96 



GREGORY 



XOTHANCrrs, AN AMERICAN EOCENE PRIMATE 



Astragalus 

 Plate XXX; Text Fig. 19 



The astragalus also is essentially similar to that of Lemur in all views. In both genera the trochlea 

 is rather narrow and extends far backward, ending above or posteriorly, in a groove for the flexor longus 

 hallucis muscle flanked by an internal and an external tubercle. The concavity of the trochlea is slight 

 and the internal lip or rim is well defined but not as large as the external rim. The facet for the fibula 



is oblique, not vertical, in correlation with the oblique position of the 

 corresponding facet on the fibula. 

 ^Mjk The movement of the astragalus on the tibia and fibula is hinge- 



^^^^^ ^^^^ limited to an anteroposterior plane. In Lemur the posterior 

 mSL ^ astragal o-fibular ligament runs transversely from the inner side of the 



lower tip of the fibula to a pit on the outer side of the astragalus just 

 behind the facet for the fibula. This ligament forms the outer part of 

 ;i trans^'erse axle upon which the astragalus turns in its hinge-like 



fjfef moA'ement upon the tibia and fibula. The inner part of the axle is 



y.M formed l)y the strong, posterior astragalo-tibial ligament, which runs 

 from a rough surface on the inner surface of the astragalus behind 

 tlie facet for the tibia to the back jmrt of the internal malleolus. 



The above-mentioned pit for the posterior astragalo-fibular liga- 



fment, on the outer side of the astragalus, and the rough surface for 

 • # the posterior astragalo-tibial ligament, on the inner side of the astra- 



'^IIH galus near the proximal end, are both well defined in Notharctus. 



%0 The neck of the astragalus in both Lemur and Notharctus is elon- 



gate and slants obliquely downward and inward in the direction of the 

 hallux, so that it crosses the straight shaft of the calcaneum. When 



t^i^^^ the foot is fully extended upon the tibia the neck of the astragalus is 



more nearly in line with the tibia, but as the foot is flexed the neck 

 1^ assumes a more and more oblique position. It is forced to do so by 



the pressure of the fibula upon the oblique astragalo-fibular facet, and 

 l)y the pressure of the navicular upon the head of the astragalus. The 

 same movement of the fibula causes the astragalus to rock upon the 

 ui-cs- H"lit •isrr''i'<'-!l'us''()V calcaneum as a base, the concave, oblique, ectal, or posterior calcaneal, 



tlKircfii.s tt nrlirnsus (Ainer. Mils. facet of the astragalus sliding over the sharply convex, ectal facet of 

 No. 13024) anil Lnimr Dioiigtn ^^le calcaneum; while the flattened oval sustentacular, or middle 

 (Amei. Mus. Aii>. ). Xatuiai calcaneal, facet of the astragalus slides over the corresponding facet 



1. Dorsal or front view. of the calcaneum. In Lemur the astragalus is tied to the calcaneum 



2. Fibular or outer side view. ^^^^^ movements by the interosseous calcaneo-astragaloid 



3. Plantar or back view. o ^ <=> 



4. Tibial or inner side view. Ugament. This short ligament arises from the "interosseous groove" 



(sulcus calcanei) on the front or upper surface of the calcaneum be- 

 tween the ectal and the sustentacular facets of the calcaneum, and is inserted on the back or lower face 

 of the astragalus, between the ectal and sustentacular facets of the astragalus. Both these pits or 

 grooves are well marked in Notharctus. 



The head of the astragalus is spheroid to ovate in shape, more or less flattened on the inner side. 



