274 



BONE OR OSSEOUS TISSUE. 



which are still occupied by the cartilage-cells, either singly or in elongated groups, 

 being termed the primary areolce (Sharpey). Simultaneously with this deposit in 

 the cartilage-matrix, a layer of osseous substance (fig. 311, im) is becoming formed 

 on the outside of the cartilage underneath the periosteum. This last is a vascular 

 membrane, containing numerous osteoblasts (0), which are chiefly collected on the 



Fig. 313. LONGITUDINAL SECTION THROUGH 



THE UPPER HALF OF THE DECALCIFIED 

 HUMERUS OF A FCETAL SHEEP, AS SEEN 

 UNDER A MAGNIFYING POWER OF ABOUT 



30 DIAMETERS. , (Drawn by Mr. J. 

 Lawrence.) (E.A.S.) 



ic, the part of the shaft which was pri- 

 marily ossified in cartilage ; what remains of 

 the primary bone is represented as dark, 

 enveloped by the clear secondary deposit. 

 The areolas of the bone are occupied by 

 embryonic marrow with osteoblasts, and 

 blood-vessels variously cut, represented as 

 dark lines. One long straight vessel (bv) 

 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 cartila,ge-cells 

 (c) may still be seen which have as yet 

 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 portion, and is bounded, 

 peripherally, by a jagged edge, the projec- 

 tions of which are indistinctly 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 cartilage of the 

 head of the bone, and in which are seen, 

 amongst numerous osteoblasts and a few 

 blood-vessels, the straight, longitudinal oste- 

 ogenic 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 granu- 

 lar ossific deposit passing up between the 

 rows of cartilage -cells is not seen in this 

 specimen. Observe the general tendency 

 of the osseous trabeculaa 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 trabeculse may afterwards become modified 

 in relation with varying physiological condi- 

 tions, and especially as the result of pressure 

 in different directions. 



inner surface next to the cartilage, 

 ic im and it is by their agency that the 



bony layer on the surface of the 



cartilage is formed and becomes increased both in thickness and length. The 

 bony_ layer, when viewed on the surface, shows the usual component fibres 

 of bony lamellas, and as other layers are deposited upon it lacunae become 

 formed between them by the inclusion of some of the osteoblasts. In this first 

 stage of ossification, we see therefore two processes going on, a deposit of earthy 



