MINUTE STRUCTURE OF BONE. 695 



canaliculi) of the different rings with each other, a continuous 

 communication is established between the central Haversian canal 

 and the outermost part of the bony rod that surrounds it, which 

 doubtless ministers to the nutrition of the texture. Blood-vessels 

 are traceable into the Haversian canals, but the ' canaliculi ' are 

 far too minute to carry Blood-corpuscles ; they are occupied, how- 

 ever, in the living Bone, by threads of sarcodic substance, which 

 bring into communication with the walls of the Blood-vessels the 

 segments of 'germinal matter' contained in the Lacunas. 



543. The minute cavities, or Lacunae (sometimes, but erroneously 

 termed 'bone-corpuscles,' as if they were solid bodies), from which 

 the canaliculi proceed (Fig. 356), are highly characteristic of the 

 true Osseous struc- 

 ture ;' being never 

 deficient in the 

 minutest parts of 

 the bones of the 

 higher Vertebrata, 

 although those of 

 Fishes are occa- 

 sionally destitute of 

 tb em. The dark 

 appearance which 



they present in sec- Lacunae of Osseous substance : — a, central cavity ; 

 tions of a dried Bone b, its ramifications, 



is not due to opacity, 



but is simply an optical effect, dependent (like the blackness of air- 

 bubbles in liquids) upon the dispersion of the rays by the highly-re- 

 fracting substance that surrounds them (§ 1 28). The size and form of 

 the Lacunas differ considerably in the several Classes of Vertebrata, 

 and even in some instances in the Orders ; so as to allow of the 

 determination of the tribe to which a bone belonged, by the Micro- 

 copic examination of even a minute fragment of it (§ 595). The 

 following are the average dimensions of the Lacuna?, in characteristic 

 examples drawn from the four principal Classes, expressed in 

 fractions of an inch : — 



The Lacunas of Birds are thus distinguished from those of Mam- 

 mals by their somewhat greater length and smaller breadth ; but 

 they differ still more in the remarkable tortuosity of their canaliculi, 

 which wind backwards and forwards in a very irregular manner. 

 There is an extraordinary increase in length in the Lacunas of 

 Reptiles, without a corresponding increase in breadth ; and this is 

 also seen in some Fishes, though in general the Lacunas of the 



