MYOLOGY OF THE ORNITIIORHYNCHUS. 



17] 



really a dismemberment of the long common fibular flexor, restrained 

 to the foot, just as the muscle that sends the single tendon to the 

 opossum's great toe is. Ordinarily, perhaps, in unguiculate mammals 

 at least, there are two long deep digital flexors, one tibial, the other 

 fibular, the distribution of the individual tendons of which is variable. 

 Thus in man, one goes to the great toe alone, the other to all the rest 

 of the digits ; and the two seem to have, as it were, exchanged places, 

 since their tendons cross to reach their respective destinations. In 

 the next animal, viz., the gorilla, it is the other muscle that gives off 

 the most tendons. In the opossum, for example, the flexor hallucis 

 occurs, and is on its proper, viz., tibial, side of the leg; but its tendon 

 aborts at the heel, being there fastened to the common tendon ; and 

 its place is supplied by a little plantar calcaneal muscle that crosses 

 the foot obliquely and gives off a tendon as large as any from the 

 common flexor; this tendon runs between the heads of the flexor 

 brevis hallucis, and is inserted in the usual manner into the base of 

 the distal phalanx of the great toe. Now in the Ornithorhynchus with 

 only one long deep flexor digitorum, we have a similar arrangement, 

 though, with a little variation. The plantar dismemberment of the 

 common long deep flexor digitorum forms a short, fleshy belly that 

 arises from the side of the os calcis, and soon becomes tendinous, di- 

 viding into two tendons that pass to terminal phalanges of the 3d and 

 4th digits. These tendons are fully as large as those coming down 

 from the leg, and have identical disposition upon the digits. 



All that has just been said has reference only to the subdivisions of 

 one — the long deep— set of digital flexors ; tliat is, it is without bring- 

 ing into the discussion the above described flexor "brevis" or sab- 

 limis. The latter corresponds to the muscle of the same name in the 

 hand. The former (flexor longns digitorum) is so variously differ- 

 entiated into two muscles and several tendons in the mammalian 

 series, that it is safest, as well as most philosophical, to regard it 

 as a morphological integer, susceptible of varying dismemberments, 

 which, as a matter of fact, supply different digits in different animals. 

 The corresponding muscle of the hand is probably in most unguicu- 

 lates single, with Jive identical tendons ; when, as in man, it is differ- 

 entiated into two, one of these is flexor longus pollicis, the other 

 flexor digitorum profundus. In the foot, the muscle is probably 

 usually divided into two that have, with different animals, different 

 digital distribution, as just stated. It will be found best to distinguish 

 these two simply as respectively 'fibular,' and 'tibial,' without refer- 

 ence to the particular digits that either supplies.* 



*In drawing antitypes of the deep digital flexors of man, it must be remem- 

 bered, that, as above mentioned, the human flexores tibialis and fibularis have, as 

 it were, changed places, so that flexor hallucis is the correlative of the flexor digi- 

 torum profundus, and flexor longus digitorum pedis of the flexor proprius pollicis. 



