NERVE IMPULSES AND REFLEX ACTION 315 



brain has been entirely removed. The fact that the brain of 

 a frog can be taken out, and yet the tissues of the body remain 

 alive for a considerable time (even the heart may go on beat- 

 ing) makes the frog an especially favorable animal for this 

 study. Of course, such a frog has no sensations and no 

 ability to make voluntary motions, still it shows some most 

 interesting reactions. If its toe be pinched, it is pulled away 

 quickly; if the tip of the toe be dipped in acid, it will be pulled 

 out promptly. The frog acts as though it had sensations of 

 pain, though, with the brain wanting, we know that it can 

 have none. It moves because the impulses excited in its 

 nerve endings, after reaching the cord, cannot go to the brain, 

 and therefore take one of the side tracks leading across arbora- 

 tions to other neuron bodies, whose axis cylinders pass im- 

 mediately out to muscles in the part of the body whence the 

 incoming impulse started. 



If the frog has its brain, a different result may follow, for 

 the conscious centers will become aware of the pinch in 

 the toe and the animal may decide to jump, instead of simply 

 pulling its foot away. In this case, the stimulus has of course 

 gone all the way to the brain and back down again, finally 

 passing into the motor cells of the cord, fibres of which extend 

 to the jumping muscles. Thus we see that the same muscles 

 are under the control of two different centers, the conscious 

 center of the brain and one in some other part of the central 

 nervous system, probably in the cord (as we have assumed 

 in the foregoing instances). Save that reflexes are usually 

 quicker, there is no appreciable difference between reflex and 

 conscious movements. 



Reflex movements m&y involve the brain, as well as the 

 cord. This would seem an impossibility; i. e. the unconscious 

 action of a conscious center. We get this impression, however, 

 merely because as a rule the brain is thought of as an organ 

 in which only conscious activity occurs. This is a mistake, for 

 not all the cells of the brain are by any means conscious centers. 



