No.2] THE HARD PARTS OF THE MAMMALTIA. 195 
Now, the lines of mammalian descent displayed by paleontol- 
ogy are characterized, among other things, in most instances, 
by the gradual elevation of the heel above the ground, so that 
the animal comes to walk on its toes. It is evident that in this 
case the concussion of running is applied more directly on the 
ends of the bones of the foot, than is the case where the foot is 
horizontal. As a consequence, we find the keel is developed 
farther forward in such animals. But in many of these, as the 
Carnivora, the hippopotamus, and the camels, there is developed 
under the toes a soft cushion, which greatly reduces this concus- 
sion. In these species the keel makes no further progress. In 
other lines, as those of the horse, the pig, and of the ruminants, 
the ends of the toes are applied to the ground, and are covered 
with larger hoofs, which surround the toe, and the cushion is 
nearly or quite dispensed with. These animals are especially 
distinguished by the fact that their metapodial keels extend 
entirely round the end of the bone, dividing the front, as well 
as the end and back, into two parts. This structure would seem 
to be a result of the greater force of the impact resulting from 
use of the legs, experienced by the end and front of the bone, 
which receives the blows. In proportion to the degree of digiti- 
gradism is the strain on the tendons at plantation and recover. 
In running, in the ditigrade Ungulata, the toe is antiflexed so as 
to bend at right angles anterior to the metapodial bone, just 
before leaving the ground (recover). It is the strain and tor- 
sion at this moment that has produced the tongue to the front 
of the metapodial. It is also evident that the elevation of the 
heel offers the condition for torsion strain on the leg immediately 
following contact with the ground. This agency has been espe- 
cially considered under the head of the fixed articulations. 
The distal phalangeal articulations are usually simply concave, 
receiving a median angle from the phalanges distad. This me- 
dian concavity is probably the result of the impact on the ground, 
of a surface narrower than the excavated extremity, viz. : of the 
apex of the unguis. This impact exerts the greatest displace- 
ment along the median line or axis of the digit. Apart from 
this, however, a determining cause of the simplicity of the pha- 
langeal groove is found in the undivided flexor tendon and the 
single sesamoid bone, both of which exercise considerable pres- 
sure on the median line, when the weight of the body is thrown 
