366 SPECIAL HISTOLOGY. 



less considerable remains even of the earlier bone-substance. 

 In this situation also, the absorption always affects not merely 

 the osseous nucleus, formed from the cartilage, but likewise the 

 periosteal layers, the latest of which only remain more in 

 their original form, as the substantia compact a. 



The Haversian canals do not originate, as is sufficiently 

 apparent from what has been said, like the cancelli of the 

 primary bone-substance, from a solution of a pre-existing tissue, 

 but are nothing more than open cavities, left from the commence- 

 ment, in the periosteal layers. They are, relatively, of a conside- 

 rable size at an early period, (vid. also 'Valentin. Entw.' p. 262), 

 measuring in the foetal humerus at five months 0016 — 0'024'", 

 in the femur at birth, according to Harting (p. 78), O10 — 0-024'", 

 just as in the most recently formed layers also of a later period. 

 Their contents have been already described. The most impor- 

 tant circumstance connected with them remaining to be noticed, 

 is the mode in which their lamellar systems originate. These 

 lamellse also are formed without the intervention of cartilage, 

 and are nothing more than deposits from the contents of the 

 canals, which substance, as has already been said, in respect 

 of its fibres and cells, entirely corresponds with the ossific 

 blastema beneath the periosteum, and, in a certain degree, is 

 merely an originally unossified remainder of it. These con- 

 ditions are easily observed in young bones, in which, the 

 periosteal layers, before they have undergone any resolution, are 

 rendered more and more compact by these new, secondary 

 lamellae; but even at a later period a more or less ossified 

 blastema (always without calcareous granules) may very fre- 

 quently be perceived on the walls of the canals in question. 

 Whilst the vascular canals are thus, on the one side undergoing 

 contraction by the deposition of these secondary layers, which, 

 just as in the periosteum itself, appear laminated, — either be- 

 cause the ossific blastema itself is so constructed, or because 

 the deposition of bone takes place with periodical pauses, — they 

 afterwards widen, or at least some of them, by absorption, as 

 for instance, the canales nutritii, the great vascular openings in 

 the apophyses, &c. ; and the compact substance, as has been 

 already remarked, is also, in many places partially, and in some 

 even entirely, absorbed. 



In what way the bone increases in thickness in the situa- 



