352 PROTOPLASM 



This is the difference in potential between the upper cut and 

 the lower intact surfaces. The region between, within the 

 apple, is a rather complicated system consisting of the upper 

 wounded surface, the lower skin surface, and the intervening 

 pulp with electrolytes. The potential measured is, consequently, 

 no simple or single one. It involves at least three potential 

 differences, viz., that between the inner and outer sides of the 

 cut surface, that between the inner and outer sides of the intact 

 skin, and that between the cut surface and the intact skin. 

 The direction of flow of current {i.e., of electrons) is from the 

 injured region at the top of the apple, through the surrounding 

 solution, the intact apple skin at the bottom, and finally through 

 the pulp of the apple. The upper, cut surface of the apple 

 is, therefore, negative; and the lower surface of the skin, positive. 

 Within the apple, the flow of current is from the inner surface 

 of the lower untouched skin, which must, therefore, be negative, 

 to the inner surface of the upper, cut area, which is, therefore, 

 positive (Fig. 154). 



Membrane potentials are generally obtained when other 

 tissues, such as frog skin or leaves, are bathed in salt solutions. 



R. Hober undertook an extensive series of experiments in an 

 effort to ascertain which ions Hving membranes permit to pass 

 and which not and, therefore, which ions are responsible for 

 membrane potentials. The responsibility cannot be wholly 

 attributed to this or that ion, but it does appear that the unequal 

 permeability of the cell membrane for potassium and chlorine 

 ions is a large factor in determining membrane potentials in 

 vital systems. 



Injury Potentials. — There remain a number of potentials 

 capable of measurement and therefore, in this sense, real, which 

 the physicist will regard as involving other factors in addition 

 to those to which they are ascribed. One such potential is that 

 due to injury. The actual physical basis of an injury potential 

 is unknown; it is due probably to an upset in ionic equilibrium 

 caused by the injury. Whatever the ultimate cause may be, 

 the potential is the direct result of injury and is capable of 

 measurement. It is not a simple potential but involves others. 

 It does, however, manifest itself clearly and was one of the first 

 forms of vital potentials discovered. If tissue is injured, there 

 is a flow from the injured to the uninjured region. The potential. 



