REFLEX ACTIONS. 137 



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 If the toe is pinched the foot is withdrawn in a 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 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 seemingly purposeful 

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

 exactness with which a definite stimulus will give a definite response. 

 This definite relationship holds only for sensory stimulation of 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 

 impulses. (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 



