24 PHYSIOLOGY 



pure watery solutions. It is possible that, in order to serve the nutrient 

 needs of the cells, more extensive changes may take place in the permeability 

 of the surface layer under limited conditions of time and space. There is no 

 doubt, for instance, that dextrose, to which the surface layer is apparently 

 impermeable, can yet serve as a very efficient food for the cell, and one 

 might ascribe the fact that the cell assimilates only the food which it requires 

 and no more, to such limited changes in permeability. An important factor 

 in the process of assimilation, at any rate by lowly organised cells, must be the 

 relative solubility of the absorbed substances in the cell and its surrounding 

 medium respectively. When a watery solution of iodine is shaken up with 

 chloroform, the latter sinks to the bottom, carrying with it the greater part 

 of the iodine. If a watery solution of organic acid be shaken with ether, the 

 latter fluid will extract the greater quantity of the acid. In no case will the 

 extraction be complete, but there will be a definite ratio between the 

 amount dissolved by the ether and the amount dissolved by the water, the 

 so-called ' coefficient of partage,' depending on the different solubilities of 

 the dissolved substance in the two menstrua. In the same way a mass of 

 protoplasm will tend to absorb from the surrounding medium and to con- 

 centrate in itself all those substances which are more soluble in the colloidal 

 system of the protoplasm than in the surrounding fluid, and this process of 

 absorption may be carried to a very large extent, if the dissolved substances 

 meet in the cell with any products of protoplasmic activity with which they 

 form insoluble compounds so that they are removed from the sphere of 

 action. It is probably by such a process as this that we may account for 

 the accumulation of calcium or silicon in such large quantities in connection 

 with the bodies of various minute organisms. 



Whereas assimilation by a living cell is ultimately conditioned by the 

 permeability of the surface protoplasm, its form is determined by the tension 

 of this layer. If the tension is uniform at all parts of the surface the form of 

 the cell will be spherical. Any diminution of the surface tensibn at one 

 point must tend to cause a bulging of the fluid contents at this point, just as 

 on distending a rubber tube with one weak spot in its wall this suddenly 

 gives way with the production of a large balloon, which rapidly extends in 

 size and ruptures unless the pressure be diminished. Diminution of the sur- 

 face tension at one point of the cell will be attended by a contraction of all the 

 rest of the surface and a driving out of the contents through the weak part. 

 This process will not as a rule result in destruction of the cell ; the resulting 

 protrusion will be limited by the distortion of the internal alveolar structure 

 of the protoplasm caused by any alteration of the spherical form of the cell. 

 Change of form in living structures thus depends ultimately on alterations 

 in surface tension, return to normal being effected by the elastic reaction of 

 the structural arrangement of the protoplasm. This point we shall have to 

 consider more fully when dealing with muscular contraction. At present 

 it is sufficient to see how any slight alteration in the chemical environment, 

 such as might be due to the presence of a particle of foodstuff, may 

 cause local variations in the surface tension of the plasma -skin and thus 



