No. 2. ] THE HARD PARTS OF THE MAMMALIA. 183 
outwards on the humerus, the animal being knock-elbowed in 
this act. The effect of this position during impact is to extend 
both radius and humerus outwards, and to push from its primi- 
tive position that one of the 
two elements which has the 
least longitudinal support 
(Fig. 23). 
The history of the elbow- 
joint in the Lagomorphous 
Rodentia is the same as that 
which I have described in the 
Diplarthra, excepting that the 
applied surfaces of the head 
of the radius and the ulna do 
not mutually interlock. The 
expansion of the head of the 
radius is identical in the two 
cases, as is the form of the 
condyles of the humerus, in- 
cluding the trochlear crest 
and the extension external to 
it (Fig. 42).. This is an ad- 
mirable illustration of the pro- 
duction of lrke effects by like 
causes, in two widely different 
lines of descent. figure 23. — Cervus canadensis, trot- 
The elbow joint of the bat ting, from behind; from Muybridge, Ani- 
has some points of resem- mal ESEOmOrOn; showing position of fore 
Planecito thateotithe higher foot in support of the body. 
Diplarthra and Rodentia. The ulna is reduced to a splint below 
the proximal part of the radius, to which it is not co-ossified,! 
and the olecranon is represented by a sesamoid bone in the ten- 
don of the extensor muscle. The humeral condyle is developed 
external to the external convexity, but the internal condyle has 
no extension beyond the flange, thus differing from Bovidz and 
Leporide. In Pteropus and Vespertilio the ulnar groove of the 
humerus is not deep. In Pteropus the external convexity of the 
condyle is wide; in Vespertilio it is narrowed into a weak troch- 
1See W. Leche, Ueber der Entwickelung des Unterarms u. Unterschenkels bet 
Chiroptera. K. Svenska Akad. Handlingar, V., 1879. 
