OF VITAL PHENOMENA n 



small ones, the number of molecules and the osmotic pressure are 

 increased and water is drawn out of the blood. But when the 

 organ rests, or grows, and large molecules are formed of small 

 ones, the reverse process does not occur. At least the water does 

 not flow in the opposite direction. The water that was drawn 

 out of the blood in the first instance finally passes into the lymph 

 vessels and then back into the blood, causing an increased flow 

 of lymph, but the lymph flow never changes in direction. 



The restricted permeability (called semipermeability) of the 

 plasma membrane of muscle and nerve cells gives the clue for 

 understanding the electric phenomena of these tissues. The rest- 

 ing plasma membrane seems to be permeable to some cations but 

 impermeable to all anions. The cations of some substances that 

 are more concentrated within the cell than outside diffuse through 

 the plasma membrane, but they cannot go far, being held back 

 by the attraction of the opposite electric charge of the anions. 

 These cations on the outer surface of the plasma membrane give 

 the whole cell surface a positive charge. If the plasma membrane 

 is stimulated at one point, this altered region becomes permeable 

 to anions and therefore negative in comparison with the unaltered 

 portion. If the altered and unaltered portions are connected by 

 means of a wire, a current flows through the wire from the un- 

 altered to the altered region. This electric current is called the 

 action current or the current of injury, according to whether 

 it is caused by stimulation or injury of the plasma membrane. 

 Such bioelectric currents are very strong in the electric organs 

 of certain fish, are also noticeable in the sensitive plants and in 

 glands, and' perhaps occur generally. 



Since the body is made up of numerous cells and each cell is 

 composed of several phases, the surface phenomena are very 

 important. While the interiors of solutions are homogeneous, 

 the surface film has peculiar properties, due to the strained rela- 

 tions of the molecules. Dissolved substances (solutes) that re- 

 duce the surface tension of the solvent tend to become more 

 concentrated in the surface film, whereas solutes that increase 

 the surface tension are less concentrated in the surface film. If 

 a solute that decreases the surface tension of one phase, becom- 

 ing more concentrated in its surface film, is soluble in the second 

 phase, it diffuses into the second phase (is absorbed by it) ; but 



