No. 2.] THE HARD PARTS OF THE MAMMALIA. 199 
fore leg only. This brings the line of pressure through the 
external part of both the head of the radius and the humeral 
condyle (Fig. 23). 
Figure 42.— Cervus elaphus ; A, B, C, humero-radial articulation; 4 and C, with 
the radius in position; 4, with radius twisted; D, £, metatarsophalangeal articula- 
tion; D, front; /, distal views. 
In conclusion, the tongue-and-groove articulations of the 
mammalian limbs and their origin may be summarized. 
I. /ntertrochlear crest of humerus of Anthropomorpha. Orig- 
inated from plastic adaptation to space between head of ulna 
and coronoid process of ulna. 
Il. Yvochlear crest of humerus of Diplarthra. Originated from 
impacts, torsions, and exterior divergence of fore arm on hume- 
rus. 
III. Sutercarpal crests of radius of Artiodactyla; from plastic 
adaptation to intercarpal grooves. 
IV. Metapodial keels; from impact, torsion, and pressure of 
two posterior ligaments and sesamoid bones, and if unsymmet- 
rical, as in Artiodactyla, modified by divergence of digits. 
