STRUCTURE OF BONE. 163 



other towards, the Haversian canal. Their long diameter is commonly from 

 [Fig. 62. [Fig. 63. 



Transverse section of the compact tissue 

 of along Bone; showing 1, the periosteal 

 layer ; 2, the medullary layer, and the inter- 

 mediate Haversian systems of lamellae, each 

 perforated by an Haversian canal. Mag- 

 nified about 15 diameters.] 



Transverse section of the compact tissue of a Tibia 

 from an aged subject, treated with acid ; showing the 

 appearance of lamellae surrounding the Haversian ca- 

 nals. Portions of several systems of lamella are seen. 

 The appearance of the lacunae, when their pores are 

 filled with fluid, is also seen, as well as the radiation 

 from the canals which then remain. From Mr. Tomes.] 



l-2400th to l-1600th of an inch ; their short diameter is about one-third, and 

 their thickness about one-sixth, of their length. 



a. It has been lately shown by Mr. J. Quekett, that there are differences in the form and 

 size of the lacunae, in the several classes of animals, sufficiently characteristic to allow of the 

 assignment of minute fragments of bone, with the aid of the microscope, to their proper class. 

 The lacuna? of Reptiles are distinguishable by their large size, and long oval form; and those 

 of Fish, by their angular form and the fewness of the radiating canaliculi. The osseous 

 lacuna? of the Bird maybe distinguished from those of the Mammal, partly by their smaller 

 size, but chiefly by the remarkable tortuosity of their canaliculi, which wind backwards and 

 forwards in such a manner, as frequently to destroy the concentric lamellar appearance. It 

 is interesting to remark further, that the sizes of the lacuna; in the four classes of the Verte- 

 brated animals, bear a close relation to the sizes of their blood-corpuscles. Here, as else- 

 where, the dimensions of the ultimate parts of the tissue are tolerably constant in each group 

 of animals, and show little variation in accordance with the size of the species ; thus there 

 is little or no perceptible difference in the size of the elements of the osseous tissue of the 

 enormous extinct Iguanodon, and of the smallest Lizard now inhabiting the earth. 



Fig. 64. 



194. From all parts of these cavities, 

 but especially from their two largest 

 surfaces, proceed a large number of 

 minute tubuli, which traverse the sub- 

 stance of the bone, and communicate 

 irregularly with one another. Their 

 direction, however, possesses a certain 

 degree of determinateness ; for those 

 passing off from the inner surface con- 

 verge towards the Haversian canal ; 

 whilst those passing off from the outer 

 surface diverge in the contrary direc- 

 tion, so as to meet and inosculate with 

 those proceeding inwards from the 



cavities of the next annulus. In this manner, a communication is kept up 

 between the Haversian canal, and the most external of its concentric lamellae 



Lacunae of Osseous Substance; magnified 

 500 diameters : a, central cavity ; b, its ramifica- 

 tions. 



