146 THE OSTEOLOGY AND MYOLOGY 



it need not be said, there are grave objections. These, and other considerations, lead me to 

 sec a peroneus tertius in this extensor of the little toe ; and I base my search for its homo- 

 logue in the fore limb upon this assumption. I cannot see the grounds of Prof. Wilder's 

 reference of the peroneus tertius to the " extensor" (flexor) carpi radialis brevier, which 

 seems to be naturally accounted for otherwise as I have just endeavored to show ; and this 

 view, moreover, necessitates his reference of the peroneus longus to tlie "flexor " (extensor) 

 carpi radialis. There seem to be but two muscles upon the back of the hand to which the 

 peroneus tertius can reasonably be referred ; one is the extensor pollicis et indicis, the 

 other the extensor metacarpi pollicis. For reasons given below, I identify the former of 

 these with the extensor brevis digitorum pedis, and am consequently obliged to find the 

 peroneus tertius in the extensor of the pollical metacarpal. It may be further urged, in 

 support of this view, that these two muscles belong to corresponding groups, radial and 

 fibular ; occupy nearly similar positions upon corresponding sides of the hand and foot ; and 

 operate exclusively upon corresponding digits. Whatever objections may be brought 

 against this view — and it is not denied that there are such — appear to be over balanced by 

 evidence in its favor. 



The plantaris is obviously homologous with the palmaris longus. A palmaris brevis is 

 scarcely if at all demonstrable in the opossum ; it probably has no homologue upon 

 the sole of the foot. 



Both fingers and toes are flexed by two sets of muscles, called in the arm "deep" and 

 "superficial," and in the leg, "long" and "short." The short flexors of the toes, in the 

 human subject confined to the sole, are in the opossum carried up the leg, and arise from, 

 and lie upon, the conjoined tendon of the long flexor. The muscular parts of the superfi- 

 cial and deep flexors of the fingers are much blended. But however the origins and 

 upper parts of these muscles may vary, in either arm or leg, or both, they preserve with 

 remarkable constancy certain characteristics at their distal extremities and insertions. 

 These are : — the tendons of the deep flexor of the fingers, and of the long flexor of the 

 toes, are large and stout, are accompanied by accessory muscles (lumbricales) and proceed 

 through the tendons of the other set to be inserted into ungual or distal phalanges ; while 

 the superficial flexor of the fingers, and the short flexor of tlie toes, are smaller and slender, 

 unaccompanied by accessories, and proceed, by splitting, upon either side of the tendons of 

 the other set, to be inserted into middle, or second phalanges. Such definite indications as 

 these would seem to go so far toward establishing the homology of the deej) with the long, 

 and of the superficial with the short sets of flexors, that opposing evidence, to be valid, 

 would have to be of the strongest kind. 



While the correspondence of the deep digital flexor system of the hand with that of the 

 foot is thus obvious and unquestionable, yet on carrying it out in details we meet with in- 

 dividual correlations as interesting as they are unexpected. For the four-tendoned long 

 flexor of the toes is not, as would be thought at first glance, the homotype of the four- 

 tendoned long flexor of the fingers, but of the single long flexor of the thumb, which in 

 the opossum remains undeveloped ; and similarly, the single long flexor of the great toe in 

 the opossum, developed as to its muscular part, but abortive as to its tendinous part, is the 

 homotype of tlie four-tendoned flexor of the fingers. This proposition is susceptible of 

 pretty conclusive argument. In the arm the profound digital flexors consist essentially of 

 a single muscle with five tendons, one for each digit ; and when this is difierentiated into 



