BONES 359 



called the patella. The shin is formed by one long bone 

 called the tibia, upon the outside of which is a very slender 

 bone called the fibula. The lower end of the fibula forms- 

 the outer ankle bone, while a projection from the tibia 

 forms the inner ankle bone. 



Seven rounded bones, called tarsal bones, form the instep 

 of the foot. Five slender bones beyond them, called meta- 

 tarsal bones, form the ball of the foot. The tarsal and meta- 

 tarsal bones are bound together so as to form an arch 

 which bears upon the ground only at the heel and ball of 

 the foot. The arch is somewhat elastic and prevents jar- 

 ring of the body in walking. It sometimes becomes flat- 

 tened, producing the painful deformity called flat foot. 

 Fourteen slender bones, called phalanges, form the 

 toes. 



644. Structure of bones. Bones are dense and brittle 

 upon the outside. All are covered with a very tough mem- 

 brane, called \ht periosteum. In flat or rounded bones the 

 hard outside surfaces are scarcely thicker than paper, and 

 the two surfaces are connected together by a network of 

 bone which looks like a honeycomb, and is called cancel- 

 lous bone. 



In long bones, the central shaft is composed of a thick 

 shell of hard bone surrounding a cavity filled with fat; 

 their ends consist of a thick shell of hard bone covering a 

 large mass of cancellous bone. 



A bone is about twice as strong as a piece of oak of the same size. 

 It is elastic and can bend considerably without breaking. Any given 

 weight of a substance is stronger when made into a hollow shaft, like 

 a bicycle frame, than when formed into a solid rod of the same length. 

 So the strength of a bone is still further increased by its being either 

 hollow, as in the shaft of a long bone, or else braced with cancellous 

 bone, as in a flat bone and the ends of a long bone. 



