THE CEREBKO-SPINAL NERVOUS SYSTEM. 487 



To prevsnt 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 or 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 simi- 

 larly restrained. 



Darwin has mentioned an interesting example of the way in which, 

 on the other hand, such an instinctive reflex act may override the 

 strongest effort of the will. He placed his face close against the glass 

 of the cobra's cage in the Reptile House at the Zoological Gardens, and 

 though, of course, thoroughly convinced of his perfect security, could 

 not by any effort of the will prevent himself from starting back when the 

 snake struck with fury at the glass. 



It has been found by experiment that in a frog the optic lobes and 

 optic thalami have a distinct action in inhibiting or delaying reflex ac- 

 tion, and also that more generally any afferent stimulus, if sufficiently 

 strong, may inhibit or modify any reflex action even in the absence of 

 these centres. 



On the whole, therefore, it may, from these and like facts, be con- 

 cluded that reflex acts, performed under the influence of the reflecting 

 power of the spinal cord, are essentially independent of the brain and 

 may be performed perfectly when the brain is separated from the cord: 

 that these include a much larger number of the natural and purposive 

 movements of the lower animals than of the warm-blooded animals and 

 man: and that over nearly all of them the mind may exercise, through 

 the higher nerve centres, some control; determining, directing, hinder- 

 ing, or modifying, them, either by direct action, or by its power over 

 associated muscles. 



To these instances of spinal reflex action, some add yet many more, 

 including nearly all the acts which seem to be performed unconsciously, 

 such as those of walking, running, writing, and the like: for these are 

 really involuntary acts. It is true that at their first performances they 

 are voluntary, that they require education for their perfection, and are 

 at all times so constantly performed in obedience to a mandate of the 

 will, that it is difficult to believe in their essentially involuntary nature. 

 But the will really has only a controlling power over their performance; 

 it can hasten or stay them, but it has little or nothing to do with the 

 actual carrying out of the effect. And this is proved by the circumstance 

 that these acts can be performed with compkte mental abstraction: and, 

 more than this, that the endeavor to carry them out entirely by the ex- 

 ercise of the will is not only not beneficial, but positively interferes with 

 their harmonious and perfect performance. Any one may convince him- 

 self of this fact by trying to take each step as a voluntary act in walking 



