STRUCTURE OF BONE. 



269 



258. From all parts of the lacunae, but especially from their two largest sur- 

 faces, proceed a large number of minute canaliculi, which traverse the substance 

 of the bone, and communicate irregularly with one another (Fig. 62). Their 

 direction, however, possesses a certain degree of determinateness ; for those 

 passing off from the inner surface converge towards the Haversian canal j whilst 

 those passing off from the outer surface diverge in the contrary direction, so as 

 to meet and inosculate with those proceeding inwards from the lacunae of the 

 next annulus. In this manner, a communication is kept up between the Haver- 

 sian canal and the most external of its concentric lamellae of bone. It is not 

 to be imagined, however, that Hood can be conveyed by these canaliculi, their 

 size being far too small ; for their diameter, at their largest part, is estimated at 

 from 144,000th to l-20,000th of an inch, whilst that of the smaller branches 

 is from l-40,000th to l-60,000th of 



an inch ; so that the blood-corpuscles Fig- 62. 



could not possibly enter them. But 

 it may be surmised that they draw 

 fluid from the nearest bloodvessels, 

 and thus keep up a sort of circulation 

 through the osseous substance, which 

 may contribute to its growth, and 

 may keep it in a state fit for repair- 

 ing itself, when injured by disease 

 or violence. 1 



259. Although a large quantity 

 of blood is sent to Bone, the vessels 

 do not penetrate its minute parts ; 

 being confined to the medullary 

 cavity, and to the Haversian canals 

 and the cancelli, which are prolon- 

 gations of it. The substance of the 



Bone, therefore, is really as non- Portion of a transverse section of the Human Clavicle, 

 Vascular as that Of Cartilage , the more highly magnified, to show the lacunae and canaliculi. 



only difference being, that it is 



channelled out by more numerous inflections of the external surface, and that 

 the vessels are thus brought into nearer proximity with its several parts. The 

 delicate osseous lamellae, which form the walls of the cancelli, and of the large 

 " cells" excavated in some of the cranial bones, have a structure precisely analo- 

 gous to that of the cylindrical laminae surrounding the Haversian canals of the 

 long bones ; and derive their nourishment from the vascular membrane cover- 

 ing their surface, through the medium of a similar set of lacunse and canaliculi. 

 They do not themselves contain Haversian canals or cancelli ; because no part 

 of their substance is far removed from a vascular membrane. The cylindrical 

 rods, that make up the hollow shaft of a long bone, are connected together by 

 solid osseous substance, which is composed of lamellse running parallel to the 



1 The lacunae and canaliculi of Bone were formerly supposed, on account of the dark 

 aspect they exhibit under the Microscope, when viewed as transparent objects, and their 

 white appearance when viewed by reflected light, to be filled with opaque matter ; but the 

 former is common to all cavities excavated in a highly-refracting substance (being shown 

 by a bubble of air in water), and ceases when a very thin section of Bone is examined, 

 especially if it have been placed in Canada balsam ; and the latter seems due to some 

 peculiar state of the earthy deposit immediately surrounding the cavities, as it may be 

 removed by the agency of an acid. In the bones of Mummies, these cavities are found to 

 be filled with a waxen material ; whilst in those which have lain in bogs, they are rendered 

 peculiarly distinct by the infiltration of some of the surrounding black matter ; and it is 

 not difficult to make them imbibe liquids, even whilst they are under observation beneath 

 the Microscope. 



