140 THE HUMAN BODY 



ing to the jumping muscles, while the paths to muscles not con- 

 cerned in any way in an adaptive response have too high resistance 

 to be passed at all, we can account for reflex actions of very great 

 complexity. 



Simple Reflexes Mediated by the Spinal Cord. The simple re- 

 flexes described in the preceding paragraphs are all of a sort that 

 can be carried on through the lowest part of the central nervous 

 system, the spinal cord. A frog whose brain has been destroyed 

 and which is therefore wholly devoid of feeling and consciousness 

 can still perform highly complicated reflex acts; he will retract a 

 foot which is pinched; he will wipe off a bit of acid-soaked paper 

 from his flank, and if unable to reach it with one foot will bring 

 the other into service. All his acts, however, are purely mechan- 

 ical, and are determined by the spread of impulses over reflex 

 paths of less or greater complexity. A very striking thing about 

 such a " reflex " frog is the accuracy with which the character of 

 his responses to various sorts of stimuli can be predicted. He is 

 a pure automaton. The most apparent difference between the 

 responses of such a " reflex " frog and those of a normal one, with 

 brain intact, is that in the case of the latter one cannot predict 

 certainly what his response to any particular stimulus will be. 

 Looking at the nervous system purely from the standpoint thus 

 far assumed, namely, as an adaptive conducting mechanism, this 

 property that the brain has of interfering with the orderly progress 

 of reflex actions is the first of its properties to require considera- 

 tion. 



The Cerebrum the Controlling Organ. In the preceding par- 

 agraph the "reflex" frog was described as having its entire brain 

 destroyed. Such a procedure reduces the animal to the " reflex " 

 state, but does not enable us to judge whether the brain acts as 

 a whole in modifying the reflex activities of the spinal cord or 

 whether this function is confined to certain parts of the organ. 

 Studies upon higher animals, especially upon pigeons, have dem- 

 onstrated that the particular part of the brain which has the power 

 to modify reflexes is the cerebrum. A pigeon whose cerebrum has 

 been removed is as truly a " reflex " animal as is a frog whose en- 

 tire brain has been destroyed. Since the cerebrum is also the seat 

 of consciousness an animal treated in this way is wholly insensi- 

 tive to pain. 



