118 THE CANCELLI OF BONES 



ingly to the intervening portions ; a second series 

 at right angles to the last and which, as the 

 former radiate from a common centre, will ne- 

 cessarily assume a curved direction. By far the 

 largest portion of the first are directed towards 

 the tuberosity of the heel, which serves the dou- 

 ble purpose of a base and lever. That portion 

 which is just beneath the articulating surface, 

 and which does not come within the range of 

 either of the surfaces of support, may be regarded 

 as forming an inverted arch. 



The os calcis of man contrasts with that of 

 other animals, not only in its size and relation to 

 the rest of the foot, but in its minute and inter- 

 nal arrangement, so that the assertion made by 

 Mr. Lawrence many years ago, independently of 

 its structure within, that " ex calce hominem ' 

 would be a safer rule than " ex pede Herculem" 

 gains additional force. 1 



In the above descriptions the minute structure 

 of several bones has been described as well as the 

 nature of the force which they are intended to 

 resist. It is not always safe to attempt to assign 

 the final cause of animal structures, to indicate 

 the intention of nature in certain conditions of 

 things though there can be no risk in describ- 

 ing in connection such conditions of organization 



1 Lectures on Physiology, Zoology, and the Nat. Hist, of Man, 

 p. 124. London, 1822. 



