PHYSIOLOGICAL 381 



it may still be able to transmit the impulse to an unaltered portion 

 of the tissue, which contracts; this shows that conduction and the 

 typical response (contraction) are not inseparable. The importance 

 of the electric currents is that they are able to pass from one cell 

 to another. For example, even the spermatozoa in a group with no 

 organic connection between them are found to influence each 

 other's movements, so that they are soon all vibrating in unison; 

 it is hard to explain this except by an electrical transmission. 



We must not omit a note on the subject of cell-rhythm. Many 

 cells display a tendency to rhythmicity in their activities, for 

 example, in the beating of cilia. Again, the cells of the heart-muscle 

 in Vertebrates have a tendency to contract rhythmically quite 

 apart from nervous stimulation, which they show even when iso- 

 lated from the organ and grown in "tissue cultures". In other cases 

 the rhythm originates in the nervous system, which sends out 

 periodic impulses. W. J. Crozier has recently studied a number of 

 cases of this sort, such as the rhythmic flashing of fire-flies, the 

 chirping of crickets and the beating of the heart of Arthropods: in 

 all these cases temperature has an effect on the frequency: and 

 (by the use of the "Arrhenius-equation" for the effect of temperature 

 on the velocity of reactions) Crozier has shown that it is likely that 

 in all these cases the motor impulses from the nervous system, 

 which determine the activities of the organs controlled, are the 

 same, whatever their nature may be. 



In concluding this survey of the structure and activities of the 

 cell, we may well return to an insistence on the idea of films of 

 protoplasm forming surfaces, certainly at the exterior, and perhaps 

 also within the cell. We have seen that the network produced by 

 the histologist's fixation of cells is an artificial structure; and also 

 that dyes and salts diffuse freely through the protoplasm; so that 

 the idea of a network of partitions dividing liquid protoplasm into 

 a honeycomb is very improbable. On the other hand, there is no 

 doubt as to the importance of surface for chemical reactions, more 

 especially for those catalysed by enzymes. Furthermore, Warburg 

 has pointed out that the surface of porous charcoal may serve as a 

 model; since various substances, otherwise resistant, are oxidised 

 thereon, always provided that iron is present to act as a catalyst, 

 as it does in the living cell. The lowering of the chemical activity of 

 the living matter which follows upon mechanical injury has been 

 mentioned; and in this section the connection between permeability 

 of membranes, irritability and especially electrical response, has been 

 hinted at. Narcotics, which depress the chemical activity of the cell 

 and lower its reactivity, are typically "surface-active" compounds, 

 soluble in fat-solvents. Moreover, it is known that after death, when 

 the permeability of the membranes is increased, enzymes pre- 

 viously locked up in some way attack and destroy the constituents 



