Muscles and Motion. 17 



3. Span the muscle, placing the tips of the fingers in the angle of the 

 elbow, and the tip of the thumb as far as you can up the arm ; again 

 bend the arm. What change in the muscle does this show ? Any 

 muscle that bends a limb, as does the biceps, is called a flexor muscle. 



4. Clasp the back of the upper right arm ; forcibly straighten the 

 arm. The muscle lying along the back of the arm is the triceps muscle. 

 It is called an extensor muscle because it extends, or straightens, the arm. 



5. Clasp the upper side of the right forearm near the elbow ; clench 

 the right hand quickly and forcibly ; repeat rapidly. 



6. Notice the mass of muscle at the base of the thumb ; pinch the 

 forefinger and thumb strongly together. What changes can be seen and 

 felt? 



7. Place the hand on the outside of the shoulder ; raise the arm to 

 the horizontal position ; repeat with a weight in the hand. 



8. Stand erect with the heels close to each other, but not quite 

 touching ; let the arms hang freely by the sides ; rise on tiptoes, with- 

 out moving otherwise ; repeat ten times. 



9. Place the tips of the fingers on the angles of the lower jaw ; shut 

 the teeth firmly, and note the bulging of the masseter muscle. 



10. Press the fingers on the temples ; again shut the jaws firmly, and 

 feel the action of the temporal muscles. 



By these experiments we" learn that when a muscle works 

 it becomes shorter, thicker, and harder. 



The Action of Muscle. The action of muscle is always 

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

 ing and hardening. It must be kept clearly in mind that 

 the work of the muscle is done by its shortening and not 

 by either the hardening or thickening. But the hardening 

 and thickening are often more noticeable than the shorten- 

 ing, and by means of them we may locate the muscle that 

 is producing any motion. 



Action of Frog's Muscle. The action of muscle may be seen much 

 more clearly in a frog\s calf muscle, as shown in Fig. 11. When the 

 nerve is stimulated at u A " a nerve impulse runs along the nerve to the 

 muscle and makes it shorten and widen, raising the weight as shown in 

 the right half of the figure. 



