62 



GENERAL ANATOMY. 



wall of the space (Fig. 34). This layer of osteoblasts forms a bony stratum, 

 and thus the wall of the space becomes gradually covered with a layer of true 

 osseous substance. On this a second 

 layer of osteoblasts arrange them- 

 selves, and in their turn form an os- 

 seous layer. By the repetition of this 

 process the original cavity becomes 

 very much reduced in size, and at last 

 only remains as a small circular hole 

 in the centre, containing the remains 

 of the embryonic marrow that is, a 

 blood-vessel and a few osteoblasts. 

 This small cavity constitutes the Ha- 



FIG. 34. Transverse section from the femur of a 

 human embryo about eleven weeks old. a. A med- 

 ullary sinus cut transversely; and b, another, long- 

 itudinally, c. Osteoblasts. a. Newly-formed osseous 

 substance of a lighter color, e. That of greater age. 

 /. Lacunae with their cells, g. A cell still united to 

 an osteoblast. 



FIG. 35. Vertical section from the edge of the 

 ossifying portion of the diaphysis of a metatar- 

 sal b'one from a foetal calf. (After Miiller.) a. 

 Ground-mass of the cartilage, b. Of the bone. e. 

 Newly-formed bone-cells in profile, more or less 

 imbedded in intercellular substance, d. Medul- 

 lary canal in process of formation, with vessels 

 and medullary cells, e. f. Bone-cells on their 

 broad aspect, g. Cartilage-capsules arranged in 

 rows, and partly with shrunken cell-bodies. 



versian canal of the perfectly ossified bone. The successive layers of osseous 

 matter which have been laid down and which encircle this central canal, consti- 

 tute the lamellae of which, as we have seen, each Haversian system is made up. 

 As the successive layers of osteoblasts form osseous tissue, certain of the osteo- 

 blastic cells remain included between the various bony layers. These continue 

 persistent, and remain as the corpuscles of the future bone, the spaces enclosing 

 them forming the lacuna (Fig. 34). The mode of the formation of the canaliculi 

 is not known. 



Such are the changes which may be observed at one particular point, the centre 

 of ossification. While they have been going on here a similar process has been 

 proceeding in the same manner toward the end of the shaft, so that in the ossify- 

 ing bone all the changes described above may be seen in different parts, from the 

 true bone in the centre of the shaft to the hyaline cartilage at the extremities. 

 The bone thus formed differs from the bone of the adult in being more spongy and 

 less regularly lamellated. 



Thus far, then, we have followed the steps of a process by which a solid bony 

 mass is produced, having vessels running into it from the periosteum, Haversian 



