BOXE 63 



canals in which those vessels run. medullary spaces filled with foetal marrow, 

 lacunae with their contained bone-cells, and canaliculi growing out of these 

 lacunae. 



This process of ossification, however, is not the origin of the whole of the 

 skeleton, for even in those bones in which the ossification proceeds in a great 

 measure from a single centre, situated in the cartilaginous shaft of a long bone, a 

 considerable part of the original bone is formed by intramembranous ossification 

 beneath the perichondrium or periosteum ; so that the girth of the bone is increased 

 by bony deposit from the deeper layer of this membrane. The shaft of the bone 

 is at first solid, but a tube is hollowed out in it by absorption around the vessels 

 passing into it, which becomes the medullary canal. This absorption is supposed 

 to be brought about by large " giant-cells." which have long been recognized as a 

 constituent of foetal marrow, and which are believed by Kolliker to have the power 

 of absorbing or dissolving bone, and he has therefore named them " osteoclasts " 

 (Fig. 33. /). They vary in shape and size, and are known by containing a large 

 number of clear nuclei, sometimes as many as twenty. The occurrence of similar 

 cells in some tumors of bones has led to such tumors being denominated 

 " uiyeloid." 



A- more and more bone is removed by this process of absorption from the 

 interior of the bone to form the medullary canal, so more and more bone is 

 deposited on the exterior from the periosteum, until at length the bone has attained 

 the shape and size which it is destined to retain during adult life. As the ossifi- 

 cation of the cartilaginous shaft extends toward the articular ends it carries with 

 it. as it were, a layer of cartilage, or the cartilage grows as it ossifies, and thus the 

 bone is increased in length. During this period of growth the articular end, or 

 epiphysis, remains for some time entirely cartilaginous ; then a bony centre appears 

 in it. and it commences the same process of intracartilaginous ossification; but 

 this process never extends to any great distance. The epiphyses remain separated 

 from the shaft by a narrow cartilaginous layer for a definite time. This layer 

 ultimately ossifies, the distinction between shaft and epiphysis is obliterated, and ' 

 the bone assumes its completed form and shape. The same remarks also apply to 

 the processes of bone which are separately ossified, such as the trochanters of the 

 femur. The bones, having been formed, continue to grow until the body has 

 acquired its full stature. They increase in length by ossification continuing to 

 extend in the epiphysial cartilage, which goes on growing in advance of the ossi- 

 fying process. They increase in circumference by deposition of new bone, from 

 the deeper layer of the periosteum, on their external surface, and at the same 

 time an absorption takes place from within, by which the medullary cavity is 

 increased. 



The medullary spaces which characterize the cancellous tissue are produced by 

 the absorption of the original foetal bone in the same way as the original medul- 

 lary canal is formed. The distinction between the cancellous and compact tissue 

 appears to depend essentially upon the extent to which this process of absorption 

 has been carried ; and we may perhaps remind the reader that in morbid states of 

 the bone inflammatory absorption produces exactly the same change, and converts 

 portions of bone naturally compact into cancellous tissue. 



Intramembranous Ossification. The intramembranous ossification is that by 

 which the bones of the vertex of the skull are entirely formed. In the bones 

 which are so developed no cartilaginous mould precedes the appearance of the 

 bone-tissue. In the membrane which occupies the place of the future bone, a little 

 network of bony spiculae is first noticed, radiating from the point of ossification. 

 When these rays of growing bone are examined by the microscope, they are found 

 to consist of a network of fine clear fibres and granular cells with a ground- 

 substance between. The fibres are termed osteogenic fibres, and soon become 

 dark and granular from calcification, and as they calcify they are found to enclose 

 the granular cells or u osteoblasts " (Fig. 36). The calcification not only involves 

 the osteogenic fibres, but also the ground-substance of the tissue in which they 



