212 IRRITABILITY 



stances wherein this period lasts a considerable time after stimu- 

 lation, that is, before the irritability returns to the original level, 

 as, for example, the smooth muscle. Indeed, depending upon 

 the specific properties of a system, a short or a long interval is 

 required before a stimulus of a given intensity is again operative. 

 Finally, in one and the same living system the duration of the 

 refractory period can be very different, depending upon the 

 momentary state of the system. Above all we know that the 

 refractory period is considerably prolonged in fatigue and like- 

 wise after the influence of other agents, as narcotics, lowering of 

 the temperature, etc. In such states a second stimulus remains 

 inoperative when it follows at a definite interval from the first, 

 whereas under normal conditions the same stimulus applied at the 

 same interval would be operative. 



Finally, there is another factor to be considered, namely, that 

 the latent period of the second stimulus is more and more pro- 

 longed as the second stimulus approaches more closely to the 

 absolute refractory period of the first. In the above schemes the 

 latent period was not taken into consideration because practically 

 for all the intervals of stimulation considered at that time it could 

 be assumed to be the same. When, however, a decrease of the 

 intervals between the individual stimuli takes place, the prolonga- 

 tion of the latent period can then not be overlooked, as it leads to 

 a retardation of response. (Figures 29, 30.) This fact was shown 

 in the classic investigations of Marey^ upon the refractory period 

 of the heart, and more recently has been the subject of study 

 by Samojloff,^ Keith Lucas^ and Gotch* in the muscle and 

 nerve. These, then, are the essential factors which bring about 

 interference, and although there are special details which deserve 



1 Marey : "Des excitations artificielles du coeur." Trav. du lab. de M. Marey II, 

 1875. The same: "Des mouvements que produit le coeur lorsqu'il est soumis a des 

 excitations artificielles." Compt. rend, de I'acad. des sciences T. LXXXVII, 1876. 



2 Samojloff: "Actionsstrome bei summierten Muskelzuckungen." Arch. f. Physio- 

 logie Suppl. 1908. The same: "t)ber die Actionsstromkurve des quergestreiften 

 Muskels bei zwei rasch aufeinanderfolgenden Reizen." Zentralblatt f. Physiol. 1910. 



3 Keith Lucas: *'On the refractory period of muscle and nerve." Joum. of Physiol- 

 ogy, XXXIX, 1909-10. The same: "On the recovery of muscle and nerve after the 

 passage of a propagated disturbance." Ibid. XXXXI, 1910-11. 



4 Gotch: "The delay of the electrical response of nerve to a second stimulus." 

 Joum. of Physiology, XXXX, 1910. 



