THE MUSCULAR ANATOMY AND THE RESTORATION OP THE TITANOTHERES 



721 



but only to a slight extent. The popliteus area is 

 bounded below by the indistinct popliteal or oblique 

 line, here indicated chiefly by a low prominence 

 extending from the upper external edge obliquely 

 downward and inward across the middle third of the 

 back of the shaft. To this oblique line may very 

 possibly have been attached remnants of the soleus 

 (which in Tapirus has lost its sheathlike character), 

 a muscle degenerated into an elastic band and shifted 

 in origin upon the popliteal surface of the femur 

 (Murie). 



The lower third of the back of the shaft terminates 

 below in a posterodistal process which in the standing 

 pose of the horse juts down between the back of the 

 astragalus and the sustentaculum of the calcaneum. 

 Postero-externally to this process the flexor longus 

 hallucis and probably the tibialis posticus passed 

 downward. 



The flexor longus hallucis and the flexor longus 

 digitorum have been confused by certain authors. 

 In man the flexor longus digitorum passes through 

 the groove behind the internal malleolus of the tibia, 

 while the flexor longus hallucis passes down behind 

 the sustentaculum of the calcaneum (Cunningham ; 

 1903.1, pp. 229, 236, 178, fig. 170), and the same 

 is true of the horse (Schmaltz, 1909.1, Taf. 56, fig. 1); 

 and yet Murie mistakenly calls the muscle which 

 "glidesln the groove behind the malleolus of the tibia " 

 "flexor longus hallucis," while the tendon which lies 

 "behind the os calcis" he calls "flexor longus digi- 

 torum," thus reversing the implied homologies with 

 man, as very clearly shown in his figures. Even 

 Windle and Parsons seem to be wrong, for they 

 homologize with the flexor longus hallucis the muscle 

 which "winds round the internal malleolus" (1903.1, 

 p. 281). The flexor longus hallucis in the tapir joins 

 the flexor longus digitorum, the latter forming the 

 perforating tendons (flexor profundus) of the three 

 digits. The tibialis posticus is not recognizable in 

 the tapir (Murie, 1872.1, p. 166) nor in some artio- 

 dactyls and Hyrax (Windle and Parsons, 1903.1, p. 

 282), but is probably fused with the flexor longus 

 hallucis. In the horse it is large and distinct 

 (Schmaltz, 1909.1, Taf. 59, 53, 56). In the Sumatran 

 rhinoceros it is fused with the flexor longus hallucis 

 and flexor longus digitorum, these three together form- 

 ing the flexor communis digitorum (Beddard, 1889.1, 

 p. 22). In the elephant it is distinct. It seems not 

 unlikely that in Eocene graviportal ungulates such as 

 Palaeosyops the muscle was present and more or less 

 appressed to the flexor longus hallucis. It may then 

 have had an area on the back of the tibia just below 

 the popliteal line and possibly another on the back 

 of the fibula, behind the flexor longus hallucis. 



The soleus is another muscle of somewhat doubtful 

 form and attachments. In man this broad muscle 



lies beneath the gastrocnemius and is broadly attached 

 to the back of the flbula and to the popliteal or oblique 

 line of the tibia; it is inserted into the tendo Achillis 

 and assists in lifting the heel. In many ungulates, 

 however, the soleus is of small size (Windle and 

 Parsons, 1903.1, p. 280) and more or less cordlike; it 

 probably contributes by its elasticity to the quick 

 backward jerk or kick of the foot. In the tapir the 

 soleus is very small and cordlike; it has shifted its 

 origin up to the femur, in the popliteal space, behind 

 the gastrocnemius area, and is here closely associated 

 with the cordlike tendinous plantaris (Murie, 1872.1, 

 p. 163 and pi. 10, fig. 13). In this extreme modifica- 

 tion of the soleus the tapir is more specialized than 

 either the horse, in which a slender belly of the soleus 

 remains attached to the tibia behind the head of the 

 fibula, or the rhinoceros, in which as figured by 

 Beddard (1889.1, p. 21) the soleus is fleshy and 

 apparently arises below the popliteus. In the ele- 

 phant the soleus is well developed. It therefore seems 

 likely that Palaeosyops and the Oligocene titanotheres 

 avoided the extreme degeneration of the soleus which 

 is characteristic of the tapir and retained a fleshy 

 soleus, attached to the oblique or popliteal line of 

 the tibia and possibly to the back of the upper part of 

 the fibula. 



MUSCULAR ATTACHMENT OF THE FIBULA 



As compared with that of Tapirus the fibula of 

 Palaeosyops has both proximal and distal ends larger 

 and not so flat; the middle of the shaft is flatter. To 

 the outer side of the head of the fibula was attached 

 the peroneus longus, which extended down the front 

 outer face of the shaft and passed to the foot through 

 a deep peroneal groove. The top of the fibula (caput 

 fibulae) behind the surface for articulation with the 

 tibia shows an expanded smooth prominence, postero- 

 internal to which was a groove; over both prominence 

 and groove slid the tendon of the popliteus. Below 

 the popliteal prominence the flexor longus hallucis 

 probably took up most of the posterior surface of the 

 shaft. The inner or medial face of the fibula was also 

 largely fifled by the flexor longus hallucis. The front 

 apex of the external malleolus was probably attached 

 to the "ligamentum collaterale longum" running to 

 the calcaneum. 



MUSCULAR ATTACHMENTS OF THE PES 



The distal rugosity of the tuber calcis served as the 

 insertion area for the foUowing tendons: (1) The true 

 tendo Achillis, which consists of the paired tendons of 

 the gastrocnemius; (2) the complex of tendinous 

 sheaths which descend from the biceps, gracilis, and 

 semitendinosus and enwrap the tendon of the gastroc- 

 nemius; (3) the tendon of the plantaris. This tendon 



