48 



THE HUMAN BODY 



which the blood-vessel passes with its surrounding rings of bony 

 material. 



The increase in length of the long bones is brought about by 

 plates of cartilage which persist between the shaft of the bone and 

 its extremities. There is a continual growth 

 of bone into these cartilages from both sides, 

 but they grow in thickness with equal rapidity 

 until the adult length of the bone is reached 

 when their growth stops and they are gradually 

 replaced by bone. 



At the same time that the bone is growing 

 by additions to its outer surfaces a continu- 

 ous absorption of its inner portions is going 

 on. This absorption is carried on by large, 

 multinuclear cells known as osteoclasts. It 

 serves the purpose of preventing the bone from 

 becoming so heavy as to be unmanageable, 

 without sacrificing unduly its strength. As 

 the result of this absorption many adult bones, 

 especially long ones, contain little or no spongy 

 bone except at their ends, the shaft being 

 hollow as shown in Fig. 13. 



The Repair of Fractured Bone. When, as 

 happens with unfortunate frequency, a bone is 

 fractured, a sequence of processes is set in mo- 

 tion at the point of injury which results finally 

 in the mending of the break. As an inevitable 

 incident of the injury which caused the frac- 

 ture there is marked laceration of the perios- 

 teum and of the other adjacent tissues. These 

 lacerated tissues pour out a mixture of blood 

 and lymph, known as the exudate, which per- 

 meates the region of injury. This exudate 

 gradually stiffens until it affords considerable 

 support to the injured bone. Osteoblasts 

 from the inner surface of the periosteum and from the fractured 

 ends of the bone penetrate the exudate. These osteoblasts, little 

 by little, replace the exudate with spongy bone, which holds the 

 injured parts even more firmly in place, and which in turn is grad- 



FIG. 13. The hu- 

 merus bisected length- 

 wise, a, marrow-cav- 

 ity; 6, hard bone; 



ticular cartilage. 



