MOVEMENTS, ETC., OE EllESH-WATER PLANAHIANS. 605 



Finally, the positive reaction might in some way be a 

 specific function of the brain, which is removed by the 

 operation. As the evidence seems to be decisive against the 

 first two possibilities this seems probably true. Is this 

 because the brain contains a special '' centre " whose function 

 it is to produce the reaction ? 



There is no reason to think of the reaction as a function of 

 the brain in the sense that that organ forms a centre which 

 originates the impulses Avliich cause the reaction. On the 

 contrary, it seems much more probable that the loss of the 

 brain causes the loss of reaction for the following reason. 

 It has been shown that removal of the brain causes a general 

 lowering of the tonus of the organism, and further that the 

 appearance of the reaction in a normal animal is closely 

 dependent on the tonic condition of the organism. Probably, 

 then, the chief reason for the non-appearance of the positive 

 reaction in posterior pieces is that in these the conditions of 

 general tonus are so changed by the loss of the brain that the 

 reaction is no louger possible. Expressing it in another way, 

 the animal is too sluggish to give the positive response. 

 This being the case, it would be expected that it might be 

 possible to induce the positive reaction in a decapitated 

 specimen provided the tonus were raised in some way. As a 

 matter of fact, as will be shown later, positive reactions to 

 certain chemical stimuli have been observed in a few cases 

 (cf. p. 649). In its form and mechanism the positive reaction 

 is not directly dependent upon the brain. 



Summing up the evidence on the relation of the brain to 

 the reactions of the flat-worm, it may be said that all the 

 reactions to mechanical stimuli shown by the normal animal, 

 with the single exception of the positive reaction, are given 

 by specimens from which the brain has been removed. 

 The relation of the brain to the positive reaction is, in large 

 part, so far as evidence can be obtained, an indirect one, viz. 

 it is necessary for the maintenance of the proper tonic 

 conditions of the organism. Thus far there is no evidence of 

 any special " centi'e " functions of the brain, similar to those 



VOL. 46, PART 4. — NEW SERIES. RR 



