260 INFLAMMATION 



a dilute acid because it "likes it," that an ameba rejects a piece of 

 ^lass because it "does not taste good," as we explain similar mani- 

 festations in higher forms; furthermore, it has been shown by Verworn 

 "that minute enucleated fragments of protozoan cells react to stimuli 

 ^ust as does the entire cell, and, therefore, it seems that the only possi- 

 ble explanation of movements in protozoa must be a direct reaction 

 of the stimulated part to the stimulus. The nature of the stimulus 

 and the nature of the stimulated substance must determine the nature 

 of the resulting reaction, and most of the observations so far made 

 suggest that these reactions can be explained according to the known 

 laws of the physics of fluids. An ameba, or a leucocyte, may be looked 

 upon as a drop of a colloidal solution, surrounded by a delicate sur- 

 face layer which is more or less readilj^ permeable to solvents and 

 to substances in solution, and suspended in a fluid of quite different 

 composition. 



Siirface Tension. — Such a drop of fluid suspended in another different fluid 

 obeys well-known laws of physics. The particles of each fluid are all under the 

 influence of a very considerable force, called the cohesion pressure, which tends 

 to draw them together closely. Within the drop each particle is subjected to 

 this force alike from all sides, so that the forces neutralize one another, and each 

 particle is as free as if there were no cohesion pressure. But the particles on the 

 surface are subjected to unequal pressure, for that of the fluid outside the drop 

 is different from that inside, and so the pressure on the surface particles is equal 

 to the difference of the cohesion pressure of the two fluids; this constitutes the 

 surface tension. It is this tension that pulls in upon the surface continually, 

 causing it to tend always to reduce the free surface to a minimum, which condition 

 exists perfectly in the sphere. The amount of cohesion affinity is very different 

 in different fluids, and therefore some have a high surface tension and some a 

 low. When a substance dissolves in another the surface tension is a resultant 

 of the surface tension of the two substances, and hence the surface tension of a 

 liquid may be raised or lowered by dissolving various substances in it. 



Artificial Imitations of Ameboid Movement 



Imagine a drop of fluid suspended in water — let it be a drop of 

 protoplasm, or oil, or mercury; the drop owes its tendency to assume 

 a spherical shape to the surface tension, which is pulling the free 

 surface toward the center and acting with the same force on all sides. 

 The result is that the drop is surrounded by what amounts to an 

 elastic, well-stretched membrane, similar to the condition of a thin 

 rubber bag distended with fluid. If at any point in the surface the 

 tension is lessened, while elsewhere it remains the same, of necessity 

 the wall will bulge at this point, the contents will flow into the new 

 space so offered, and the rest of the wall will contract; hence the drop 

 moves toward the point of lowered surface tension. Conversely, if 

 the tension is increased in one place, the wall at this point will con- 

 tract with greater force than elsewhere, driving the contents toward 

 the less resistant part of the surface, and the drop will move away 

 from the j)oint of increased tension. The rcsemblaiice of these ciianges 

 of form and tlic type of motion produced, to ameboid movement, is 

 apparent, and nmch experimenting has been done to determine how 



