20 PROTOPLASMIC ACTION AND NERVOUS ACTION 



The question^ Why is living matter so characteristi- 

 cally cellular in structure, seems to be equivalent to the 

 question, Why is it partitioned, subdivided into minute, 

 usually microscopical, portions (cells), or structurally 

 discontinuous ? Each portion of protoplasm is separated 

 from its surrounding medium or from adjoining cells 

 by a thin, structurally distinct boundary layer usually 

 called the ''plasma membrane"; and the presumption is 

 that some definite physiological advantage attaches to 

 this peculiarity. The most evident general answer is 

 that this layer serves to separate or insulate the hving 

 protoplasm from the surroundings, and thus to protect 

 it from the disintegrative or otherwise adverse influence 

 of the latter. This view regards the plasma membrane 

 as primarily a protective structure. Through its 

 presence each separate portion of living substance, or 

 cell, is enabled to retain its special composition and 

 individuality. But this answer, while undoubtedly 

 correct in part, is too vague and general to be satisfactory. 

 The recent experimental studies on protoplasmic per- 

 meabiUty have thrown a more definite light on the 

 problem. They have shown that in typical living cells 

 the external protoplasmic layer has the properties of a 

 semi-permeable membrane; i.e., it is impermeable or 

 difficultly permeable to the water-soluble substances of 

 low molecular weight present in the protoplasm and 

 surroundings (and to chemically similar substances), 

 while freely permeable to w^ater. Free diffusion of 

 soluble substances either into or out of the cell is thus 

 prevented; the protoplasm can preserve a chemical 

 composition different from that of the surrounding 

 medium without the interference that would result 



