S. S. MAXWELL 21 



comes quiet. After a few minutes of immobilit)^ the cords can be 

 gently loosened and removed and the experiment can go on for some 

 time without any need of artificial restraint. This is important be- 

 cause experiments on contact stimuli should not be complicated by 

 possible inhibitions or reenforcements from the presence of the binding 

 cords. It is true that the ventral surface of the body is still in contact 

 with the board, but this is not an unnatural situation since the ani- 

 mal when free often rests for long periods on the bottom of the aquar- 

 ium. In order better to observe the movements of the paired fins 

 the animal was usually placed above the board on a thick piece of 

 wood no wider than the body, thus allowing the pectorals to project 

 like Vv^ings. 



The reactions about to be described were obtained by stroking or 

 scratching the outer margin of the head from near the snout to a 

 point just below the eye. It was not necessary that the stroke 

 be carried the whole distance; a short stroke or sometimes a mere 

 touch anywhere within the region mentioned gave the same result. 

 It is not to be inferred that analogous reactions are not elicited by 

 contact stimuli applied to other regions. I have confined this paper 

 to reactions from the parts mentioned for the sake of definiteness of 

 description and interpretation. 



Strength of Stimulus and Sense of Reaction. 



For most dogfish a stroke with a finger wet with sea water was 

 sufficient to produce a definite response. As a more severe stimulus I 

 used a scratch with the points of a small pair of forceps. The first 

 of these usually corresponds to the designation ''weak" the other 

 "strong" stimulus. 



It soon became apparent that fairly constant responses could 

 be obtained if the stimuli were of uniform intensity. In fact under 

 favorable conditions the movements could be repeated over and 

 over with machine-like regularity. The following portion of the 

 record of an experiment is typical (Table l). The pauses between the 

 successive trials were merely the time necessary to set down the 

 results. 



Weak Stimuli. — Inspection of the results of the above experi- 

 ment shows that when a weak stimulus is used the dorsal fins and the 



