68 



THE ESSENTIALS OF HISTOLOGY. 



absorption of bony matter wherever it occurs, by large multi-nucleated 

 cells (fig. 77, /, /) which are termed osteoclaxts. They are cells of the 



FIG. 76. LONGITUDINAL SEC- 

 TION THROUGH THE UPPER 

 HALF OF THE DECALCIFIED 

 HUMERUS OF A FCETAL SHEEP, 

 AS SEEN UNDER A MAGNIFYING 

 POWER OF ABOUT 30 DIA- 

 METERS. (From a drawing by 

 J. Lawrence.) 



ic, the part of the shaft which was 

 primarily ossified in cartilage ; 

 what remains of the primary bone 

 is represented as dark, enveloped 

 by the clear secondary deposit. 

 The areola; of the bone are occu- 

 pied by embryonic marrow with 

 psteoblasts, and blood-vessels var- 

 iously cut, represented as dark 

 lines. One long straight vessel 

 (bv) passes in advance of the line 

 of ossification far into the cartila- 

 ginous 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 absorp- 

 tion. ii, the part of the bone 

 that has been ossified in mem- 

 brane, that is to say, in the osteo- 

 blastic tissue under the perios- 

 teum. It is well marked off from 

 the central portion, and is bound- 

 ed, peripherally, by a jagged edge, 

 the projections of which are in- 

 distinctly seen to be prolonged by 

 bunches of osteogenic fibres. A 

 row of osteoblasts covers the 

 superficial layer of the bone. The 

 subperiosteal layer is prolonged 

 above into the thickening (p), 

 which encroaches upon the carti- 

 lage of the head of the bone, and 

 in which are seen, amongst nume- 

 rous 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 pass- 

 ing up between the rows of carti- 

 lage-cells is not seen in this speci- 

 men ; it would have extended as 

 far as a line joining the marks 

 x x . Observe the general ten- 

 dency of the osseous trabeculge 

 and the vascular channels between 

 them to radiate from the original 

 centre of ossification. This is 

 found to prevail more or less in 

 all bones when they are first 

 formed, although the direction of 

 the trabeculae may afterwards be- 

 come modified in relation with 

 varying physiological conditions, 

 and especially as the result of 

 pressure in different directions. 



same nature as the myeloplaxes of the marrow, and are as characteristic 

 of absorption surfaces as are the osteoblasts of surfaces where bony 

 deposit is proceeding (fig. 78). 



