52 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



now closed, an induction shock Avas thrown into 

 the tissue at the same instant that the electro- 

 magnet writer recorded the fact, by altering its 

 position on the cylinder. Again, as soon as the 

 paralyzed Medusa responded to the induction shock, 

 the radii of the vacant segment were drawn apart, 

 and in this way a curve was obtained by the other 

 writer on the rotating cylinder. Now, by after- 

 wards dropping a perpendicular line from the point 

 at which the electro-magnet writer changed its 

 position, to the parallel line made by the other 

 writer, and then measuring the distance between 

 the point of contact and the point on the last- 

 mentioned line on which the curve began, the period 

 of latent stimulation was determined. A glance at 

 Figs. 3 and 4 (p. 55) will render this description 

 clear to any one who is not already acquainted with 

 the method, when it is stated that the upper line 

 is a record of the movements of the electro- magnet 

 writer, and the lower line that of the movements of 

 the other writer. It will be observed that the 

 point a in the upper line marks the point at which 

 the induction shock was thrown in ; so that by 

 first producing the perpendicular till it meets the 

 lower line at 6, and then measuring the distance 

 between the point h and the point c, .at which 

 the curve in the lower line first begins, the latent 

 period (h c) is determined — the time occupied by 

 the rotation of the cylinder from 6 to c being 

 known. 



Summation of Stimuli. — In this way I have 

 been able to ascertain the period of latent stimula- 



