THE CANCELLI OF BONES 115 



When the small size of the surface on which the 

 tibia rests is borne in mind, it will be readily anti- 

 cipated 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 areolas in 

 its interior of large size. The astragalus rests 

 below on the os calcis, by means of two artic- 

 ulating surfaces of different sizes, and in front 

 on the scaphoid bone, so that whatever pres- 

 sure 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 greater articulating surfaces, rests 

 mainly on the os calcis, the larger amount is trans- 

 ferred in the direction of this bone. 



On making a longitudinal section of this bone 

 (Fiy. 32), two series of cancelli are distinguishable 

 at sight one, a nearly vertical series (a), one 

 end of which sustains the arched portion of the 

 astragalus on which the tibia bears, and the other 

 rests on the surface beneath, which articulates 

 with the os calcis ; the second (&), a horizontal 

 series nearly at right angles to the preceding, one 

 end of which rests on the vertical series and the 

 other on the surface articulating with the sea- 



