DEVELOPMENT OF BONE. 



69 



The bone which is first formed is more reticular and less regularly 

 lamellar than that of the adult, and contains no Haversian systems. 

 The regular lamella? are not deposited until some little time after 

 birth, and their deposition is generally preceded by a considerable 

 amount of absorption. It is about this time also that the medullary. 



FIG. 77. PAKT OF A LONGI- 

 TUDINAL SECTION OF THE 

 DEVELOPING FEMUR OF THE 



BABBIT. (Klein.) (Drawn un- 

 der a magnifying power of 350 

 diameters. ) 



<i, rows of flattened cartilage-cells : 

 b, greatly enlarged cartilage- 

 cells close to the advancing bone, 

 the matrix between is partly 

 calcified ; c, d, already formed 

 bone, the osseous trabeculaj 

 being covered with osteoblasts 

 (e), except here and there, 

 where an osteoclast (/) is seen, 

 eroding parts of the tuberculae ; 

 g, h, cartilage-cells which have 

 become shrunken and irregular 

 in shape. From the middle of 

 the figure downwards the dark 

 trabeculaa, which are formed of 

 calcified cartilage-matrix, are be- 

 coming covered with secondary 

 osseous substance deposited by 

 the osteoblasts. The vascular 

 loops at the extreme limit of the 

 bone are well shown, as well as 

 the abrupt disappearance of the 

 cartilage-cells. 



canal of the long bones is formed by the absorption of the bony tissue 

 which originally occupies the centre of the shaft. 



After a time the cartilage in one or both ends of the long bones begins 

 to ossify independently, and the cpipliyses are formed. These are not 

 joined to the shaft until the growth of the bone is completed. Growth 

 takes place in length by an expansion of the cartilage (intermediate carti- 

 lage} which intervenes between the shaft and the epiphyses, and by the 

 gradual extension of the ossification into it ; in width entirely by the 

 deposition of fresh bony layers under the periosteum. In the terminal 



