88 



THE HUMAN BODY. 



united by numerous cross or oblique branches as seen in the 

 longitudinal section (Fig. 42). The outermost ones open on 

 the surface of the bone beneath the periosteum, and in the 

 living bone blood-vessels run from this through the Haversian 

 canals and convey materials for its growth and nourishment. 



B 



FIG. 41. A, a transverse section of the ulna, natural size; showing the medullary 

 cavity. B, the more deeply shaded part of A magnified twenty diameters. 



The average diameter of the Haversian canals is 0.05 mm. 

 (^ of an inch). 



Around each Haversian canal lies a set of plates, or lamellae, 

 of hard bony substance (see the transverse section Fig. 41), 

 each canal with its lamellae forming an Haversian system : 

 and the whole bone is made up of a number of such systems, 

 with the addition of a few lamellae lying in the corners be- 

 tween them, and a certain number which run around the 

 whole bone on its outer and inner surfaces. In the spongy 



