26 PHYSIOLOGY 



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 tension 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 surface 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 pro- 

 trusion 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 food-stuff, may cause local variations in the 

 surface tension of the plasma skin and thus result in the protrusion of 

 pseudopodia and the ingestion of the food particle. 



VITAL PHENOMENA OF CELLS. A. Assimilation. The activity 

 of every living being, whether uni- or multicellular, can be regarded 

 as compounded of two phases, assimilation and dissimilation. By 

 assimilation we mean the building up of the living substance at the 

 expense of material obtained from the external world. In this process 

 substances are formed of high potential energy, and this energy can 

 be obtained only at the expense either of energy imparted to the 

 system at the moment of assimilation, as, e.g. in the assimilation of 

 carbon from carbon dioxide under the influence of the sun's rays, or 

 of energy contained in the food-stuffs themselves. In all living orga- 

 nisms, except those provided with chlorophyll corpuscles, it is the 

 latter method which is adopted, and a food-stuff therefore connotes 

 some substance which can be taken in by the cell and can serve to it 

 as a source of chemical energy. The evolution of energy, which is 

 required for the movements and other vital activities of the cell, is 

 derived from a disintegration or dissimilation of the protoplasm and 

 is generally associated with the process of oxidation. In assimilation, 

 besides the building up of living protoplasm, there may also be a 

 synthesis of more complex from less complex compounds, without 

 their necessary entry into the structure of the living molecule. In the 

 absence of any definite criteria by which we may judge as to the living 

 or non-living condition of parts of the cell, it is a little dangerous to 



