102 HAND-BOOK OF PHYSIOLOGY. 



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 action, 

 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, in- 

 cluding 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 complete mental abstraction: and, more 

 than this, that the endeavor to carry them out entirely by the exercise 

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

 harmonious and perfect performance. Any one may convince himself of 

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

 stairs, or to form each letter or word in writing by a distinct exercise of 

 the will. 



These actions, however, will be again referred to, when treating of 

 their possible connection with the functions of the so-called sensory gan- 

 yliti, p. 115 et seg., Vol. II. 



Morbid reflex actions. -The relation of the reflex action to the strength 

 of the stimulus is the same as was shown generally in the action of gan- 

 glia, a slight stimulus producing a slight (p. 87, Vol. II.) movement, and a 

 greater, a greater movement, and so on; but in instances in which we must 



