1893.] Evolution of Metapodial Keels of Diplarthra. 431 
that is, if the first phalanx be articulated with the metapodial, 
it will been seen that when the phalanx is placed in a line 
with the metapodial or very nearly so, there is equal bearing 
upon all parts of the two articular surfaces of the bones ; but 
if the phalanx is flexed even to a slight extent, then the bear- 
ing falls largely upon the keel for the metapodial surface and the 
shallow groove for the phalangeal surface. When I say flexed 
I mean bent in the direction of the plantar side of the foot. 
FIGURE 4.—Modern horse. Distal view of median metatarsal or cannon bone, 
showing complete keel. 
How now can we apply these facts to the mechanical 
explanation of the further extension of the keel toward the 
dorsal side of the metapodials? When the foot assumed the 
ungaligrade position permanently, we can understand, and 
havé in fact already seen how the keel impinges upon and 
produces the notch in the phalanx. Any flexure of the pha- 
lanx upon the metapodial, under pressure would bring the 
keel to a more advanced position with reference to the phalan- 
geal articulation, and would cause an extension of the notch 
in the phalanx so as to form a groove. ` That the phalanx is 
so flexed is evident to anyone who has ever studied carefully 
the movements of the foot of a horse in the act of pulling a 
heavy load up an inclined surface. Here the phalanx is seen 
to be greatly flexed when the foot strikes the ground and is 
therefore under great strain and pressure. I speak of it par- 
ticularly in a horse pulling a load, because it is more noticea- 
ble in this case than in the unencumbered animal, but what 
is true of one is also true of the other. Now in an animal in 
which the keels and grooves were little or not at all advanced, 
such as was the case with theancestor of the horse when he 
emerged from the digitigrade and assumed the unguligrade 
position of the foot, the flexure of the phalanx brought the 
keel forward and produced a groove in the phalanx. As the- 
