'27 '2 MICROSCOPIC A. \AT(>MY OF THE ORGAXS. 



another. These are at first separated from one another by a 

 thin layer of cartilage, the epiphyseal line. By means of this 

 cartilage the bone is enabled to increase in length, and not 

 until all growth in length has ceased does the epiphyseal 

 line disappear. 



In addition to this process of bone formation there is also a 

 destruction of bone. In this process of absorption the so-called 

 osteoclasts play an active part. These are giant cells (Fig. 



FIG. 198. 

 Lacnia 



Bone 



Marrow cells 



' * '4*** 



"^ $' 



* " * 



Giant cell ______ &_ $ / ^tJ ** 



(osteodofrf} * * % t ,. V** , 



*.* . *!* *' 



^V^v.-.vvv-' 



From a longitudinal section of the femur of a rabbit's embryo. X 335. 



containing many nuclei and situated in small hollowed-out 

 spaces in the bone known as Ifowship's lacunce. It is believed 

 generally that these osteoclasts in some way absorb or destroy 

 the bone after it has been formed by the osteoblasts, and that 

 in this way the medullary cavity is increased in size. They 

 are to be observed not only in growing bones, but also in those 

 that are fully Developed. All the bones are formed from carti- 

 lage, with the exception of the bones of the roof of the skull, 

 the lateral part of the skull, most of the face bones, and a 

 small part of the base of the skull. 



