BONE. 313 



developed canals may generally be seen along the outer mar- 

 gin of the cutting. 



This view of the formation of the Haversian canals also 

 accords well with other characters presented by transverse 

 sections of bone. Thus, in all such, it will be seen that the 

 smallest Haversian canals are situated in the external part, 

 while the larger canals are placed internal to these : it will 

 also be observed, that the smaller canals are surrounded by 

 the fewest number of concentric lamella, and the larger by 

 the greatest number. (See Plate XXXII. fig. 1.) Now 

 the fact of an additional number of lamella encircling the 

 larger canals proves two things : first, that these large Haver- 

 sian channels are of older formation than the small ; and 

 second, that each lamella grows or expands after its depo- 

 sition, whereby the calibre of such canals becomes increased, 

 which is contrary to the generally entertained notion of the 

 formation of the Haversian canals, viz. by the filling up of 

 the large cancelli, brought about by the continual deposition 

 of new osseous lamellse, the outermost of which, for such a 

 result to ensue, must remain stationary in point of size. 



ACCIDENTAL OSSIFICATION. 



The abnormal growth and development of bone is a very 

 common pathological occurrence. Thus we have it occurring 

 on the surface of the bones themselves in the form of exostoses, 

 in the permanent cartilages, in the cellular tissue of muscles, 

 glands, the ovaries, membranes, as the coats of the arteries, 

 and probably occasionally also in that of every other tissue and 

 organ of the body. 



It is not, however, every ossific deposit which presents 

 all the characters of bone : thus those contained in the ovaries, 

 in the mesenteric glands, and in the coats of the arteries, 

 usually want the more conspicuous elements of bone, the bone 

 cells and lamella?, although these have been met with in ossific 

 depositions remote from all connexion with bone. 



In the reparation of fractures we have a development of 

 true bone preceded by the formation of cartilage. 



c c 2 



