164 The Unity of the Organism 



two for the conductor, and one for the effector. An illus- 

 tration would be a reflex-arc consisting of a tactile cell of 

 a touch organ, the two cells constituting the conducting 

 path, and an effector cell in a muscle. A point of special 

 interest in connection with such an arc is that the dependence 

 of the parts on one another is such, in the highest develop- 

 ment of the arc, that the specific action of each part is 

 dependent on the specific and connected action of the other 

 parts. "The optic nerve itself," to quote Sherrington, "is 

 unable to enter into a heightened phase of its own specific 

 activity on the application of light. Initiation of nervous 

 activity by light is the exclusive (in this instance) function 

 of cells in the retina, retinal receptors." 1 And of course 

 without brain cells as effectors for vision the specific activi- 

 ties of receptors and conductors would be impossible. Not 

 only would an optic nerve fiber's conducting ability be useless 

 without a retinal cell on the one hand and a brain cell on the 

 other, but the very conductive act itself would not be fully 

 performed. 



This specific dependence, as it might be called, of the 

 parts of a reflex-arc is so significant that another example 

 may be profitably noticed. A striking one is afforded by 

 the effects of the passage of gall stones through the gall 

 ducts, instanced by Sherrington, partly on the basis of his 

 own studies. The excruciating pains associated with this 

 malady are due to the distention of the wall of the gall duct 

 by the passage through it of the mineralized organic con- 

 cretions which constitute the gall stones. The point in 

 this for us is the fact that though the stimulus which pro- 

 duces the pain is mechanical and acts upon the wall of the 

 duct, this stimulus is so peculiar that other sorts of me- 

 chanical stimuli of the same tissue, even to cutting and 

 wounding, give no sense of pain. Though the duct may 

 be cut without causing pain, pain may be produced by 

 injecting the duct to distention with neutral fluid. "Marked 



