260 ADVANCED PHYSIOLOGY 



not wasted spaces but are filled with a soft yellowish-red sub- 

 stance called marrow. In addition to much fat, this ma- 

 terial contains little bodies known as marrow cells, which, as 

 has been pointed out, produce red blood corpuscles. 



Composition of Bone. Bone itself is composed of two very 

 different substances: (1) mineral substance, chiefly phosphate 

 of lime, which is hard and brittle, and gives rigidity; this is 

 secreted by cells called osteoblasts, and constitutes about 35% 

 of the entire body substance; (2) animal matter, a tougher 

 material than lime, and neither hard nor brittle. Being par- 

 tially made of organic materials, when a bone is placed in the 

 fire the animal matter is burned away, leaving the lime only. 

 The animal matter gives strength and toughness to the bone 

 so that it is not easily broken ; this substance is called collagen 

 and makes up about 16% of a bone. Collagen is more familiar 

 as the material which gives rise to gelatine when bones are 

 boiled. The rest of bone material (about 50%) is chiefly 

 water. 



Bones of Children and Adults. The relative amount of 

 animal matter in the bones of children is much greater than 

 in those of adults. Indeed, in very early childhood the 

 bones consist wholly of animal matter and it is not until 

 later that lime is gradually deposited in them; the bones of 

 children can therefore be bent considerably without breaking. 

 This is a manifest advantage, for the numerous falls and mis- 

 haps which a child suffers would produce many disastrous 

 results were the bones as brittle as in later life. But for this 

 very reason it is necessary that the child be taught habits of 

 holding the body erect arid in proper position. If the body 

 is habitually allowed to stoop, or assume a position which 

 keeps the bones bent out of their proper form, they will be 

 almost certain to retain this shape when the lime is deposited 

 in them and they become hardened. A good straight form with 

 erect shoulders adds much, not only to a person's appear- 

 ance but also to his health and consequent happiness in life. 



