TISSUES OF THE BODY. 241 



Several Haversian canals may often be seen also uniting with their enlarged 

 ends to form a small medullary cavity, between which and the larger 

 kind many intermediate forms exist. 



REMARKS. Beside the German works on the subject, compare Tome's article, 

 " Osseous Tissue," in the Cyclopedia of Anatomy and Physiology, as well as the 

 excellent treatise of Tomes and De Morgan in the Phil. Transact, for the year 1853, 

 part i. p. 109. 



141. 



The hard homogeneous osseous tissue between these Haversian canals 

 has a laminated structure, explained by the mode of origin and formation 

 of the mass in successive portions. These lamellae are united in the most 

 intimate manner with one another, but may be separated in macerated 

 bone which has been deprived of its mineral constituents. 



The systems of laminae, however, are of two classes. In one of 

 these the leaves affect the whole thickness of the bone, in the other 

 they are arranged round the individual Haversian canals. We may 

 designate the first as general or fundamental, the others as special or 

 Haversian lamellae. 



Nowhere can this be better seen than in a transverse section of the 

 middle portion of a hollow bone, such as we have in fig. 233. The 

 general lamellae are here distinguishable as a system of concentric layers 

 traversing the whole thickness of the piece : commencing internally (b) 

 around the central medullary canal of the bone, whose walls they form 

 (medullary lamellae) ; then usually less distinct in the middle portion (d), 

 with numerous interruptions (intermediate lamellae), and on the other 

 hand appearing in the most distinct manner again externally towards the 

 periosteum (d) (periosteal lamellae). Of course these stratifications belong 

 to one and the same system of lamellae. The number and the thickness 

 of the individual leaves is subject to variation. The latter amounts to 

 0'0077-0'0156 mm. and upwards. The special lamellae surround the 

 Haversian canals in varying number from 6-18, with extremes in both 

 directions (c). Their thickness is, on an average, 0'0065-0'0127 mm., 

 and their arrangement is, as a rule, more or less concentric, the most 

 internal of them constituting the walls of the Haversian canals. The 

 latter are not unfrequently situated eccentrically in their systems of 

 lamellae. Should this be the case to any great extent, the latter may be 

 incomplete towards one side, and it occasionally happens also that the 

 systems of two Haversian canals are enclosed again in secondary lamellae 

 (Tomes and De Morgan). The strength of these systems further round 

 the canals is very variable. Those of the latter, which have a medium 

 calibre, usually possess the strongest. In the heavier cylindrical bones 

 of the human skeleton, the Haversian canals usually lie so close 

 together that their concentric lamellae almost entirely obscure the inter- 

 mediated ones ; not so, however, in the smaller bones of the metacarpus 

 and fingers, where the distance between them remains greater, as is the 

 case generally among other mammals. 



If we prepare a longitudinal section of the compact substance of a long 

 bone, the extended network of the Haversian canals will be seen sur- 

 rounded by lines running parallel with their contour, and at the same 

 distance from one another as those concentric ones of the transverse 

 section. Thus the lamellae appear to be a system of tubes of consider- 

 able length, disposed one within the other, and placed, as a rule, per- 



