STRUCTURE AND COMPOSITION OF BONE. JOINTS. 89 



parts of the bone the Haversian canals are very large and the 



intervening lamellae few in number. 



Between the lamellae lie small cavities, the lacunce, 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 canals, the canali- 

 culi , radiate and penetrate the 

 bony lamellae in all directions. 

 The innermost canaliculi of each 

 system open into the central Ha- 



. FIG. 42.-A thin'ion^tudinai^ec- versian canal; and those of various 



tion of bone, magnified about 350 lop 1iri jYi infprpnmTrmrnnufiTirr fViaao 

 diameters, oo, Haversian canals. 1RCl1 miCdtlllg, tnese 



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 and canaliculi are shown in Fig. 42. 



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 latter 

 being really an intercellular substance or skeleton formed 

 around and by these cells, much in the same way as a calca- 

 reous skeleton is formed around a Foraminifer 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 

 the parts, an outer dense layer and an inner spongy portion. 

 In the flat and irregular bones there is no medullary cavity, 

 and the whole centre is filled up with cancellated tissue with 

 red marrow in its spaces. For example, in the thin bones 

 roofing in the skull we find an outer and an inner hard layer of 

 compact bone known as the outer and inner table respectively, 

 the inner especially being very dense. Between the two tables 

 lies the spongy bone, red in color to the naked eye from the 

 marrow within it, and called the diploe. The interior of the 

 vertebrae also is entirely occupied by spongy bone. Every- 

 where, except where a bone joins some other part of the skel- 

 eton, it is covered by the periosteum. 



Chemical Composition of Bone. Apart from the bone- 

 corpuscles and the soft contents of the Haversian canals and 



