58 



THE ESSENTIALS OF HISTOLOGY 



advances pari passu with the endochondral calcification, but beyond 

 this the uncalcified cartilage grows both in length and breadth, so that 

 the ossification is always advancing into larger and larger portions of 

 cartilage ; hence the endochondral bone as it forms assumes the shape 

 of an hour-glass, the cylindrical shape of the whole bone being main- 

 tained by additions of periosteal bone to the outside (see fig. 66). The 



FIG. 67. PART OF A LONGI- 

 TUDINAL SECTION OF THE 

 DEVELOPING FEMUR OF THE 



BABBIT. (Drawn under a 

 magnifying power of 350 

 diameters.) 



a, 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 trabeculte 

 being covered with osteoblasts 

 (<-), except here and there, 

 where an osteoclast (/) is seen, 

 eroding parts of the trabeculse; 

 <7, h, cartilage-cells which have 

 become shrunken and irregular 

 in shape. Prom the middle of 

 the figure downwards the dark 

 trabeculfe, 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. 



absorption of the calcified cartilage-matrix appears to be effected, as is 

 the case with absorption of bony matter wherever it occurs, by large 

 multinucleated cells (fig. 67, /, /) which are termed osteoclasts. They 

 are the same as the myeloplaxes of the marrow. 



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 lamellae 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 canal of the 

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

 originally occupies the centre of the shaft. 



