THE NERVOUS SYSTEM. 1U1 



evident adaptation and purpose in the movements of the cold-blooded 

 animals, have led some to think that they must be conscious and capable 

 of will without their brains. But purposive movements are no proof of 

 consciousness or will in the creature manifesting them. The movements 

 of the limbs of headless frogs are not more purposive than the 

 movements of our own respiratory muscles are; in which we know that 

 neither will nor consciousness is at all times concerned. It may not, in- 

 deed, be assumed that the acts of standing, leaping, and other move- 

 ments, which decapitated cold-blooded animals can perform, are also 

 always, in the entire and healthy state, performed involuntarily, and 

 under the sole influence of the cord; but it is probable that such acts 

 may be, and commonly are, so performed, the higher nerve-centres of the 

 animal having only the same kind of influence in modifying and direct- 

 ing them, that those of man have in modifying and directing the move- 

 ments of the respiratory muscles. 



Inhibition of Reflex Actions. The fact that such movements as 

 are produced by irritating the skin of the lower extremities in the human 

 subject, after division or disorganization of a part of the spinal cord, do 

 not follow the same irritation when the mind is active and connected 

 with the cord through the brain, is, probably, due to the mind ordinarily 

 perceiving the irritation and instantly controlling the muscles of the irri- 

 tated and other parts; for, even when the cord is perfect, such involun- 

 tary movements will often follow irritation, if it be applied when the mind 

 is wholly occupied. When, for example, one is anxiously thinking, even 

 slight stimuli will produce involuntary and reflex movements. So, also, 

 during sleep, such reflex movements may be observed, when the skin is 

 touched or tickled; for example, when one touches with the finger the 

 palm of the hand of a sleeping child, the finger is grasped the impres- 

 sion on the skin of the palm producing a reflex movement of the muscles 

 which close the hand. But when the child is awake, no such effect is 

 produced by a similar touch. 



Further, many reflex actions are capable of being more or less con- 

 trolled or even altogether prevented by the will: thus an inhibitory action 

 may be exercised by the brain over reflex functions of the cord and the 

 other nerve centres. The following may be quoted as familiar examples 

 of this inhibitory action: 



To prevent the reflex action of crying out when in pain, it is often 

 sufficient firmly to clench the teeth or to grasp some object and hold it 

 tight. When the feet are tickled we can, by an effort of will, prevent the 

 reflex action of jerking them up. So, too, the involuntary closing of the 

 eyes and starting, when a blow is aimed at the head, can be similarly 

 restrained. 



Darwin has mentioned an interesting example of the 

 on the other hand, such an instinctive reflex act may/C-foerridfc the 



