62 PSYCUOLOOI. 



the physiology of the future, save iu respect to a few points 

 of which ii word must uow hv said. And first of 



THE SUMMATION OF STIMULI 



in the same nerve-tract. This is a propert}' extremely im- 

 portant for the uuderstandint^ of a great many phenomena 

 of the neural, and consequently of the mental, life ; and it 

 behooves us to gain a clear conception of what it means be- 

 fore we proceed any farther. 



The law is this, that a stirmdus which ivould he inadequate by 

 itself to excite a nerve-centre to effective discharge may, by acting 

 with one or more other stimuli [equally ineffectual by themselves 

 alone) bring the discharge about. The natural way to con- 

 sider this is as a summation of tensions which at last over- 

 come a resistance. The first of them produce a 'latent 

 excitement ' or a ' heightened irritability '—the phrase is 

 immaterial so far as practical consequences go ; the last is 

 the straw which breaks the camel's back. Where the 

 neural process is one that has consciousness for its accom- 

 paniment, the final explosion would in all cases seem to 

 involve a vivid state of feeling of a more or less substantive 

 kind. But there is no ground for supposing that the ten- 

 sions whilst yet submaximal or outwardly inefi'ective, may 

 not also have a share in determining the total conscious- 

 ness present in the individual at the time. In later 

 chapters we shall see abundant reason to sujDpose that they 

 do have such a share, and that without their contribution 

 the fringe of relations which is at every moment a \ital in- 

 gredient of the mind's object, would not come to conscious- 

 ness at all. 



The subject belongs too much to physiology for the 

 evidence to be cited in detail in these pages. I will throw 

 into a note a few references for such readers as may be in- 

 terested in following it out,* and simply say that the direct 



* Valentin : Archiv f. d. gesammt. Physiol., 1873, p. 458. Stilling: 

 Leipzig Acad. Berichte, 18T5, p. 372 (Journal of Physiol., 1875). J. 

 Ward : Archiv f. (Anat. n.) Physiol., 1880, p. 72. H. Sewall : Johns 

 Hopkins Studies, 1880, p. 30. Krouecker u. Nicolaides : Archiv f. 

 (Anat. u.) Physiol.. 1880, p. 437. Exuer : Archiv f. die ges. Physiol., Bd. 

 28, p. 487 (1882). Eckhard : in Hermann's Hdbch. d. Physiol., Bd. i. Thl. 

 II. p. 31. Frangors-Franck : Le(,ons sur les Fonctions motrices du Ccr- 



