THE HUMAN BODY AS A MACHINE 221 



Method c. Bend the head backward. 



Observations. 1. Where is the muscle located that pulls 

 the head backward ? 



2. Locate the power, weight, and fulcrum. 



Conclusions. 1. What class of levers is represented here ? 

 Make a sketch to show your conclusion. 



2. Show that all three classes of levers are in the foot. 



4. MICROSCOPIC STRUCTURE (OPTIONAL) 



Materials. Boiled lean beef, microscope slides, portable or stationary 

 compound microscope, and hand lens. 



Method a. Try pulling the lean beef in strips. Also examine with 

 hand lens. 



Observations. 1. What is the structure of the muscle ? 



2. Find thin whitish bai^ds scattered throughout the muscle. Describe 

 their appearance. 



Conclusions. 1. How are the fibers of muscles held in place ? 



2. Compare these fibers with bundles of rubber bands. 



Method b. Examine a bit of muscle with low power of a compound 

 microscope. 



Observation. What do you see concerning the structure of muscle, 

 not seen well with the hand lens ? Can you make out the fibers ? 



Conclusion. What is lean meat ? What is its structure ? 



c. General Adaptations of the Skeleton 



Materials. Shank bones of calves or sheep from the market 

 some sawed crosswise, others lengthwise, skeleton of frog 

 or other vertebrate, skeleton of man, charts, HC1, Bunsen 

 burner or alcohol lamp, crucible, scales. 



1. COMPOSITION 



Method. Weigh two small pieces of bone. Drop one into 

 HC1 and allow it to remain until the action ceases. Burn a 

 second piece in a crucible. Weigh what remains in both cases. 



Observations. 1. What is the original weight of each piece ? 



2. What happens when the bone is put in acid ? Its per- 

 centage of loss in weight ? 



