354 



SCIENCE 



[N. S. Vol. LIII. No. 1372 



this extremely thin layer of protoplasm^ it 

 would seem that the electrical resistance may 

 be regarded as a measure of the permeability 

 of the protoplasm to ions. It is of interest 

 in this connection to find that the measure- 

 ments of the permeability of the protoplasm 

 by a variety of other methods (plasmolysis, 

 exosmosis, diffusion of salts through the 

 tissue, entrance of dyes, etc.) confirm the re- 

 sults obtained by electrical measurement. 



In view of these facta tlie simplest assump- 

 tion which we can make concerning M. is that 

 it is a substance at the surface of the proto- 

 plasm which determines the resistance: as M 

 increases in amount and forms a thicker layer 

 the resistance increases, and vice versa. 



Tissue which has developed under normal 

 circumstances is found to be rather constant 

 in its electrical resistance. This is of con- 

 siderable practical importance as it enables 

 us to test material as it comes into the lab- 

 oratory and to reject any which has been in- 

 jured or is in .any way abnormal. 



"We may therefore speak of a normal degree 

 of resistance as indicating a normal state of 

 the tissue. If injury occurs and the resist- 

 ance falls we may consider that the loss of 

 resistance gives a measure of the amount of 

 injury. Thus if the tissue loses ten per cent, 

 of its normal resistance we may say that the 

 injury amounts to ten per cent. This enables 

 us to place the study of injury upon a quan- 

 titative basis. 



In the case of Laminaria we find that if 

 the injury in a solution of sodium chloride 

 amounts to five per cent, the tissue recovers 

 its normal resistance when replaced in sea 

 water. If however the injury amoimts to 

 twenty-five per cent, the recovery is incom- 

 plete: instead of rising to the normal it re- 

 covers to only ninety per cent, of the normal. 

 The greater the injury the less complete the 

 recovery. When injury amounts to ninety 

 per cent, there is no recovery at all. 



2 Some of the current passes between the masses 

 of protoplasm (i.e., in the cell wall) but allowance 

 can be made for this since the relative proportion 

 of cell wall and protoplasm remains unaltered 

 throughout the experiment. 



This is of practical interest in view of the 

 fact that in physiological literature it seems 

 to be generally assumed that when recovery 

 occurs it is always complete, or practically 

 so, as if it obeyed an " all or none " law. 

 It is evident that partial recovery may be 

 easily overlooked im.less accurate measure- 

 ments are possible. This may serve as an- 

 other illustration of the fact that quantita- 

 tive methods are indispensable in the study of 

 fundamental processes. 



It is evident that injury presents two 

 aspects. One is the temporary loss of resist- 

 ance which disappears, wholly or in part, 

 when the tissue is placed under normal condi- 

 tions: this may be called temporary injury. 

 The other is the permanent loss of a part of 

 the resistance which is observed after more 

 prolonged exposure: this may be called per- 

 manent injury. By exposing tissue for 

 various lengths of time to a toxic solution 

 and observing the amount of recovery each 

 time we may construct a time curve of per- 

 manent injury. This curve may be subjected 

 to the same kind of mathematical treatment 

 as the time curve of temporary injury, al- 

 ready discussed. The mathematical analysis 

 leads to the conclusion that if we adopt the 

 scheme O^S— >A-r>M— »B we must re- 

 gard temporary injury as due to the loss of 

 M. while permanent injury is due to the loss 

 of O. Recovery occurs when the loss of M is 

 replaced by a fresh supply of M derived from 

 O, but if O is itself depleted recovery will 

 be incomplete. 



It may be added that an equation has been 

 found which enables us to predict the re- 

 covery curves under a great variety of con- 

 ditions with considerable accuracy. 



If we accept the conclusions stated above 

 we are obliged to look upon recovery in a 

 somewhat different fashion from that which 

 is customary. Recovery is usually regarded 

 as due to the reversal of the reaction which 

 produces injury. The conception of the 

 writer is fundamentally different; it assumes 

 that the reactions involved are irreversible 

 (or practically so) and that injury and re- 

 covery differ only in the relative speed at 



