GENERAL CONDITIONS OF BRAIN- ACTIVITY. 83 



electrical irritation of the cortical centres sufficiently proves 

 the point. For it was found by the earliest exiDerimenters 

 here that whereas it takes an exceedingly strong current 

 to produce any movement when a single induction-shock 

 is used, a rapid succession of induction-shocks (' faradiza- 

 tion ') will produce movements when the current is com- 

 paratively weak. A single quotation from an excellent 

 investigation will exhibit this law under further aspects : 



" If we continue to stimulate the cortex at short intervals with the 

 strength of current which produces the minimal muscular contrac- 

 tion [of the dog's digital extensor muscle], the amount of contraction 

 gradually increases till it reaches the maximum. Each earlier stimula- 

 tion leaves thus an effect behind it, which increases the efficacy of the 

 following one. In this summation of the stimuli .... the following 

 points may be noted : 1) Single stimuli entirely inefficacious when 

 alone may become efficacious by sufficiently rapid reiteration. If the 

 current used is very much less than that which provokes the first begin- 

 ning of contraction, a very large number of successive shocks may be 

 needed before the movement appears — 20, 50, once 106 shocks were 

 needed. 2) The summation takes place easily in proportion to the 

 shortness of the interval between the stimuli. A current too weak to 

 give effective summation when its shocks are 3 seconds apart will be 

 capable of so doing when the interval is shortened to 1 second. 3) 

 Not only electrical irritation leaves a modification which goes to swell 

 the following stimulus, but every sort of irritant which can produce a 

 contraction does so. If in any way a reflex contraction of the muscle 

 experimented on has been pi-oduced, or if it is contracted spontaneously 

 by the animal (as not unfrequently happens ' by sympathy,' during a 

 deep inspiration), it is found that an electrical stimulus, until then 

 inoperative, operates energetically if immediately applied.'" * 



Furthermore : 



" In a certain stage of the morphia-narcosis an ineffectively weak 

 shock will become powerfully effective, if, immediately before its appli- 



veau, p. 51 ff., 339. — For the process of summation in nerves and imiscles, 

 cf. Hermuun : ibid. Thl. i. p. 109, and vol. i. p. 40. Also Wuudt: 

 Physiol. Psych. , i. 243 ff . ; Richet : Travaux du Laboratoire de Marey, 1877, 

 p. 97 ; L'Homme et I'lntelligence, pp. 24 ff., 468 ; Revue Philosophique, 

 I XXI. p. .564. Kronecker u. Hall: Archiv f. (Auat. u.) Physiol., 1879; 

 Schoiileiu . ibid. 1882. p. 357. Sertoli (Hofmanu and Schwalbe's Jahres- 

 bericht, 1883. p. 25. De Watteville : Nourologisches Ceutralblatt, 1883, 

 No. 7. Griiuhagen : Arch. f. d. ges. Physiol., Bd. 34, p. 301 (1884). 



* Bubuoff und Heidenhain : Ueber Erreguugs- und Hemmungsvorgange 

 innerhalb der motorischen Hirnceutren. Archiv f. d. ges. Physiol., Bd. 

 26, p. 156(1881). 



