USES OF THE BONES. 349 



to the infinity of movements of which the latter are capable. The 

 ribs are thus subservient to the movements of respiration. 



3. The long bones especially manifest the power to resist fracture 

 before illustrated, and to this effect the hollow cylindrical form of 

 their diaphyses greatly contributes. Indeed, if the weight of the 

 shaft, and the length, be the same, its strength as a lever varies 

 (within certain limits) directly as its diameter, i. e. if the shaft of the 

 os femoris, weighing sixteen ounces, and being seventeen inches 

 long, is hollow and one inch in diameter, it is twice as strong as 

 if of the same length and weight, and condensed into a solid rod 

 half an inch in diameter. Thus we perceive the advantage result- 

 ing from the function of the medullary canal in the long bones ; 

 viz., to increase the strength of the bone, the amount of material in 

 it being given. Every blade of grass, indeed, is constructed on the 

 same principle. This arrangement is also apparently more conso- 

 nant with the power of rapid repair after fracture in the long bones; 

 in which fractures must necessarily be most frequent. Finally, the 

 existence of the cavity necessitates the presence of a substance to 

 fill it, which is accomplished by the marrow, as above explained 

 (p. 341). 



4. But the long bones are also for support, especially in the lower 

 extremities ; and in this relation some interesting facts are ascer- 

 tained. The strength of bone as a mere column of support, its 

 weight and general conformation being the same, varies inversely 

 with the square of its length; i. e. an os femoris 8J inches long, and 

 weighing 17 ounces, would be four times as strong as one twice as 

 long, but of the same weight. Hence the short bones are vastly 

 stronger in proportion, in this respect. Indeed, we may imagine 

 the lower extremities of an animal to become incapable of sustain- 

 ing its weight, from a slight increase of their length, while the bones 

 become no heavier. Hence only animals having light bodies have 

 long extremities ; while the very heavy have proportionally short 

 columns of support. Hence, also, the tarsus and carpus of the 

 lower animals, as well as man, consist of short bones. Moreover, 

 the speed of animals cannot be increased in proportion to size; the 

 skeleton becoming at length so heavy that much muscular force is 

 exhausted in merely sustaining it. 



5. Man has comparatively a long column of support, especially so 

 far as the os fernoris is concerned ; and thus the power of rapid 

 locomotion is secured. The diminution of strength consequent on 



