44 The Phylogcny of the Crura] Flexors 



it in dissections of Phrynosoma. On the other liand Perrin finds it in 

 Varanus and apparently in Lacerta and Gongylus, and Gadow, 82, re- 

 gards it as a typical portion of his flexor longiis digitorum of which it 

 forms the caput accessoriimi. It seems to be a muscle separated from the 

 fibular border of the plantaris superficialis lateralis, a view which re- 

 ceives confirmation from the statement of Gadow that it is sometimes 

 fused above with that muscle. Its apparent absence in certain forms 

 may upon this view be regarded as due to its failure to separate from the 

 parent muscle. 



The plantaris superficialis lateralis (Fig. 3, Psl) is a rather large 

 muscle which takes its origin by a tendon from the posterior surface of 

 the lateral condyle of the femur. A sesamoid cartilage is imbedded in 

 the tendon just above the line where the muscle fibres begin to make 

 their appearance and, as the tendon is traced downwards, it is found 

 to broaden out into a thin sheet covering the anterior (deep) surface 

 of the muscle and gradually fading out below, with the exception of a 

 narrow band which continues on to the region of the ankle joint, be- 

 coming enclosed by the muscle substance (Fig. 4). Just when the 

 tendon begins to fade out an aponeurotic layer (Fig. 4, a) appears on 

 the posterior (superficial) surface and increases in strength as it passes 

 downwards, becoming a part of the plantar aponeurosis. It is with the 

 inner border of this that the plantaris superficialis medialis and the ten- 

 don which descends from the border of the flexor tibialis externus and 

 gives rise to the plantaris superficialis tenuis, unites. 



As the muscle substance is traced downwards it is seen to be continued 

 past the ankle joint into the plantar region of the foot. In the upper 

 part of the muscle the fibers are all parallel, arising from the tendon of 

 origin, but lower down fibers arise from the slender tendon which con- 

 tinues the tendon of origin downwards and have a somewhat radiating 

 arrangement (Fig. 4). Tracing out the two sets of fibers, it is found 

 that the upper ones insert into the upper part of the plantar aponeurosis, 

 while it is those which arise from the prolongation of the tendon of 

 origin that form exclusively the lower part of the muscle and are con- 

 tinued over the tarsus to be inserted into the plantar aponeurosis, and 

 the sesamoid cartilage developed in it over the fifth metatarsal. 



In addition to these three muscles there is yet a fourth (Psa) which 

 is apparently to be reckoned as a portion of the plantaris superficialis. 

 It arises from the posterior surface of the external condyle of the 

 femur below the plantaris superficialis lateralis and passes downward 

 under cover of that muscle to about the middle of the crus, where it 

 unites with the plantaris profundus III-II, or rather, inserts into the por- 

 tion of the plantar aponeurosis covering that muscle (Fig. 4, a'). 



