1106 GENERAL ANATOMY OR HISTOLOGY. 



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," the so-called osteoclasts of Kblliker 

 (Fig. 633). 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 

 "myeloid." 



As 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. 



The number of ossific centres is different in different bones. In most of the 

 short bones ossification commences by a single point in the centre, and proceeds 

 toward the circumference. In the long bones there is a central point of ossifica- 

 tion for the shaft or diaphysis ; and one or more for each extremity, the epiphysis. 

 That for the shaft is the first to appear. The union of the epiphyses with the 

 shaft takes place in the reverse order to that in which their ossification began, with 

 the exception of the fibula, and appears to be regulated by direction of the 

 nutrient artery of the bone. Thus the nutrient arteries of the bones of the arm 

 and forearm are directed toward the elbow, and the epiphyses of the bones form- 

 ing this joint become united to the shaft before those at the opposite extremity. 

 In the lower limb, on the other hand, the nutrient arteries pass in a direction from 

 the knee : that is, upward in the femur, downward in the tibia and fibula ; and in 

 them it is observed that the upper epiphysis of the femur, and the lower epiphysis 

 of the tibia and fibula, become first united to the shaft. 



Where there is only one epiphysis, the medullary artery is directed toward 

 that end of the bone where there is no additional centre, as toward the acromial 

 end of the clavicle, toward the distal end of the metacarpal bone of the thumb 

 and great toe, and toward the proximal end of the other metacarpal and meta- 

 tarsal bones. 



Besides these epiphyses for the articular ends, there are others for projecting 

 parts or processes, which are formed separately from the bulk of the bone. For 



