42 THE HOUSE I LIVE IN. 



you could blow hard enough, you might blow 

 through any kind of wood whatever. The 

 philosopher and chemist will force water and 

 even quicksilver through the pores of almost 

 any sort of wood. Surely then air can go 

 through. At all events, whether we can get 

 the air through or not, there are indeed holes 

 through it. 



But you cannot blow through any of the 

 timbers of the house I live in. This shows 

 that the internal structure of bone is very differ- 

 ent from that of wood. I will endeavor to 

 show you wherein it is different. 



SHAPE OF BONES. Bones are of three 

 kinds ; long bones, broad or flat bones, and 

 round bones. The long bones have a hollow 

 through them containing marrow or pith ; but 

 the other two sorts of bones have no such 

 hollow. They have, however, a great many 

 little holes or cells in the inside. Some of 

 them look, on breaking them, almost like sponge 

 or honey-comb. Some of the long bones, be* 

 sides being hollow, are also spongy. They 

 are largest and most spongy at the ends, and 

 smaller and more firm at the middle. 



