BONE. 307 



tinually being developed in the epiphysis ; they are also 

 constantly marshalling themselves into rows or columns, 

 the lowermost cells of which dip into the cancelli and be- 

 come absorbed ; on the other side the cancelli of the bone 

 are continually invading the intercellular spaces of the 

 cartilage. 



It is thus that a bone grows in length. Each epiphysis 

 of a long bone, however, after a time becomes a centre of 

 ossification ; this proceeds to meet that of the shaft ; a layer 

 of cartilage usually, however, intervenes between the two, 

 until the period of the full development of the osseous 

 system, when this layer becomes absorbed, and the shaft and 

 the epiphysis become consolidated by bony union. The first 

 trace of ossification of the epiphysis in the human subject 

 is usually apparent at about the ninth month. 



We have now to ask ourselves the question, how does 

 the bone increase in diameter ? We have shown that it is 

 generally considered as proved by the madder experiments 

 already referred to, that a long bone increases in breadth by 

 the deposition of new lamellae at the circumference, as well 

 as in the cavities of the medullary cells and Haversian 

 canals ; but we have seen also from what has been already 

 said in reference to the different sizes of the external and 

 internal Haversian canals and their mode of formation, that 

 each of these canals is continually undergoing a process of 

 expansion, and it is by this expansion that the chief increase 

 of the diameter of a bone takes place. It was formerly sup- 

 posed that a layer of cartilage existed in all growing bones on 

 their external surfaces, but we now know that such is not 

 the case, and that the new osseous deposit takes place in 

 fibres. If it were necessary that a layer of cartilage should 

 exist in the situation named, it would be equally requisite 

 that it should be present in each medullary cell and in each 

 Haversian canal. 



The small granular corpuscles already referred to as 

 occurring in the cancelli of all bones, but especially in those 

 of the fostus, it would thus appear are developed in con- 

 siderable quantities at a very early period of the develop- 



