THE SKELETON 



377 



continually, both from the inside and the outside surface of bone. 

 Early in life, when the skeleton grows rapidly, the living part of 

 bone exceeds the mineral matter in it. Bones of young people are 

 frequently quite soft and flexible. Very early in life, almost all 

 bones are formed of a tissue called cartilage. 



Microscopic Structure of Bone. — Gristle, the common name for car- 

 tilage, is seen under the microscope to be made up of living cells surrounded 

 by a clear, tough material, 

 the latter formed by the 

 action of the cells embedded 

 in it. Formation of bone 

 takes place in much the 

 same way. In a piece of 

 bone examined under the 

 microscope are found hun- 

 dreds of little irregular 

 spaces. From these spaces 

 radiate many tiny canals. 

 These spaces, during the 

 life of the animal, were 

 occupied by bone-forming 

 cells, irregular processes 

 from which passed out into 

 the canals. Later, limy mat- 

 ter was deposited around certain of the cells. Thus true bone was formed. 



Hygiene of the Skeleton; Differences in Structure of the Skele- 

 ton OF A Child and an Adult. — During childhood, because of the small 

 amount of mineral matter and consequent lack of rigidity in bone, we must 

 exercise some care in the treatment of the skeleton. A round-shouldered 

 condition, brought on by sitting in an incorrect position, may cause a promi- 

 nent deformity of the skeleton. Children are frequently allowed to walk 

 before the leg bones are capable of supporting the weight of the body, thus 

 causing bow-leggedness. Seats at school which force a child to take an incor- 

 rect attitude when at rest are frequently the means of permanently deforming 

 the vertebral column. Later in life, when the activity of the bone-forming 

 cells ceases, more mineral matter is present than animal matter. A broken 

 bone is thus much more serious at this time than in a young person. 



Bone, as seen under the microscope. 



Joints 



Movement in the Hind Leg of the Frog. — In the hind leg of the frog pre- 

 viously studied, cut through the muscles of the thigh to the bone; make 

 out exactly how and where the muscles of the thigh are attached to the 



