290 ANNALS NEW YORK ACADEMY OF SCIENCES 



short foot, conditioning a low knee joint, also enable the very heavy 

 animal to rise from the ground with comparative ease. At the same 

 time, excessive shortening of the tibia (as in Ground Sloths) probably 

 conditions very slow motion, which would be very disadvantageous to an 

 animal that has to wander far in search of food. 



GEAVIPOETAL AND CUESOBIAL TYPES OP PEMUE 



The femur of primitive animals ( Creodonts, Condylarths, Stem Peris- 

 sodactyls, etc.) is long, when compared with the size of the body. It 

 has a prominent third trochanter situated in the upper fourth of the 

 shaft and serving for the insertion of the glutseus superficials (seu 

 maximus) . This muscle arises from the enlarged tuber coxa? . of the 

 ilium and apparently functions as an adductor of the femur. The 

 trochanter major projects high above the shaft. 



These characters persist in the cursorial types, the chief difference 

 being that here, as a rule, the femur is relatively short when compared 

 with the size of the body. The short femur is associated with a sub- 

 horizontal innominate bone and with comparatively open angles of in- 

 sertion of the long muscles (Fig. 6, I), which pull across the long axis 

 of the femur and give high components of rotation and relatively low 

 centripetal components (in the direction of the shaft of the bone). 

 Such an arrangement, associated as it is with cursorial adaptations, 

 seems less adapted for the support of a great dead weight than that no- 

 ticed above in graviportal animals. In the standing pose, a sharp angu- 

 lation at the knee conditions a tendency for the leg to collapse, which is 

 counteracted by the stretching and tension of the opposing flexors and 

 extensors of the thigh (Fig. 7). In the horse, these opposite tensions 

 are transmitted by means of special tendons on opposite sides of the leg, 

 which also serve to tie as it were the limb in position and thus relieve 

 the muscles to a considerable extent. In graviportal animals, the long 

 muscles are also stretched in standing and thus serve as ligaments, and 

 since they are inserted at very narrow angles, their centripetal compo- 

 nents are relatively high (p. 278). 



In the femur of graviportal animals, the third trochanter is often 

 reduced, absent or confluent with a long ridge running down from the 

 great trochanter and situated well down the shaft. The functional 

 meaning of this is possibly that the glutseus superficialis is less developed 

 in graviportal animals, its place, perhaps, being usurped by the greatly 

 enlarged glutseus medius. Again, it is conceivable that the third tro- 

 chanter may have been crowded out, so to speak, by the enlargement of 

 the powerful vastus muscles, which arise on the femur just above the 



