REFLEX ACTION 111 



stimulated will move toward the location of the acid, and this will 

 be quickly followed by movements of the hind leg on the same side. 

 These movements, tending to remove the source of stimulation, 

 are made possible by the passage of the impulse over adjuster 

 neurones that transmit it posteriorly to efferent neurones leading 

 to muscles of the hind leg. The reactions described occur on the 

 side of the animal to which the acid has been applied. If, under 

 such conditions, the hind leg that is contracting be prevented 

 from moving, the hind leg of the opposite side will respond to the 

 original stimulus by contracting. This effect is the result of 

 adjuster neurones that carrj' impulses from one side of the spinal 

 cord to the other and make possible bilateral coordination. 



Compound Reflexes. — In the examples given we have been 

 concerned with isolated reflexes. That is, particular reflex arcs 

 have been discussed as if they were separable from the remainder 

 of the nervous system. Such is obviously not the case. In fact, 

 if any reflex reaction is completely analyzed it is found to be 

 dependent upon many reflex arcs. This compounding of reflexes, 

 or the interaction between reflex arcs, is the principal method 

 of nervous coordination (Fig. 62). 



Any organism at any particular moment is being subjected to 

 many different kinds of stimuli; yet its reactions are orderly and 

 exhibit correlation. One may say, without becoming involved in 

 too theoretical a consideration, that if more than one reflex arc be 

 stimulated at the same time one of two things may happen. The 

 reflexes may combine for the production of a harmonious effect, in 

 which case they are said to be allied reflexes. In contrast to this 

 condition, cases are found where stimuli that occur simultaneously 

 do not produce reflexes that reinforce one another. Instead, one of 

 the reflexes may prevent the others from becoming effective, that 

 is, it may inhibit the others. Such a reflex is said to be antago- 

 nistic with respect to the others. When an antagonistic reflex or 

 group of reflexes occurs, such reflexes dominate the animal's reac- 

 tions until the}^ in turn are inhibited or their stimulus is removed. 

 The succession of reflexes, or their occurrence in sequence, is very 

 well understood in some situations and must be a very important 

 factor in reflex coordination. The procedure by which a frog 

 obtains its food involves a sequence or chain of reflexes which has 

 been analyzed. The visual stimulus produced by a moving 

 insect brings about the protrusion of the tongue. If the insect 



