DEVELOPMENT OF BONE 



57 



only by remains of the calcined cartilage-matrix (fig. 67, c), but they 

 soon become thickened by lamellae of fibrous bone (b) which are 



FIG. 66. LONGITUDINAL SEC- 

 TION THROUGH THE UPPER 

 HALF OF THE DECALCIFIED 

 HUMERUS OF A FCETAL SHEEP, 

 AS SEEN UNDER A MAGNIFY- 

 ING POWER OF ABOUT 30 

 DIAMETERS. 



ic, the part of the shaft which was 

 primarily ossified in cartilage ; 

 what remains of the primary 

 bone is represented as dark, en- 

 veloped by the clear secondary 

 deposit. The areolaa of the bone 

 are occupied by embryonic mar- 

 row with osteoblasts, and blood- 

 vessels variously cut, represented 

 as dark lines. One long straight 

 vessel (ftc) passes in advance of 

 the line of ossification far into 

 the cartilaginous head, most of 

 the others loop round close to the 

 cartilage. At one or two places 

 in the older parts of the bone 

 elongated groups of cartilage- 

 cells (c) may still be seen, which 

 have escaped absorption, im, the 

 part of the bone that has been 

 ossified in membrane, that is to 

 say, in the osteoblastic tissue 

 under the periosteum. It is well 

 marked off from the central por- 

 tion, and is bounded, peripherally, 

 by a jagged edge, the projections 

 of which are indistinct!}' seen to 

 be prolonged by bunches of osteo- 

 genic fibres. A row of osteoblasts 

 covers the superficial layer of the 

 bone. The subperiosteal layer is 

 prolonged above into the thicken- 

 ing (p), which encroaches upon 

 the cartilage of the head of the 

 bone, and in which are seen, 

 amongst numerous osteoblasts 

 and a few blood-vessels, the 

 straight longitudinal osteogenic 

 fibres ( of), and some other fibres 

 (_pf) crossing them, and perhaps 

 representing fibres of Sharpey. 

 The calcareous salts having been 

 removed by an acid, the granular 

 ossific deposit passing up between 

 the rows of cartilage-cells is not 

 seen in this specimen ; it would 

 have extended as far as a line 

 joining the marks x x . Observe 

 the general tendency of the osse- 

 ous trabecula? and the vascular 

 channels between them to radiate 

 from the original centre of ossifi- 

 cation. This is found to prevail 

 more or less in all bones when 

 they are first formed, although 

 the direction of the trabeculse 

 may afterwards become modified 

 in relation with varying physio- 

 logical conditions, and especially 

 as the result of pressure in dif- 

 ferent directions. 



ic 



deposited by the osteoblasts, and between which bone-corpuscles 

 become included, as in the case of the subperiosteal bone. The latter 



