86 OSTEOLOGY. 



tion spreads eccentrically towards the margins of the bone, where ultimately the 

 sutures are formed. Here the growth rendered necessary by the expansion of the 

 cranium takes place through the agency of an intervening layer of vascular 

 connective tissue rich in osteoblasts ; but in course of time the activity of this is 

 reduced until only a thin layer of intermediate tissue persists along the line of the 

 suture ; this may eventually become absorbed, leading to the obliteration of the suture 

 by the osseous union of the contiguous bones. Whilst the expansion of the bone in all 

 directions is thus provided for, its increase in thickness is determined by the activity 

 of the underlying and overlying strata. These form the periosteum, and furnish 

 the lamellse which constitute the inner and outer compact osseous layers. 



Ossification in Cartilage. Cartilage bones are those which are preformed in 

 cartilage, and include most of the bones of the skeleton. Their growth is often 

 described as endochondral and ectochondral, the former term implying the 

 deposition of membrane bone in the centre of the cartilage, while the latter 

 signifies a deposit of membrane bone on the surface of the cartilage, the osteo- 

 genetic layer on the surface of the cartilage being named the perichondrium till 

 once bone has been formed, when it is called the periosteum. 



In a cartilage bone changes of a similar nature occur. The cartilage, which may 

 be regarded histologically as white fibrous tissue + chondro-sulphuric acid and a 

 certain amount of lime salts, undergoes the following changes : First, the cartilage 

 cells being arranged in rows, become enlarged ; secondly, the matrix between the 

 cartilage cells becomes calcified by the deposition of an additional amount of lime 

 salts ; thirdly, the rows of cells become confluent ; and, fourthly, into the spaces so 

 formed extend the blood-vessels derived from the vascular layer of the periosteum. 

 Accompanying these vessels are osteoblasts and osteoclasts, the former building up 

 true bone at the expense of the calcified cartilage, the latter causing an absorption of 

 the newly formed bone, and leading to its conversion into a marrow cavity, so that 

 in due course all the cartilage or its products disappear. At the same time that this 

 is taking place within the cartilage, the perichondrium is undergoing conversion into 

 the periosteum, an investing membrane, the deeper stratum of which, highly vascular, 

 furnishes a layer of osteoblast cells which serve to develop the circumferential 

 lamellse of the bone. It is by the accrescence of these layers externally, and their 

 absorption internally through the action of the osteoclast cells, that growth takes 

 place transversely. A transverse section of the shaft of a long bone shows this 

 very clearly. Centrally there is the marrow cavity, formed primarily by the 

 absorption of the calcified cartilage; around this the spongy tissue produced 

 by the partial erosion of the primary periosteal bone is disposed, whilst externally 

 there is the dense envelope made up of the more recent periosteal growth. 



Growth of Bone. The above description, whilst explaining the growth of bone 

 circumferentially, fails to account for its growth in length ; hence the necessity in 

 long bones for some arrangement whereby ossification may take place at one or both 

 extremities of the body. This zone of growth is situated where the ossified body 

 becomes continuous with the cartilaginous epiphysis. In addition, within these 

 epiphysial cartilages calcification of the cartilage takes place centrally, just as in the 

 diaphysis. The two parts of the bone, viz,, the diaphysis and epiphysis, are thus 

 separated by a layer of cartilage, sometimes called the cartilage of conjuga- 

 tion, as yet uncalcified, but extremely active in growth owing to the invasion 

 of vessels and cells from a vascular zone which surrounds the epiphysis. The 

 nucleus of the epiphysis becomes converted into true bone, which grows 

 eccentrically. This arrangement provides for the growth of the shaft towards 

 the epiphysis, and the growth of the epiphysis towards the shaft; so that as 

 long as the active intervening layer of cartilage persists, extension of growth in 

 a longitudinal direction is possible. As might be expected, experience proves that 

 growth takes place more actively, and is continued for a longer time, at the end 

 of the bone where the epiphysis is the last to unite. In consequence, surgeons 

 sometimes term this the " growing end of the bone." Subsequently, however, at 

 variable periods the intervening layer of cartilage becomes calcified, and true bony 

 growth occurs within it, thus leading to complete osseous union between the shaft 

 and epiphysis. When this has taken place all further growth in a longitudinal 



