TB,A>fSACT10NS OP THE SECTIONS. 1 JD 



almost invariably responds to a single flash by giving one or more contractions. 

 On removing the margin such responses cease on the part of the bell, although they 

 continue on tlie part of tho severed margin. But on removing the so-called " eye- 

 specks" from the margin such responses cease; and that these "eye-spocks" 'are 

 true visual organs is further proved by the fact that, while unmutilated<b'ff ?•«/«; will 

 throng into the path of a beam of light, and even follow the beam wherever it is 

 moved through the water, i^'arsiee vfith their "eye-specks " removed will no longer 

 do so. Any one of the luminous rays of the spectrum acts as a stimulus, but not 

 so the rays which lie on either side of the luminous spectrum. 



The period of latent stimulation was determined in the case oiAurcUa aurt'ta 

 by employing the induction-shock. It was foimd to vary greatly, according to the 

 temperature at which the tissue was kept. Thus, while in water at 20'^ it was 

 5 sec, in water at 70° it was } sec. It was also found to vary gi-eatly under the 

 influence of so-called summation of stimuli. Thus, while in v/ater at 4o° the latent 

 period was g sec. in the case of the first of a series of stimuli supplied in regular 

 succession at two seconds' interval, it was only | sec. in the case of the tenth stimulus 

 of the series. In every such series of stimuli supplied at short intervals the latent 

 period becomes progressively less and less until it attains its minimum, while the 

 strength of the contraction becomes progressively greater and greater until it attains 

 its vuiximum, the intensity of the stimulation, of course, remaining constant 

 throughout the series. If more than one minute is allowed to elapse between any 

 two successive stimuli of a series, this beneficial or arousing effect of summation no 

 longer asserts itself ; the tissue has, as it were, forgotten the occurrence of the 

 previous stimuli. That the arousing effect in question is due to the occurrence of 

 the successive stimidatiom, and not to the occurrence of the successive contractions, 

 appears to be indicated by the fact that if induction-shocks be employed whicli 

 are of less than minimal intensity at the commencement of a series, they first become 

 of minimal and evontuallj^, at the end of a series, of more than minimal intensity. 

 Now, as in this case no contraction occurs in response to the first three or four 

 stimuli, it is evident that the summating influence must have reference to the 

 process of stimulation as distinguished from that of contraction. Nevertheless, 

 that the summating effect is a general one pervading the whole extent of the re- 

 sponding tissue, and not confined to the area occupied by the electrodes, is proved 

 by the fact that if, during the administration of a series of stimuli, the electrodes 

 be suddenly shifted to another part of the excitable tissue (perhaps eight or nine 

 inches from their previous seat), the summating effect is resumed from the point at 

 which it was left by the previous stimulus. The author further proved by various 

 experiments that during the natural swimming motions of the JMeduste every con- 

 traction exerts a beneficial influence on its successor, which resembles both in kind 

 and degree that whicJi is exerted by a contraction due to an artificial stimulus. 



Artificial Rhythm. — When the paralyzed disk of ^(w-e/^rtfoo-i'i'rtissubmitted to strong 

 faradaic stimidation,it goes into a tolerably well-pronounced tetanus. If the strength 

 of the current be now diminished, the tetanus assumes a wild and tumultuous charac- 

 ter, somewhat resembling that of a heart under similar circumstances. If the strength 

 of the current be again progressively diminished the character of the tetanus bo- 

 comes progressively less and less tumultuous, until at last it ceases to be tetanus 

 and passes into rhythm. This artificial rhythm is quite as regular and quite as 

 sustained as is the natural rhythm of the animal. Its rate varies in difiereut spe- 

 cimens, but usually corresponds with that of rapid swimming. Progressively di- 

 minishing the strength of the faradaic stimulation has the ertect of progi'essively 

 decreasing the rate of the rhythm down to the point at which all response ceases'^; 

 but between the slowest rhythm obtainable by minimal stimulation and the most 

 rapid rhythm obtainable before the appearance of tetanus there are numerous 

 degrees of rate to be observed. The artificial rhythm may be obtained with a 

 portion of any size of irritable tissue, and whether a" small or a large piece of the 

 latter be included between the electrodes. The persistency of any given rate of 

 rhythm under the same strengtli of cm'rent is wonderfully great ; 'for it generally 

 requires more than an hour of continuous faradization before the rh-\thm beo-ins 

 to become irregular, owing to incipient exhaustion. At first only one systole is 

 omitted at long intervals, but afterwards these omissions become frequent and all 



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