Wyman on the Cancelli of Bones. 135 
has only a thin shell, but here its diameter is largest and filled 
with the cancellated structure, which especially in the lateral 
portions has a very definite arrangement; the cancelli forming 
a series of pillars, which ascend very nearly vertically from the 
surfaces of the condyle to the walls of the bone above them, 
which are bent inwards as the bone diminishes its diameter 
towards the middle of the shaft. A corresponding arrange- 
ment exists in the two extremities of the tibia, where the 
surface which is the seat of pressure is sustained by columns 
of bony fibres extending to the walls above or below it, accor- 
ding as the upper or lower portion of the bone is examined. 
This structure has been distinctly figured and described by 
Bourgery & Jacob. The cancelli are, as in the parts before 
described, prevented from lateral flexion by braces which are 
interposed at right angles to their direction. 
IV. ASTRAGALUS. 
The tibia alone bearing vertically on the astragalus, this 
last bone will necessarily sustain in each foot one half the 
weight of the body, or the whole of it when it is supported 
on one foot. When the small size of the surface on which 
the tibia rests is borne in mind, it will be readily anticipated 
that in its internal structure it will give us another illustration 
of mechanical adaptation. The astragalus, though it receive 
so many shocks in the violent movements of the body and is 
called upon to resist so much vertical force, is nevertheless a 
light bone and presents areolz in its interior of large size. 
The astragalus rests below on the os calcis, by means of two 
articulating surfaces of different sizes, and in front on the 
scaphoid bone, so that whatever pressure is transmitted to it is 
in turn transferred to the surfaces of the bones just named, 
with which it is in contact. The pressure is therefore trans- 
mitted in two directions, but as the astragalus, by means of its 
oe articulating surfaces, rests mainly on the os calcis, the 
“amount is transferred in the direction of this bone. 
' Op. cit. Tome I. pp. 119 and 121, also Pl. 43, Figs. 3, 4, and 7. 
