OF DIDELPHYS VIRGINIANA. 147 



two, the inner (ulnar) part of the muscle has four of the tendons for the four fingers, and 

 the outer (radial) part has one tendon for the thumh. Likewise, in the leg, the originally 

 five-tendoncd flexor is differentiated into two muscles, an inner (tibial) muscle with one 

 tendon for the great toe, and an outer (fibular) muscle with four tendons for the other toes. 

 Now bearing in mind the relative positions of each of these special dismemberments, and 

 remembering that our correspondences are to be drawn according to position and relation, 

 not according to size, number of tendons, etc., it is necessary to conclude that the two 

 inner (ulnar and tibial) moieties are homologous with each other, and that the two outer 

 (radial and fibular) moieties are likewise mutually correlated. That is to say, the flexor 

 longus proprius pollicis of the foot is the homotype of the flexor profundus digitorum of 

 the hand ; and the flexor longus digitorum of the foot is the homotype of the flexor longus 

 proprius pollicis of the hand. In the opossum there is no actual long flexor of the thumb, 

 unless, as is probably the case, the abortive fasciculus described as lying on the substance 

 of the common deep flexor be that muscle ; and similarly, the long flexor of the great toe, 

 though distinct as to its muscular part, really sends no tendon to the toe ; but this con- 

 stitutes no valid objection whatever. 



In further illustration of the homotypy of the long, deep, digital flexors, the reader is 

 referred to my article (No. viii) in the Medical Record, as above cited, where the curious 

 special condition that occurs in man is especially treated of These flexors are so notori- 

 ously inconstant in their amount of development, their number of tendons, and the particu- 

 lar digits that each serves, that confusion has arisen, which would be happily obviated by a 

 little more precise nomenclature. Ignoring number of tendons, and the digits they serve, 

 the following names are proposed : In the arm, flexor digitorum ulnaris for the inner mo- 

 iety, and flexor digitorum radialis for the outer moiety; in the leg, flexor digitorum 

 tihialis for the inner moiety, and flexor digitorum jibularis for the outer moiety. It is 

 probable that similar considerations are applicable in special determinations of the superficial 

 and short flexor systems, if not also the sets of digital extensors ; but at present I am not 

 prepared to follow up the subject. 



Like the flexor, the extensor system consists essentially of two sets of muscles, which 

 may be denominated the ''long" and "short" in both limbs. The former is the more con- 

 stant and important ; it gives off, on the back of both hand and foot, four tendons, which 

 supply the four fingers and the four lesser toes, the thumb and great toe wanting tendons, 

 as in the instance of the flexor set. The same idea of a typical extensor muscle with five 

 tendons— one to each digit— is more perfectly carried out than in the analogous case of 

 the flexors. For here we have, not merely teleologically correspondent, but morphologi- 

 cally homologous muscles liberated on the same (inner) side of the hand and foot; they 

 are the extensor proprius minimi digiti of the hand, and the extensor longus proprius polli- 

 cis of the foot. This identification of these two last named muscles is indisputable, and 

 is not affected, apparently, by the question whether they really are dismemberments of 

 the common extensor. 



The extensors of the other set, the "short" or accessory, are more obscure, and their ho- 

 mologies more difiicult to determine. In man the short extensor lies wholly upon the dor- 

 sum of the foot, and supplies the same digits that receive tendons from the long extensor ; 

 but in the opossum it is carried up the leg, and its tendons descend behind the external 

 malleolus. In man, we have as the only muscles of the hand to correspond to this of the 



