372 AGRICULTURE ON THE PACIFIC SLOPE 



Burn a piece of bone ; notice the beautiful structure, 

 strong and light. This is mineral matter such as plants 

 take up from the soil (page 57), and makes splendid 

 plant food. Bone is widely used for fertilizing. 



Place another piece of bone in acid (one part muri- 

 atic acid to six of water) ; this will dissolve away the 

 mineral matter and leave the gristle. This is the 

 part which was burned away in the first experiment. 



What is muscle? If you look at a thick piece of 

 steak taken from a leg of beef, you see that the meat or 

 muscle is made up of separate strands. Each of these 

 is made up of smaller strands, and these of still smaller. 

 The smallest strands, made of fibers much finer than 

 the finest hair, are called muscle fibers. Blood vessels 

 go between the fibers to supply food and air (oxygen), 

 so that food may be burned and force or energy set 

 free to do work. Nerves also go to them to control 

 the work by making them start and stop, or go faster 

 or slower. 



Some muscles are under the control of the will (vol- 

 untary muscles); the brain sends a message along 

 nerves to the muscle and it contracts. Such are, for 

 example, the muscles of the hand. But you cannot by 

 willing it make your heart beat faster or slower; it is 

 not under the control of the will; such muscles as those 

 of the heart are called involuntary. 



Heart action. The heart is the most important 

 muscular organ of the body. It contracts on the aver- 



