IMPACTS AND STRAINS OX THE FEET OF MAMMALIA. 377 



displacement as the protrusion of tlie inferior, commencing with 

 a concavity (Eleplias) ; becoming more concave (Fig. 77), and 

 becoming finally a groove. (3) When the dense edge of a bone, 

 as in the case of the lateral walls of the astragalus, is presented 

 upward, a groove is produced in the down-looking bone ; e. g., 

 the lateral grooves of the distal end of the tibia. (4) When the 

 inferior bones are the denser, the superior articular face yields ; 

 e. g., the distal end of the radius to the first row of carpals 

 (Fig. 81). 



(5) The metapodial keels commence in the lower types on the 

 posterior side of the distal extremity of the bone. This is j^artly 

 due to the presence there of a pair of sesamoid bones which, with 

 the tendons in which they are developed, sustain and press on the 

 lateral parts of the extremities, and leave the middle line without 

 support (Figs. 79-81). 



