38 MAMMALIAN ANATOMY 



A flat bone increases in diameter through growth along its edges. 

 An early union of two contiguous bones results in an arrest of growth 

 and a narrowing of the part. This is observable in the human skull, 

 wherein early union of the two frontal bones produces a narrow forehead. 



Growth in thickness is produced by the deposition from the peri- 

 osteum of new bone on the external surface and an absorption of old 

 bone from the internal surface. Thus the medullary cavity, if present, 

 enlarges by the destruction of its walls. To show this method of 

 growth, a broad ring has been placed around a bone of a living 

 animal ; after the lapse of sufficient time the animal was killed, and 

 the ring found within the medullary cavity. 



Absorption of bone. It has been noted that most bones are pre- 

 formed in cartilage with the shape which they will have when com- 

 pletely ossified. Since the growth in length takes place at the ends of 

 the bones, we should expect to find the cartilaginous elevations which 

 are near the ends of the bone, but on the diaphysis, gradually assuming 

 a position nearer the middle of the shaft. This is not the case, how- 

 ever ; their relative positions are constantly maintained by a growth 

 of the side of the elevation near the epiphysis and an absorption of 

 material from the side of the elevation near the middle of the diaphysis. 

 The elevation therefore might be said to travel slowly but constantly 

 toward the end of the bone until the epiphysis coalesces with the 

 diaphysis. 



Relative Growth. It has been observed that the rate of growth 

 is not the same for all bones, and also that the growth of any one bone 

 does not proceed in all parts with the same rapidity. In long bones 

 one end grows faster than another ; the primary centre reaches the 

 epiphyseal cartilage and unites with the secondary centres earlier at 

 one end than at the other. We have noted that the nutrient canal of 

 the shaft pierces the compact tissue obliquely, and hence is directed 

 toward one end of the bone ; observation of the processes of growth 

 reveals that the end toward which the vessel is directed unites first with 

 the shaft. 



GENERAL TERMS USED IN OSTEOLOGY. 



The terms which are used to show the relations of bones to the 

 exterior, and their relations to one another, are somewhat different 

 when applied to bones (1) of the trunk, (2) of the head, and (3) of 

 the limbs. 



