No. 2.] THE HARD PARTS OF THE MAMMALTA. 181 
The notch between the head of the radius and the coronoid 
process of the ulna is fitted by a corresponding ridge of the 
humeral condyle, the intertrochlear crest, in the Anthropomor- 
pha. In the Myrmecophagide it is wanting, because the me- 
dian convexity of the ulnar cotylus is opposite the space between 
the head of the radius and the coronoid process, so that the ulnar 
concavity of the humeral condyle is immediately continuous with 
the radial surface of the same. 
In the elbow joint of the Diplarthra a different state of affairs 
exists. The head of the radius lies firmly on the coronoid pro- 
cess of the ulna, having the transverse diameter increased. This 
increase of width I suppose to be caused by the strong impact 
of the condyles of the humerus directly upon its extremity, which 
accompanies the use of the limb as a support, especially in alight- 
ing upon it in the rapid locomotion characteristic of diplarthrous 
Ungulata. The increase in diameter is both external and inter- 
nal, and so far as to equal the width of the humerus, and not 
to permit any flange on the edge of its internal condyle. The 
effect of impact on the head of the radius is greater than it is 
on the condyles of the humerus, because its energy is expended 
in the case of the former on a surface whose arc is only one- 
third that of the latter. It must be remembered, however, that 
it is not equally distributed on the humeral are. The coronoid 
process of the ulna also expands transversely, but not to the 
same extent as the head of the radius. Its external expansion 
is overpassed by that of the radius, the external portion of the 
radius in the Diplarthra occupying the position of the entire 
head of that of man. The effect of the two convexities of the 
humeral condyles has ‘been in most Mammalia to prolong in the 
direction of their movement the corresponding parts of the coro- 
noid region of the ulna. Hence there has appeared an external 
and an internal ridge of this part, enclosing a concavity or groove 
between them. The head of the radius in its external extension 
has overpassed the external ridge, and embracing it, has formed 
an interlocking fixed articulation. The interior face of the head 
of the radius has also applied itself exactly to the median groove 
of the ulna, forming another fixed key joint of much firmness. 
The articular face of the head of the radius has naturally formed 
a ridge in adaptation to this groove of the ulna, as well as to 
the groove of the humerus which corresponds to it. Opposite 
