OF DIDELPHYS VIRGINIANA. 121 



compensate for the want of other thumb extensors. It is the most deep seated of all the 

 extensors, but its tendon speedily becomes the most superficial of any, overlying the ten- 

 dons of the supinator longus and the two radial wrist extensors in its oblique passage 

 across them to gain, and be inserted into, the base of the first metacarpal. The muscular 

 part is pinnatifid, two or three times as long as the tendinous, lying Hat upon the radius and 

 ulna, and taking extensive origin from both these bones, as well as from the interosseous 

 septum. On one side fleshy fibres extend nearly to the wrist ; on the other the tendon 

 runs higher up. The tendon is very stout and flat, and gives off fascial expansions on 

 either side. 



? There is one other muscle upon the forearm that may or may not be con- 

 sidered as the representative of the anconeus. It is a short, stout little bundle of fibres that 

 passes transversely across the inner aspect of the convexity of the elbow, from the inner 

 condyle to the olecranon, filling up the notch or gap that would otherwise be apparent 

 between these two salient points. I make it out to be perfectly distinct from any of the 

 muscular surroundings. Its action must be feeble and very limited. 



The remarkable muscular similarity of the opossum's arm to that of man, as indicated 

 by the muscles just described, is carried out to a surprising extent by the little muscles 

 proper to the hands. The palm is very fleshy; there is a well developed "ball" upon the 

 thumb, and a corresponding massa carnea along the ulnar border, in both of which sub- 

 stantially the same muscles thnt are found in man may be demonstrated. In addition to 

 these, and to the lumbricals already noticed, there are two sets of interossei — palmar and 

 anconal. The fingers are freely movable in all dkections, and the palm may be hollowed 

 from side to side, as well as closed in the ordinary way. 



Adductor polUcis. Adductor minimi digiti. — On stripping off the flexor tendons and 

 lumbricals, we come upon a broad, thin plane of transverse muscular fibres, resting upon 

 the palmar interossei, and covering the whole palm. This has a median raph{3, along the 

 whole length of the third metacarpal, from which and from the carpal bone that the latter 

 articulates with, the muscle takes origin. Passing across the hand, on either side from 

 the median line, the fibres converge to be inserted into, respectively, the inner side of the 

 the phalanx of the thumb and of the little finger, forming the two muscles whose names 

 head this paragraph. They contract the palm from side to side. 



Ah diictor poinds. Flexor hrevis poUicis. — A large flattened mass, forming the chief bulk 

 of the ball of the thumb, arising from the trapezium, annular ligament, base of metacarpal, 

 and inner surface of the appressed cartilaginous spur, and inserted into the base of the 

 first phalanx of the thumb, beside the preceding. A few fasciculi may be demonstrated as 

 partially distinct, though with much the same origin and insertion. These represent the 

 short flexor. 



Opponens pollicis. — ^A small fusiform muscle, perfectly distinct from the preceding, lying 

 upon the extreme radial border of the hand, in intimate relation with the tendon of the 

 flexor ossis metacarpi pollicis, along the first metacarpal ; inserted into the outside of the 

 base of the first pollical phalanx. 



Abductor minimi digiti. — Similar to the corresponding muscle of the thumb ; arising 

 from the pisiform bone, lying along the ulnar side of the fifth metacarpal, and inserted 

 into the outside of the base of the first phalanx of the little finger. A true flexor brevis 

 is scarcely or not demonstrable as distinct. 



MEMOIRS HOST. 80C. NAT. HIST. VOL. II. 31 



