MOTION'. 9 



11. Press the fingers on the temples ; again shut the jaw firmly, and 

 feel the action of the temporal muscles. 



12. Make a narrow band of paper that will snugly fit the forearm 

 when the hand is open ; now clench the fist strongly. 



13. With a tape measure take the circumference of the upper arm 

 when the arm hangs free ; again when the forearm is strongly flexed. 



14. In the same way measure the forearm when the hand is open, 

 and when the hand is clenched. 



By these experiments we learn that when a muscle works it becomes 

 shorter, thicker, and harder. 



Nerves and Muscles of a Rabbit's Leg. In the hind leg of a 

 rabbit the sciatic nerve may be found by separating two large 

 muscles on the sides of the thigh, beginning behind the knee joint. 

 The shape and connections of the muscles may be learned, and also 

 the distribution of the nerve. 



The Action of Muscle. The action of muscle is always 

 a " pull." The muscle shortens, at the same time thick- 

 ening and hardening. These changes in muscle are 

 roughly shown in the preceding experiments of feeling 

 the arm during its action. But the isolated calf muscle of 

 the frog may be made to prove the characteristic changes 

 with great clearness. 



Action of Frog's Muscle. A frog may be killed painlessly by put- 

 ting a teaspoonful of ether into a fruit jar of water, immersing the frog 

 and capping the jar. When the frog becomes motionless, its head 

 should be cut off and a wire run down the spinal column to destroy the 

 spinal cord. After cutting the skin around the base of one thigh the 

 skin may easily be stripped from the whole hind limb. If the muscles 

 on the back of the thigh be gently separated there will be found a white 

 thread running lengthwise, the sciatic nerve. It should be severed 

 near the hip and carefully turned down upon the calf muscle. It should 

 not be pinched or dragged. The muscles of the thigh should now all 

 be cut away, being careful not to sever the nerve near the knee. The 

 hip joint should be unjointed. With the handle of the scalpel the calf 

 muscle should be separated from the shin bone, and just below the 

 knee the shin bone and all the muscles except the calf muscle severed. 



