42 THE VOYAGE OF H.M.S. CHALLENGEE. 



The deep muscles on the back of the leg show very material differences in the two 

 animals. 



Tibialis 'posticus. — In the Thylacine this muscle is small in size and intermediate in 

 position between the flexor longus hallucis and the flexor longus digitorum. It has 

 apparently no direct bony attachment at its origin, but springs from the fascia which 

 covers the popliteus, whilst externally it is inseparably connected with the flexor longus 

 hallucis. At the inner ankle its tendon passes under cover of that of the flexor longus 

 digitorum, and proceeds along the inner margin of the foot to be inserted into the 

 internal cuneiform bone, and into the base of the first metatarsal bone. 



In the discus the tibialis posticus is apparently double. The largest or internal part 

 (PI. V. fig. 2,f) occupies the position of the flexor longus digitorum which is absent. 

 It arises by two heads — a tibial and a fibular. The latter springs from the posterior 

 aspect of the head of the fibula, and its fibres which pass obliquely downwards and 

 inwards are partially fused with the upper part of the subjacent popliteus. The tibial head 

 takes origin from the upper half of the posterior surface of the tibia, and after being- 

 joined by the oblique fibular fibres, the muscle gives rise to a tendon which passes over a 

 groove on the back of the internal maleolus to reach its insertion into the inner aspect 

 of the scaphoid bone. It is possible that this muscle may be the representative of 

 the flexor longus digitorum, but it is difficult to reconcile this view with its tarsal inser- 

 tion. 



The smaller and external part of the muscle (PL V. fig. 2, g) is a very minute fleshy 

 belly, which lies deeply under shadow of the huge flexor muscle, and arises from the head 

 of the fibula and the fascia over the popliteus. Its slender tendon passes over the 

 groove on the back of the internal maleolus, under cover of the tendon of the preceding 

 muscle, and is inserted into the plantar aspect of the scaphoid bone. 1 



Flexor muscles. — In the Thylacine there is a small flexor longus digitorum and a 

 large flexor longus hallucis ; in the Cuscus the flexor longus digitorum is apparently 

 absent, but the flexor longus hallucis is a huge muscle, and it gives off from its super- 

 ficial surface, whilst still in the leg, the representative of the flexor brevis digitorum. 



The flexor longus digitorum in the Thylacine springs from the posterior aspect of the 

 shaft of the tibia in its upper two-thirds, and also by a pointed and separate process from 

 the back of the internal tuberosity of the tibia and the posterior ligament of the knee- 

 joint. At the inner ankle it crosses the tendon of the tibialis posticus obliquely, and 

 entering the sole ends by joining the large tendon of the flexor longus hallucis. 



The flexor longus hallucis is a very large and powerful muscle. It occupies the outer- 

 most place of the deep muscles on the back of the leg, and it takes origin from the whole 

 length of the posterior surface of the fibula. The thick rope-like tendon in which it ends 

 enters the sole by passing forwards in a groove on the back of the astragalus, and after 



1 Young notes that the tibialis posticus is double in the Koala, loc. cit., p. 41. 



