OSSIFICATION AND GROWTH OF BONES. 71 
arrangement. As growth goes on these osteoblasts become embedded in the ossify- 
ing matrix, and remain as the corpuscles of the future bone, the spaces in which 
they are lodged corresponding to the lacune and canaliculi of the fully-developed 
osseous tissue. From the primary centre ossification 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, which 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 
lamellee which constitute the inner and outer compact osseous layers. 
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 
lamellz 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 cancellous 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. 
Such a 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 
shaft. This zone of growth is situated where the ossified shaft 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, 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. 
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 direction ceases. In cases where the epiphysis enters into 
the formation of a joint, the cartilage over the articular area persists and undergoes 
neither calcification nor ossification. In long bones the ossific nucleus for the shaft 
or diaphysis is the first to appear, and is hence often called the primary centre of 
ossification. The centres for the epiphyses appear subsequently at variable periods, 
and are referred to as the secondary centres of ossification. 
From what has been said it will be gathered that the vascular supply of the 
bone is derived from the vessels of the periosteum. These consist of fine arteries 
