INTRA CA RTILA GIXO US OSSIFICA TTON 



45 



surface of which the cells become osteoblasts, or bone-forming cells. By the agency of these 

 cells a thin layer of bony tissue is being formed between the outer membrane, now the periosteum, 

 and the cartilage by the intramembranous mode of ossification just described; this constitutes 

 the first periosteal lamella. These two processes go on simultaneously. The second stage 

 (vascularization) consists in the prolongation into the cartilage of processes of the deeper or 

 osteogenetic layer of the periosteum, these processes consisting of bloodvessels and cells 

 osteoblasts, or bone formers, and osteoclasts, or bone destroyers. The latter are similar to 

 the giant cells (myeloplaques) found in marrow, and they excavate passages through the new- 

 formed bony layer by absorption, and pass through it into the areolae. Wherever these processes 

 come in contact with the calcified walls of the primary areola 3 they absorb it, and thus cause a 

 fusion of the original cavities and assist in the formation of larger spaces, which are termed the 

 secondary areolae (Sharpey), or medullary spaces. These secondary spaces become filled with 

 embryonic marrow, consisting of osteoblasts, vessels, a few leukocytes, and a few myelocytes. 

 The first periosteal lamella is rapidly followed by the formation of others of the same nature, 

 the osteoblasts secreting the lamella 3 remaining between the successive layers with their pro- 

 cesses passing from one to the other. The spaces occupied by these cells are the lacunas and the 

 small channels occupied by the processes are the canaliculi. A periosteal lamella is not smooth 

 and regular and does not extend completely around the developing bone, but meets others 

 that aid in completing the circle. The irregularities are due to projecting processes of bone that 

 meet others and enclose small, irregu- 

 lar, longitudinal canals, which contain 

 vessels and primitive marrow, and 

 are the primitive Haversian canals. 

 These are als_o seen at the junctions 

 of the lamella 3 (Fig. 13). The osteo- 

 clasts of the primitive marrow apply 

 themselves to the walls of the canals 

 and absorb the osseous tissue until a 

 comparatively large and regular 

 canal is formed, and within this canal i 

 the osteoblasts secrete successive 

 concentric layers of bone until a 

 small central canal alone remains 

 which contains a little marrow and ^ 

 the vessels. This canal is the true 

 Haversian canal. The concentric 

 lamella 3 are the Haversian lamella 3 , 

 between which the osteoblasts remain 

 in their lacunae and radiating can- 

 aliculi. The remains of the peri- 

 osteal lamella 3 between the Haversian 

 systems constitute the interstitial 

 lamellae (Fig. 6). Within the centre 

 of the rod of developing bone the osteoclasts meanwhile destroy the trabecula 3 of calcific 

 material covered by osseous tissue, and thus is formed one common cavity the beginning 

 of the medullary cavity. The marrow then forms one common mass in the centre of the bone, 

 and the surrounding fibrous tissue becomes a second periosteum, or endosteum, which sur- 

 rounds the marrow and secretes incomplete lamellae, thus bounding the marrow cavity as the 

 perimedullary lamellae. All of the above osseous tissue is merely temporary in the growth of 

 the bone thickness. As can now be readily seen, the long bones increase evenly in thickness by 

 the periosteal method, while increase in length is due entirely to the intracartilaginous method. 

 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 ends 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. 



As more and more bone is removed by this process of absorption from the interior of the bone 

 to form the medullary canal, so more and more bone is deposited on the exterior by the peri- 

 osteum, until at length the bone has attained the shape and size which it is destined to retain 

 during adult life. As the ossification of the cartilaginous shaft extends toward the articular 

 ends it carries with it, as it were, a layer of cartilage, or the cartilage grows as it ossifies, and thus 

 the bone is increased in length. During this period of growth the articular end, or epiphysis, 

 remains for some time entirely cartilaginous; then a bony centre appears in it, and it undergoes 

 the same process of intracartilaginous ossification; the cancellous bone of the extremities of the 

 processes of the bones is never completely removed to form a single marrow cavity, but the 



FIG. 14. Osteoblasts from the parietal bone of a human enbryo 

 thirteen weeks old. a. Bony septa with the cells of the lacuna?. 

 b. Layers of osteoblasts. c. The latter in transition to bone cor- 

 puscles. (After Gegenbauer.) 



