THE HUMAN BODY. 



i 



cavity and pass insensibly into the spaces ot the spongy 

 bone around it. These openings are the cross sections of 



Fio. 23. A, a transverse section oi the ulna, natural size ; showing the medul- 

 lary cavity. B\ the more deeply shaded nart of A magnified twenty diameters. 



tubes known as the Haversian canals, which run all 

 through the bone, the majority of them in the direction of 

 its long axis, though numerous cross branches unite them. 

 The outermost Haversian canals open on the surface of 

 the bone beneath the periosteum ; from there blood-ves- 

 sels pass in, and, traversing the whole bone in these chan- 

 nels, convey materials for its growth and nourishment. 



What are the Haversian canals? Where do the outer ones open? 

 What enters them? Where do tiie blood-vessels of a bone run? 

 What do they carry n 



