222 THE HUMAN BODY AS A MACHINE 



3. Is the bone now flexible ? 



4. What happens when the bone is burned in a crucible ? 

 Percentage of loss ? 



5. Is it flexible after burning ? 



Conclusions. 1. What substance is taken out by the acid ? 

 By the heat ? 



2. What is the purpose of each of these two substances ? 



Observations. 1. Note the membrane (periosteum) covering 

 the larger pieces of bone. Is it attached to the bone loosely or 

 closely ? 



2. Is it a strong or weak membrane ? 



Conclusions. 1. For what does the periosteum seem to be 

 fitted ? 



2. The periosteum is a living and growing surface on the 

 live bone ? Would it probably mend a crack or fracture in 

 the bone ? If so, explain. 



2. JOINTS 



Observations. 1. Study a joint of any skeleton, as a frog's. 

 Move the joint. In how many directions will it move ? 



2. Can it be easily twisted or rotated ? 



3. Can you tell whether it is hingelike, ball and socket, 

 gliding, or rotary ? 



4. Note how the joints are held together by a tough tissue 

 (ligament). Is it tough? Elastic? Strong? 



Conclusions. 1. How many different kinds of joints can 

 you find in a skeleton ? 



2. How are the bones of the skeleton held together ? 



Observation. Can you find where muscles have been at- 

 tached to the bone ? Are the means of attachment (tendons) 

 soft or tough ? Elastic or unyielding ? 



Conclusion. Why are muscles attached to bones ? 



3. GENERAL SKELETON 



Observations. 1. Note that the skeleton is in two general 

 parts, the axial, of the vertebral column or backbone, and 



