological modification, rather than a morphological disparity, and should not weigh un- 

 duly against numerous other evidences that the peroneus longus is the homologue of the 

 *^ extensor " (flexor) carpi radialis longior, and the peroneus brevis the homologue of the 

 " extensor " (flexor) carpi radialis brevior. Position, relation, and even function attest the 

 correctness of this view. The peronei, like the so-called radial wrist " extensors," are 

 really and essentially flexors ; and in the ordinary position of the foot, at right angles with 

 the leg, have actually this action, though when the foot is so strongly extended as to be 

 brou<'-ht in a straight line with the leg, they may act as extensors. This difference results 

 solely from the mode of confinement of the tendons at the malleolus, and is purely teleo- 

 logical ; a similar variance in the action of the radial wrist "extensors" would occur, were 

 the tendons of the latter slipped around the head of the radius. The view here taken of 

 these muscles is further supported upon a principle of exclusion, so to speak ; the only 

 other muscles to which the peronei could be referred being satisfactorily homologized with- 

 out reference to the peronei. 



The inner, or ulnar and tibial, muscles next come to be examined. There are two 

 such muscles — a flexor and an extensor — in each limb. In the fore, the " extensor " 

 (flexor) carpi ulnaris proceeds along the inner side and back of the firearm and wrist, 

 to the base of the inner (fifth) metacarpal ; it flexes the wrist, or draws it backward. In 

 the hind, the tibialis anticus (flexor tarsi tibialis, as it has been well named), has entirely 

 correspondent position and relations ; passing over the inner side of the ankle and instep 

 to the internal cuneiform bone, at the base of the inner (fifth — "first" of usual language) 

 metatarsal, it flexes the foot, or draws it forward. Similarly, upon the other side of the 

 fore limb, the "flexor" (extensor) carpi ulnaris proceeds along the inner side and front of 

 the wrist, to the pisiform, extending the wrist or drawing it forward ; and in the hind limb 

 the tibialis posticus (extensor tarsi tibiahs) has the same relations and functions, and vir- 

 tually, if not actually, the same insertion, viz. : the navicular, which is the innermost of the 

 proximal row, here taking the place of the pisiform, which has no tarsal homologue. 



I find the " peroneus tertius " to be one of the most puzzling muscles of the limbs, and 

 believe that others have met with similar difficulty in the attempt to determine its homol- 

 ogy. In the first place, it is not easy to trace it from one animal to another, and so de- 

 termine its general homology ; for it is an inconstant muscle, wanting in many, if not the 

 majority, of^mammals, and when found it is variable in its position, relation and function. 

 Inman,'for instance, the muscle called peroneus tertius is, when present, an insignificant 

 one, looking like a detachment from the long digital extensor, and proceeding to the base 

 of the outer metatarsal. In the opossum the muscle I call, and believe to be, peroneus 

 tertius, is large and important, and the proper long extensor of the little toe ; but I am 

 not fully sati'^fied that this digital extensor is the peroneus tertius (see remarks, foot note, 

 antea, p. 130). Still this is the best identification I can make ; any other involves greater 

 objections. Thus, for example, if this little toe extensor of the opossum be not the third 

 peroneus, it is an extra muscle, that has no correspondent in the hand, nor in many other 

 mammals ; the extensor minimi digiti of the fore limb being quite another thing. Again, 

 upon the same supposition, we should be forced to view the muscle I call extensor digito- 

 rum brevis, as the peroneus tertius, in order to find the latter at all ; a view against which, 



KKM0IB8 BOST. 800. NAT. HIST. 



VOL. II. 87 



