No. 2.] THE HARD PARTS OF THE MAMMALIA. 179 
by paleontology. The elbow joint passes from simple to troch- 
lear, from the lower or less specialized Mammalia to the most 
specialized. Nowhere is the direct mechanical effect of motion 
more demonstrably evident than in the movable articulations of 
the skeleton of the Mammalia. 
a. Lhe heads of the humerus and femur represent smaller and 
larger segments of spheroids, and both look in part upwards. 
These articulations are not due to impact, but to rotary move- 
ment of the movable limb element on the fixed scapular and 
pelvic arches. This movement antedates in time the existence 
of impact, since it characterizes the movements of aquatic ani- 
mals which preceded the terrestrial. Neither of these articula- 
tions acquired their spheroidal form until the advent of the 
Mammalia. To the more constant and uniform activity of the 
Mammalia, as compared with their reptilian ancestry, must we 
ascribe the gradual rounding of the humeral and femoral heads, 
which form the most nearly universal joints in their skeleton. 
b. The Elbow Font. In lower salamanders, as Crypto- 
branchus, the ulna and radius rest normally, like the tibia and 
fibula, in the transverse plane of the distal extremity of the 
humerus. In terrestrial Batrachia and Reptilia (Theromora, 
Lacertilia), the plane of the ulna and radius is either oblique 
or at right angles to that of the distal extremity of the humerus. 
Mammalia possess the same character, the radius resting proxi- 
mally above or anterior to the ulna, instead of alongside of it as 
in the lower Batrachia. This position may be ascribed, I think, 
to the long-continued action of the supinator muscles of the fore- 
arm, together with that of the biceps flexor, in the endeavor to 
use the fore feet for purposes other than mere locomotion. 
Supination and flexion of the fore arm are necessary for any 
use of the manus in connection with the head, or in order to 
grasp any object which should project in front of it. In both 
reptiles and mammals the condyle of the humerus is divided by 
a shallow vertical groove ; that is, a groove in the line of the long 
axis of the leg. This groove is due to the fact that when the 
ulna and radius assume a relation transverse to the end of the 
humerus, their transverse diameter, especially that of the ulna, 
is less than the latter. The groove or median concavity is then 
nothing more than the result of the pressure and impact of the 
narrower ulna on the middle of the condyle, experienced during 
