50 THE HUMAN BODY. 



head of a small pin. In these spaces there is found dur- 

 ing life a substance known as the red marrow, which is 

 quite different from the yellow fatty marrow of the medul- 

 lary cavity.* 



Why bones are hollow. If the bones were solid they 

 would be extremely heavy and unnecessarily strong for 

 the common purposes of life, unless only the same amount 

 of material were used in their construction, and then they 

 would be either loose in texture and easily broken, or, 

 if dense, they would be thin rods and not give sufficient 

 surface for the attachment of muscles. It is a well-known 

 principle in practical mechanics that the same amount of 

 material will bear a greater strain if in the form of a tube 

 than in that of a solid rod of the same length ; hence iron 

 pillars are cast hollow; to fill them up solidly would make 

 them enormously heavier without anything like a propor- 

 tionate increase in strength. Take a glass tube one foot 

 long and a piece of glass rod of the same length and 

 weight; support each at its ends and hang weights on the 

 middle until it breaks; the tube will be found to bear a 

 very much greater strain before yielding. We see an ap- 

 plication of this same method of utilizing a given amount 

 of material to the best advantage for support, in the hol- 

 low stalks of grass, wheat, and barley. 



Varieties of structure found in . different bones. Bones 

 which, like the humerus and femur, present a shaft and 



What lies in the cavities of the spongy bone ? 



Why are most bones hollow ? Which will bear most weight, a 

 tube or a solid rod of the same material, weight, and length? Give 

 illustrations. Why are grass stalks hollow ? 



* Many of the bones of birds are thin-walled tubes of dense bone : the central 

 cavity contains air and no marrow, and communicates by tubes with the lungs, 

 Examine the humerus of a pigeon or a rooster, 



