136 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



may be divided into three groups by characteristics that are physio- 

 logically significant. These classes are: (1) Simple reflexes, or 

 those in which a single muscle is affected. The best example of 

 this group is perhaps the winking reflex, in which only the orbic- 

 ularis palpebrarum is concerned. (2) Co-ordinated reflexes, in 

 which a number of muscles react with their contractions so grad- 

 uated as to time and extent as to produce an orderly and useful 

 movement. (3) Convulsive reflexes, such as are seen in spasms, 

 in which a number of muscles perhaps all the muscles are con- 

 tracted convulsively, without co-ordination and with the pro- 

 duction of disorderly and useless movements. Of these groups, 

 the co-ordinated reflexes are by far the most interesting. They 

 can be obtained to perfection from the reflex frog. In such an 

 animal no spontaneous movements occur if the sensory surfaces are 

 entirely protected from stimulation. A sudden stimulus, however, 

 of sufficient strength applied to any part of the skin will give a 

 definite and practically invariable response in a movement which 

 has the appearance of an intentional effort to escape from or remove 

 the stimulus. If the toe is pinched the foot is withdrawn in a 

 gentle manner if the stimulus is light, more rapidly and violently, 

 but still in a purposeful and co-ordinated fashion, if the stimulus 

 is strong. If the animal is suspended and various spots on its 

 skin are stimulated by the application of bits of paper moistened 

 with dilute acetic acid the animal will make a neat and skillful 

 movement of the corresponding leg to remove the stimulating body. 

 The reactions may be varied in a number of ways, and in all cases 

 the striking features of the reflex response are, first, the purposeful 

 character of the movement, and, second, the almost mechanical 

 exactness with which a definite stimulus will give a definite re- 

 sponse. This definite relationship holds only for the external 

 integument, the skin and its organs. It is obvious, in fact, that 

 a muscular response can be effective only for stimuli originating 

 from the external surface. Stimuli from the interior of the body 

 exert their reactions, for the most part, upon the plain musculature 

 and the glands. The convulsive reflexes may be produced by 

 two different means: (1) By very intense sensory stimulation. 

 The reflex response in this case overflows, as it were, into all the 

 motor paths. A variation of this method is seen in the well- 

 known convulsive reaction that follows tickling. In this case the 

 stimulus, although not intense from an objective standpoint, is 

 obviously violent from the standpoint of its effectiveness in sending 

 into the central nervous system a series of maximal sensory im- 

 pulses. (2) By heightening the irritability of the central nervous 

 system. Upon the reflex frog this effect is obtained most readily 

 by the use of strychnin. A little strychnin injected under the 



