176 



STUDIES IN PHYSIOLOGY 



Advantages of Hollow Bones. In the bone structure we 

 have just described, two advantages are combined, namely, 

 lightness and strength. The framework of a bicycle best 

 illustrates the principle involved, which is that the greatest 



possible strength and lightness 

 are secured in a given amount 

 of material by using hollow cyl- 

 inders. The spongy bone within 

 the hard outside layer strength- 

 ens still further the bony cylin- 

 der without adding greatly to the 

 body weight. 



Blood Supply in Bones. In 

 the periosteum and throughout 

 the hard and spongy bone and the 

 marrow run numerous blood ves- 

 sels that bring the proteid and 

 other food materials required for 

 the nutrition of the living bone 

 cells. From the blood, also, these 

 cells take out the various mineral 

 matters needed to form the hard 

 intercellular substance, which 



gives to bone its rigidity. We 



The branching tubes are canals , , * . J , , , 



through which run blood ves- have already noted the fact that 

 seis. The irregular black new red corpuscles are produced 

 spots represent the outlines of ,1 _, TnflT . rftw nf 



] 



FIG. 75. A Longitudinal 

 tion of Bone X 200. 



living bone cells. The white 



portions of the figure show Classification of the Bones of 



the bony intercellular sub- the Human Skeleton. For COn- 

 stance (compare with Fig. 8). 



venience, the bones of the hu- 



man skeleton are divided into four groups. In the first 

 group are the long bones, the humerus, radius, ulna, femur, 

 tibia, fibula, metacarpals, metatarsals, and the phalanges of 

 fingers and toes. All these bones have a structure similar 

 to that of the soup bone just described, in that they have 

 two heads and a shaft, and a central cavity filled with yel- 



