THE SUPPORTING TISSUES 49 



ually replaced by hard bone like that which was present before the 

 injury. The spongy temporary bone is absorbed by the osteo- 

 clasts described above. 



To secure proper knitting of the fracture two things are of 

 great importance; the first of these is the reduction of the fracture_, 

 whereby the parts are brought as nearly as possible into their 

 former positions; the second is immobilization of the part by means 

 of splints, bandages, or casts to hold the broken ends in place during 

 the formation of the new bony material. 



Chemistry of Bone. Bone is composed of inorganic and organic 

 portions intimately combined, so that the smallest distinguishable 

 portion 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 organic portion 

 left after this treatment constitutes a flexible mass, retaining the 

 form of the original bone; it consists chiefly of an albuminoid, 

 ossein, which by long boiling, especially 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 man- 

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

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

 pressure in a Papin's digestor; much nutritious matter being, in 

 the common modes of domestic cooking, thrown away in the bones. 



The inorganic salts of bone may be obtained free from organic 

 matter by calcining a bone in a clear fire, which burns away the 

 organic matter. The residue forms a white very brittle mass, re- 

 taining perfectly the shape and structural details of the original 

 bone. It consists mainly of normal calcium phosphate, or bone- 

 earth [CaaCPO^l; but there is also present a considerable propor- 

 tion of calcium carbonate (CaCO 3 ) and smaller quantities of other 

 salts. 



Hormones of the Supporting System. We learned in a previous 

 chapter (p. 28) that co-ordination of many bodily processes, and 

 notably of the growth process, is secured chemically by means of 

 substances known as hormones. The size of the Body depends on 

 the size of the bones which make up the skeleton. For that rea- 

 son any clue to the mechanism which governs the growth of the 

 bones is of great interest. Some years ago the discovery was made 



