S. S. MAXWELL 27 



ual animal. Specimens were usually less sensitive and less respon- 

 sive immediately after the struggles connected with capture and 

 immobilization were over than they were ten or fifteen minutes later. 

 In fact an occasional animal remained unresponsive until roused to 

 excitability by an unusually hard scratch or a pinch or twist of the 

 tail, when it suddenly began to react in the regular way. These 

 instances forcibly reminded one of the awakening from a nap. Imme- 

 diately after the "awakening" all the responses, even to strong stimuli, 

 were likely to be positive, although occasionally just the reverse was 

 the case. Then after a few strokes the reactions would become 

 normal, that is, positive to weak and negative to strong stimuli. 



Naturally one raises the question: How is the reversal brought 

 about? The phenomena described in this paper seem to present a 

 close analogy to the observation of Sherrington' that in a spinal dog 

 the reaction to a stimulus applied to the plantar surface of the hind 

 foot differs in a way dependent on the nature of the stimulus; a firm 

 gentle pressure causes extension, a sharp prick causes flexion. Sher- 

 rington apparently assumes the existence of one kind of nerve endings, 

 nociceptors, which are excited by harmful stimuli, and which give 

 rise to flexion, and another kind which respond to bland stimuli 

 by extension. The analogy in the case of the dogfish is quite marked, 

 except that the stimulus which is "bland" in one animal or in one 

 state of the animal, is "nocuous" in another animal or in another 

 state of the same animal. 



There appears to me to be a yet closer analogy between these reac- 

 tions and the reversibility of the heliotropic reactions of certain 

 organisms; namely, those which are positive to weak and negative, 

 to strong light. All the phenomena seem to me to indicate that the 

 reversal of the stereotropic reactions of the dogfish is a central process. 

 It has been objected to the idea that the heliotropic reversals are 

 brought about in the nervous system that such reversals occur in 

 unicellar organisms where no separate nervous system exists; but it 

 has been pointed out by Loeb^ that even in unicellular organisms 



" Sherrington, Charles S., The integrative action of the nervous system, New 

 Haven, 1906. 



*Loeb J., Forced movements, tropisms, and animal conduct, Philadelphia 

 and London, 1918. 



