OSSIFICATION AND GKOWTH OF BONES. 85 



subsequent changes which take place, these are dependent on the conversion of the 

 scleratogenous tissue into- membrane and cartilage. A characteristic of this tissue 

 is that it contains elements which become formed into bone-producing cells, called 

 osteoblasts. These are met with in the connective tissue from which the membrane 

 bones are formed, whilst they also appear in the deeper layers of the investing 

 tissue of the cartilage (perichondrium), and so lead to its conversion into the bone- 

 producing layer or periosteum. All true bone, therefore, may probably be regarded 

 as of membranous origin, though its appearance is preceded in some instances by 

 the deposition of cartilage ; in this case calcification of the cartilage is an essential 

 stage in the process of bone formation, but the ultimate conversion into true bone, 

 with characteristic Haversian systems, leads to the absorption and disappearance of 

 this primitive calcified cartilage. In considering the development of bone an 

 inspection of the skeleton of a foetus will enable the student to realise that much 

 of what is bone in the adult is preformed in cartilage, whilst a part of the fully 

 developed skeleton is represented only by membrane: hence, in regard to this 

 ossification, bones have been described as of cartilaginous and membranous origin. 

 If the development of a long bone is traced- through successive stages from the 

 cartilaginous condition in which it is preformed, it will be noticed that ossification 

 begins in the body ; the part of the bone ossified from this centre is referred to as 

 the diaphysis, and, since it is the first to appear, the centre is spoken of as the primary 

 centre of ossification. As yet, the ends of the body are cartilaginous knobs, but 

 at a later stage one or more ossific centres appear in these cartilaginous extremities. 

 These centres, which are independent of the diaphysis and appear much later, at vari- 

 able periods, are termed secondary centres, and from them the epiphyses are formed. 

 If there is more than one such centre at the end of a bone, the associated centres 

 unite, and at a later stage the osseous mass so formed joins with the body or 

 diaphysis, and in this way the formation of the bone is completed. Complete 

 fusion by osseous union of the epiphyses with the diaphyses occurs at variable 

 periods in the life of the individual. Prior to this taking place, the two are bonded 

 together by a cartilaginous layer which marks the position of the epiphyseal line. 

 If the bone is macerated at this stage of growth, the epiphysis falls away from the 

 diaphysis. In the case of the articular ends of bone it will be noticed that the 

 surfaces exposed by the separation of the epiphysis from the diaphysis are not 

 plane and smooth, but often irregular, notched, and deeply pitted, so that when the 

 two are brought together they interlock, and, as it were, dovetail into each other. 

 In this way the extremities of the bone as yet ununited by osseous growth are, 

 during youth and adolescence, able to withstand the shocks and jars to which during 

 life they are habitually subjected. A long bone has been taken as the simplest 

 example, but it by no means follows that these epiphyses are confined to the 

 articular extremities of long bones. They are met with not only in relation to the 

 articular surfaces of bones of varied form, but also occur where bones may be 

 subjected to unusual pressure or to the strain of particular muscles. For this 

 reason epiphyses of this nature have been called pressure and traction epiphyses 

 (Parsons). There occur, however, secondary independent centres of ossification, 

 which cannot be so accounted for. Possibly these are of phylogenetic interest 

 only, and may accordingly be classed as Atavistic. 



Ossification in Membrane. Membrane bones are such as have developed 

 from fibrous tissue without having passed through a cartilaginous stage. Of this 

 nature are the bones of the cranial vault and the majority of the bones of the 

 face, viz., the maxillse, zygomatic (malar), nasal, lacrimal, and palate bones, as well as 

 the vomer. The medial lamina of the pterygoid process (internal pterygoid plate) 

 is also of membranous origin. In the course of the development of a bone from 

 membrane, as, for example, the parietal bone, the fibrous tissue corresponding to 

 the position of the primary centre becomes osteogenetic, because here appear the 

 bone-forming cells (osteoblasts), which rapidly surround themselves with a bony 

 deposit more or less spicular in arrangement. As growth goes on these osteoblasts 

 become embedded in the ossifying matrix, and remain as the corpuscles of the 

 future bone, the spaces in which they are lodged corresponding to the lacunae and 

 canaliculi of the fully developed osseous tissue. From the primary centre ossifica- 



