REFLEX ACTION 



4957 



REFLEX ACTION 



through which it came. If it strikes the sur- 

 face at an acute angle it will rebound at an 



n 



REFLECTION OF LIGHT 

 (a) Regrular reflection of rays from the plane 

 surface m n; (b) diffused reflection from an 

 uneven surface. 



acute angle with that surface, and the two an- 

 gles will be equal. These statements illustrate 

 concretely what 

 happens when a 

 ray of light, heat 

 or sound (a wave 

 of radiant en- 

 ergy) strikes upon A .. 

 a surface; it is "* 

 turned back, or |j 

 reflected. The 

 angle at which a 

 of radiant 

 energy strikes a 

 surface is called IMAGE FORMED BY A 



, i / MIKiiOll 



the angle oj tnci- The mirror , g repr esented by 



dence, and the i An observer at ewhoM 



, . . graze rests upon the mirror 



angle at winch it at m w m see there tlv 



i tnrnoH hnrt in fleeted light from the candle 



1CK 1S a. The line in 6 bisects the 



the angle of re- angle e m a, and helps to 



, , , . A visualize the fact that the 



/ tec ll on. AS an gie of incidence is equal to 



fttntpri ahovp the angle of reflection. The 



OVG ' same effect is produced if the 



these angles are observer is below the mirror, 



equal. The re- at * 



flection of light is discussed fully in these vol- 

 umes under a subhead in the article LIGHT. 

 See also SOUND; ECHO. 



REFLEX, re'fleht, ACTION. If you place 

 your hand on a hot stove it is withdrawn 

 before you have time to think. A sudden and 

 unexpected sound may cause you to jump and 

 possibly to scream. Did you intend to jump? 

 "No," you reply, "I did it before I knew it." 

 -c and similar acts arc good examples of 

 reflex action, which is action caused by tin- 

 communication of a sensory with a motor n 

 through a nerve center. The term is generally 

 restricted to involuntary acts like those men- 

 tioned above and to acts of which we are un- 

 conscious. 



The simplest form of reflex action is shown 

 m the accompanying diagram, which illustrates 

 what physiologists term flic r, tl, j- an The sur- 



Jnerve 



skin 



REFLEX ACTION ILLUS- 

 TRATED 



face of the skin is touched, and the impulse 

 traverses the sensory nerve to a, the sensory 

 nerve cell. At c the fibers of this cell interlace 

 with those of the motor nerve cell b, and the 

 impulse is carried along the motor nerve to the 

 muscle, which it 

 causes to con- 

 tract. A study 

 of the diagram 

 shows that the 

 following mecha- 

 nism is necessary 

 to produce a re- 

 flex act: a sensi- 

 tive surface, usu- 

 ally the skin; a 

 sensory nerve; a 

 motor nerve con- 

 necting with a 

 muscle, gland or blood vessel ; and a nerve cen- 

 ter in which these nerves connect. This is the 

 simplest form of reflex arc, and we have de- 

 scribed the simplest reflex act. There are, how- 

 ever, some reflex acts that are very complex. 

 Some of them control the action of glands and 

 others affect the circulation and respiration. 



Reflex acts of the spinal and cranial nerves 

 are performed more quickly than voluntary 

 acts, and they often protect the body from in- 

 jury. Before you were aware that your hand 

 was on a hot stove it had been jerked away, or 

 the sensory impulse may have divided at the 

 reflex center and part of it gone on to the 

 brain; in that case you felt the pain so near 

 the instant you moved your hand that the 

 sensation and the movement seemed to come 

 together. The time for a reflex act of this sort 

 is from six-hundredths to eight-hundredths of a 

 second. 



The chief reflex centers for involuntary re- 

 flex action are in the medulla oblongata and the 

 spinal cord, but smaller centers are located in 

 different parts of the body. As we have seen, 

 some of the sensory impulses may be carried 

 to the brain; therefore not all reflex acts arc 

 unconscious. Many voluntary reflexes become 

 automatic by long practice, and many acts that 

 are begun voluntarily, such as walking, 

 continued without further attention. All reflex 

 action is for the protection of the body or for 

 IK the various higher activities of the 

 brain. WJMI. 



RrUtrd Subject*. The following articles in 

 these volumes should be read in this connection : 



Hrnln. fubhead 



M'-ihitla Oblongata 



Habit 



us System 



