46 



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 osteodasts. 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. 16. 



Chemistry of Bone. Bone is composed of in- 

 organic and organic portions intimately com- 

 bined, so that the smallest distinguishable por- 

 tion contains both. The inorganic matters 

 form about two-thirds of the total weight of a 

 dried bone, and may be removed by soaking 

 the bone in dilute hydrochloric acid. The or- 

 ganic portion left after this treatment consti- 

 tutes a flexible mass, retaining the form of the 

 original bone; it consists chiefly of an albu- 

 minoid, ossein, which by long boiling, especially 

 - dar- under pressure at a higher temperature than 

 that at which water boils when exposed freely 

 to the air, is converted into gelatin, which dissolves in the hot 

 water. Much of the gelatin of commerce is prepared in this manner 

 by boiling the bones of slaughtered animals, and even well-picked 

 bones may be used to form a good thick soup if boiled under 



uta 



FIG. 16. The hu- 

 merus bisected length- 

 wise, a, marrow-cavi- 

 ty; b, 

 c, spongy 

 ticular cartilage. 



