36 The Pliylogeiiy of the Crural Flexors 



The plantaris superficialis lateralis (Psl) is a muscle which has in- 

 variably been described as the femoral head of the large superficial 

 muscle which I term the plantaris profundus III. From the remark- 

 able similarity of the muscles of the forearm and crus in the urodele 

 amphibia it might be expected that the same constancy in the relations 

 of the superficial and deep flexors of the leg to the femur and crural bones 

 as obtained in the corresponding muscles of the crus in their relations 

 to the humerus and antibrachial bones would be found, and it was on 

 this ground that I was first led to refer this muscle to the superficial 

 layer and to regard it as distinct from the plantaris profundus III. 

 Further study only served to confirm the correctness of this view by 

 revealing a consistent phylogeny for the crural flexors based upon it. 



The muscle takes its origin from the flexor surface of the external 

 condyle of the femur and is separated by a distinct interval from the 

 upper part of the origin of the plantaris profundus III. It passes down 

 the fibular side of the leg, quite distinct from the plantaris profundus 

 and passes over into a rather feeble tendon, which is inserted into the 

 outer border of the fibula near its lower extremity. Throughout its 

 whole extent, therefore, it is distinct from the plantaris profundus III 

 in Amblystoma. 



The plantaris profundus III (Pp^^^) is the largest of all the muscles 

 of the crus and is described by Humphry, 72, as the flexor sublimis 

 digitorum, by Hoffmann, 73, as the femoro-fibulse-digiti I-V, by Perrin, 

 93, as the external flexor of the digits and by Eisler, 95, as the plantaris 

 superficialis major, all these authors, as has already been noted, including 

 in the muscle the plantaris superficialis lateralis. It arises in Ambly- 

 stoma from the posterior surface of the upper part of the fibula and its 

 fibers are directed downwards and somewhat tibially to be inserted into 

 the under (dorsal) surface of the plantar aponeurosis, through which it 

 acts upon the digits. 



From the upper part of the tibial border of this muscle a slender slip 

 (^Ppiiirn) separates and passes almost vertically downwards to fade out 

 in connective tissue in the neighborhood of the ankle joint, in close 

 proximity to the tibial border of the plantaris profundus I. This is 

 evidently the muscle described by Eisler as the plantaris superficialis 

 minor and has apparently been overlooked by Perrin. It seems to be, 



interesting to note that in tlie caudo-pedal of Cryptobranchus there is not 

 only an end to end union of the ischio-flexorius proper and the plantaris 

 superficialis medialis, but also of the former and what may be termed an 

 ischio-caudalis. 



