312 THE SOLIDS. 



Others carried still deeper into the bone, these several 

 apertures will have coalesced into one large space the rudi- 

 mentary medullary cavity. 



From these several particulars, I therefore infer that the 

 canals in question are intimately connected with the form- 

 ation of the medullary cavity ; and that the absorption of the 

 cancelli situated between each of them is brought about by 

 the vessels contained within them, aided also probably by the 

 granular cells. 



Were the canals merely destined to convey to the cartilage 

 the nourishment necessary for its transformation into bone, 

 it might be expected that they would serve as so many centres 

 from which the ossification would proceed ; we have seen, 

 however, that the cartilage in their immediate vicinity is the 

 last to be removed, and its place supplied by bony cancelli. 



Medullary cells. The primary cancelli are small, studded 

 with granules, form closed cavities, and do not contain bone 

 cells. (See Plate XXXIV. Jigs. 2, 3.) The secondary and 

 larger cancelli are formed by the absorption of the numerous 

 septa of the primary cancelli ; they do not form closed cavities, 

 but communicate freely together and contain bone cells im- 

 bedded in their parietes. (See Plate XXXI Y./^. 4.) 



Haver sian canals. The Haversian canals are generally de- 

 scribed as being formed by the filling up of certain of the me- 

 dullary cells, in consequence of the successive deposition of 

 new lamina? of bony matter. It seems to me, however, to be 

 very questionable, whether they are ever formed in the manner 

 indicated, and if so, such is assuredly not the general mode 

 of their formation. 



I am induced to take a different view of their formation, 

 and consider they originate as follows : 



The surface of all bones, whether long, flat, or irregular, 

 is observed to be marked with numerous grooves of different 

 sizes and depths. In the recent state these are occupied with 

 blood-vessels, and it is around them that successive layers of 

 bone are deposited, until at length the vessels become en- 

 tirely included and a perfect canal is formed. 



In transverse sections of long bones, grooves and partially 



