No. 2.] THE HARD PARTS OF THE MAMMALIA. 1QI 
bones themselves. (1) They project in the direction of gravity. 
Constant jarring on the lower ‘extremity of a hollow cylinder 
with soft (medullary) contents, and flexible end walls would tend 
to a decurvature of both inferior and superior adjacent end walls. 
If the side walls are wide and resistent, the projection will be 
median, and will be prolonged in the direction of the flexure of 
the joint. (2) They fit entering grooves of the proximal ends of 
corresponding bones. Thesé will be the result of the same appli- 
cation of force and displacement, as the protrusion of the inferior, 
commencing with a concavity as in the astragalus (E/ephas) ; 
becoming more concave (Plate II.), and becoming finally a groove. 
(3) When the dense edge of a bone, as in the case of the lateral 
walls of the astragalus, is presented upwards, a groove is pro- 
duced in the down-looking bone; ¢.g. the lateral grooves of the 
distal end of the tibia. (4) When the inferior bones are the 
denser, the superior articular face yields; e.g. the distal end 
of the radius to the side walls and summits of the first row of 
carpals (Fig. 15). (5) All of these descending convexities have 
been converted into keels in the line of flexure by the long- 
continued torsion produced by the greater or less rotation of 
the bone with long axis. They have been thus pinched at all 
points of their length during flexures, but especially at their 
extremities. 
To examine these cases in detail. In the ruminating anima!s 
(ox, deer, camel, etc.) and in the horse, among other living 
species, the ankle joint is a very strong one, and yet admits of 
an extensive bending of the foot on the leg. It is a treble 
tongue-and-groove joint ; that is, two keels of the first bone of 
the foot, the astragalus, fit into two grooves of the lower bone 
of the leg, the tibia, while between these grooves a keel of the 
tibia descends to fill a corresponding groove of the astragalus. 
Such a joint as this can be broken by force, but it cannot be 
dislocated. Now, in all bones the external walls are composed 
of dense material, while the centres are spongy and compara- 
tively soft. The first bone of the foot (astragalus) is narrower, 
from side to side, than the tibia which rests upon it. Hence 
the edges of the dense side walls of the astragalus fall within the 
edges of the dense side walls of the tibia, and they appear to 
have pressed into the more yielding material that forms the end 
of the bone, and pushed it upward, thus allowing the side walls 
