OSSEOUS TISSUE. 

 Fig. 217. 



329 



B 



A. Transverse section of ulna, deprived of its earth by an acid. The openings of the Haversian 

 canals seen, natural size. A small portion is shaded, to indicate the part magnified in B. B. Part 

 of the section A, magnified 20 diameters. The fundamental or general lamellae are seen at a, and 

 between the concentric lamellae the lacunae appear as little dark specks. b. Portion of a cancellus. 



each lamella averages yi^ to ^ of an inch in the cranial bones, 

 and their number varies from 10 to 100. The layer formed by them 

 varies between ^ and ^ of an inch in thickness. It gives off pro- 

 cesses, B J n to T o of an inch thick, between the Haversian rods. 



2. The special lamellae those concentrically surrounding the Ha- 

 versian canals constitute, as it were, the walls of the latter, and are 

 intimately united to each other. The number surrounding the 

 canal, and the consequent thickness of the system the Haversian 

 rod formed by them, bear no constant relation to the size of the 

 canal; smaller canals being sometimes surrounded by numerous 

 lamellae, and larger ones by but few. Generally the largest canals, 

 and the most minute, have but few surrounding lamellae, and there- 

 fore have thin walls ; while those of a middle size have thick ones. 

 The thinnest walls measure j^V^ to g of an inch, and the thickest 

 jh to iltf f an inch. (Kolliker.) Each lamella is from 5 ^V^ to 

 siW of an inch thick, averaging from 4^0 <r to -goW f an inch. 



