CORRELATION BETWEEN METABOLIC GRADIENTS. 323 



brane, leaving the anions behind ; that consequently the membrane 

 is the seat of a polarization, being positively charged externally, 

 negatively charged internally ; that upon stimulation the membrane 

 becomes more permeable at the locus of stimulation, letting anions 

 through ; and that consequently the membrane is depolarized at the 

 place of stimulation, becoming temporarily negative. The diffi- 

 culties in the way of this conception have been pointed out by 

 Keith Lucas ('12), and one of its former chief adherents, R. S. 

 Lillie, has been recently strongly inclined to a chemical point of 

 view. While the current of action is readily accounted for on the 

 basis of the membrane-depolarization theory, it seems to us that it 

 is more difficult to account for the current of injury and almost 

 impossible to explain on this basis the permanent differences of 

 potential which exist along the axes of many organisms. To get 

 such a gradation of potential it is necessary to assume the existence 

 of a gradation in concentration of ions along surface and interior. 

 A difference in concentration of similarly charged ions along the 

 axis could scarcely be brought about by the properties of the mem- 

 brane; the membrane is either permeable or impermeable to a 

 class of ions. Such concentration differences must be due to dif- 

 ferences in the speed of production of such ions at different levels. 

 This differential speed of production of ions with reference to 

 level is exactly what we suggest to exist, and if it exists the 

 assumption of membranes is unnecessary. The membrane-de- 

 polarization theory -also states that the current of injury is due to 

 the exposure of the (presumably) negatively charged interior of 

 the tissue at the injured surface, thus producing a negativity at the 

 cut surface. But it has long been known that the negativity due 

 to injury is present not only at the cut surface, but also exists 

 along the uninjured external surface, decreasing with distance 

 from the site of injury. This is readily explained on our point of 

 view which regards injury as a form of stimulation, accompanied 

 by increased metabolic rate, and transmitted with a decrement to 

 adjacent regions, as are other forms of stimulation. 



So numerous are the possible sources of potential differences 

 that it is probable that such differences in organisms are due in 

 different instances to different factors or to a combination of 

 factors. All that we wish to emphasize here is that certain very 



