BOXE 



---diaph.. 



. 



through resorption, internally. This explains why a band of metal placed 

 around the bone of a young animal is later found within the marrow. 

 The internal resorption takes place in such a way that a meshwork of 

 spicules and plates, denser toward the periphery, remains within the shaft, 

 and the marrow occupies its interstices. To a limited extent new bone is 

 formed in the interior of the shaft by osteoblasts in its lining membrane, 

 called the endosteum. The deposition of new layers externally is produced 

 by osteoblasts in the periosteum, which is a specialized connective tissue 

 layer surrounding the bone. It replaces, and apparently is derived from, 

 the perichondrium of the original cartilage. The extent to which new 

 bone is formed, and its distribution, may 

 be determined by feeding madder to grow- 

 ing animals. This dye, as has long been 

 known, imparts a red color to the matrix 

 of bone deposited while it forms a part of 

 the diet. By this means Kolliker deter- 

 mined that the deposition of periosteal 

 bone is not uniform. In a given bone, 

 there will be unstained areas, where no 

 new bone is being formed, or where an ex- 

 ternal resorption is taking place. In this 

 way the bones acquire their characteristic 

 modelling. 



Growth in length occurs chiefly through 

 the activity of the uncalcified cartilage. 

 In a long bone, ossification first produces a 



band of bone encircling the cartilage, and then a hollow shaft of bone 

 with a rounded mass of cartilage at either end (Fig. 78, A, B). The cells in 

 these masses continue to divide, prolonging the longitudinal rows of cells 

 such as are seen in Fig. 75. As ossification takes place at one end of these 

 rows, new cells are formed at the other, and thus the length of the shaft or 

 diaphysis increases. Certain bones have been found to grow more at one 

 end than at the other. After a time osteogenic tissue invades the cartilages 

 at the extremities of the bone, extending into them from the marrow 

 cavity of the shaft. It forms a small bone within each, and these are 

 known as epiphyses (Fig. 78, D). Between the epiphysis and the diaphy- 

 sis there remains a layer of cartilage, called the epiphyseal synchondrosis, 

 which allows further growth in length. The cells which it produces are 

 added chiefly to the shaft. The relation of the epiphyses to the growth 

 of bone was demonstrated by early experiments, in which metal pegs 

 were placed in the bones of young animals. Pegs in the shaft scarcely 

 separate from one another during growth, but a peg in the epiphysis 

 moves away from one in the diaphysis. The epiphyses are formed at 



art.- 



D 



FIG. 78. PLAN OF OSSIFICATION IN A 

 LONG BONE, BASED UPON THE TIBIA. 



Cartilage is drawn in black, and bone is 

 stippled. art., Articular cartilage; 

 ep., epiphysis; diaph., diaphysis. 



