452 LECTURE XVIII. 



cepting at the two ends. Now a tissue once ossified 

 ceases (save under exceptional circumstances) to grow, 

 so that any increase of thickness in the diaphysis must 

 be wholly due to a development out of periosteum, in 

 which growth proceeds much more slowly than in car- 

 tilage. This is the reason why the shaft of a long bone 

 is narrower than its extremities.* Whilst in this way 

 parts which were previously either connective tissue or 

 cartilage, are converted into bone, the development of 

 the medullary tissue is going on within the bone. The 

 original bone is extremely dense, a very solid and rela- 

 tively compact mass. Subsequently the substance of 

 the bone disappears more and more, one part of it after 

 another is dissolved, and the medullary cavity [canal] 

 arises, the size of which is not in any way restricted to 

 that of the original osseous rudiment, but often conside- 

 rably exceeds it. Thus the development of bone, when 

 taken as a whole, does not consist merely in the gradual 

 apposition of a succession of fresh osseous layers derived 

 from periosteum and cartilage, but also in the continual re- 

 placement of the innermost layers of the bone by masses 

 of marrow. 



In the interpretation of these facts the blastema the- 

 ory was long appealed to as the great authority. Havers 

 and Duhamel, who made excellent investigations into 

 the history of bone, started with the supposition that a 

 nutritious juice (succus nutritius) was secreted, from 

 which the new masses arose. The development of the 

 marrow was imagined to consist in a formation of 

 cavities, into which first a viscous juice and then a fatty 

 matter was secreted cavities which were invested by 

 the medullary membrane, and whose contents varied 



* For a diagram of the growth of long bones, see Havers (Osteologia nova. 

 Francof. 1692, Tab. I., fig. 1), and KSlliker (Handbuch d. Gewebelehre, 3d edit., 

 Leipzig, 1859, p. 259). 



