THE BONE. 



1105 



Thus far there has been traced the formation of enlarged spaces (secondary 

 areolae), the perforated Avails of which are still formed by calcified cartilage-matrix, 

 containing an embryonic marrow, derived from the processes sent in from the 

 osteogenetic layer of the periosteum, and consisting of blood-vessels and round 

 cells, osteoblasts (Fig. 633). The walls of these secondary areolae are at this time 

 of only inconsiderable thickness, but they become thickened by the deposition 

 of layers of new bone on their interior. This process takes place in the following 

 manner : Some of the osteoblasts of the embryonic marrow, after undergoing 

 rapid division, arrange themselves as an epithelioid layer on the surface of the 

 wall of the space (Fig. 634). 

 This layer of osteoblasts form 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 ar- 

 range themselves, and in their 

 turn form an osseous layer. By 

 the repetition of this process the 

 original cavity becomes very 

 much reduced in size, and at 

 last only remains as a small cir- 

 cular hole in the centre, con- 

 taining the remains of the em- 

 bryonic marrow that is, a 

 blood-vessel and a few osteo- 

 blasts. This small cavity con- 

 stitutes the Haversian canal of 



the perfectly ossified bone. The transition to bone-corpuscles. 



successive layers of osseous 

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

 stitute 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 osteoblastic cells remain included between the various bony layers. These 

 persist as the corpuscles of the future bone, the spaces enclosing them forming 

 the lacunae (Figs. 634 and 636). The canaliculi, at first extremely short, are 

 supposed to be extended by absorption, so as to meet those of neighboring 

 lacunae. 



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 

 set up in the surrounding parts and has been gradually proceeding toward the end 

 of the shaft, so that in the ossifying 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 

 canals in which those vessels run, medullary spaces filled with foetal marrow, 

 lacunae with their contained bone-cells, and canaliculi growing out of these 

 lacunae. 



This process of ossification, however, is 'not the origin of the whole of the 

 skeleton, for even in those bones in which the ossification proceeds in a great 

 measure from a single centre, situated in the cartilaginous shaft of a long bone, a 

 considerable part of the original bone is formed by intramembranous ossification 

 beneath the perichondrium or periosteum ; so that the girth of the bone is increased 

 by bony deposit from the deeper layer of this membrane. The shaft of the bone 

 70 



FIG. 636. Osteoblasts from the parietal bone of a human em- 

 bryo thirteen weeks old. (After Gegenbauer.) a, Bony septa with 

 the cells of the lacunae. 6, Layers of osteoblasts. c, The latter in 



