90 



THE HUMAN BODY. 



materials for its growth and nourishment. The average 

 diameter of the Haversian canals is 0.05 mm. (-g-J-^- of an 

 inch). 



Around each Haversian canal lies a set of plates, or 

 I lamellae, of hard bony substance (see the transverse section 

 Fig. 38), each canal with its lamellae forming an Haver- 

 sian 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 between them, and a certain number which 

 run around the whole bone on its outer surface. In the 

 spongy parts of the bone the Haversian canals are very 

 large and the intervening lamellae few in number. 

 I Between the lamellae lie small cavities, the lacuncB, each 

 / of which is lenticular in form, somewhat like the space which 



would be inclosed by two 

 watch-glasses joined by their 

 edges. From the lacunae many 

 extremely fine branching ca- 

 nals, the canaliculi, radiate 

 and penetrate the bony la- 

 mellae in all directions. The 

 innermost canaliculi of each 

 system open into the central 

 Haversian canal; and those of 

 various lacunae intercommu- 

 nicating, these fine tubes form a set of passages through 

 which liquid which has transuded from the blood-vessels 

 in the Haversian canals can ooze all through the bone. 

 The lacunae ami canaliculi are well seen in Fig. 39. 



In the living bone a granular nucleated cell lies in each 

 lacuna. These cells, or bone corpuscles, are the remnants 

 * of those which built up the bone, the hard parts of the lat- 

 ter being really an intercellular substance or skeleton 

 formed around and by these cells, much in the same way 

 as a calcareous skeleton is formed around each Foraminifer 

 (see Zoology) by the activity of its protoplasm. By the 

 co-operation of all the bone corpuscles, and the union of 

 their skeletons, the whole bone is built up. 



In other bones we find the same general arrangement of 



FIG. 39. A thin longitudinal sec- 

 tion of bone, magnified about 350 

 diameters, aa, Haversian canals. 



