38 THE FUNCTIONS OF ANIMALS 



certainly far from homogeneous. The complex colloidal 

 system is delicately poised between two states, the one 

 truly fluid, though viscous, the other truly solid, though 

 gelatinous. The fluctuations between these ' gel ' and ' sol ' 

 states are largely governed by local variations in concentra- 

 tion of the electrically charged ions of acids, bases, and 

 inorganic salts. Within the cell certain areas, the nucleus, 

 for example, seem to be enclosed by invisibly thin semi- 

 solid membranes, and a similar membrane, more easily 

 studied, forms the external boundary of the cell. 



This " plasma membrane " exhibits the phenomenon 

 of semi-permeability — that is to say, it permits the passage 

 of some substances, but retains others. Molecules of 

 water and of gases pass through readily, and so also do 

 many organic substances of the class of the " fat-solvents," 

 such as alcohol, ether, chloroform, benzene. All these 

 soak into the substance of the plasma membrane and have 

 a narcotic action, depressing the activity of the cell ; 

 for, as Lillie has said, the plasma membrane is more than 

 a mere partition : it is a sensitive intermediary between the 

 cell and the external world. On the other hand, water- 

 soluble organic substances, such as sugars and amino- 

 acids and urea, and all inorganic salts, are held back by the 

 healthy plasma membrane. But the plasma membrane 

 is a most delicate structure, whose semi-permeable pro- 

 perties are easily impaired ; for example, a pure solution 

 of sodium chloride is harmful to most cells, but the addi- 

 tion of a little calcium chloride '' balances " the solution 

 and renders it harmless ; sea water is a perfectly balanced 

 solution. The question of the permeability of the plasma 

 membrane enters into almost every physiological problem, 

 and will be referred to again. 



Generalising from his studies on colour sensation, 

 Professor Hering was led to regard all life as an alterna- 

 tion of two kinds of activity, both induced by stimulus, 

 the one tending to storage, construction, assimilation of 

 material, the other tending to explosion, disruption, dis- 

 assimilation. 



Generalising from his studies on nervous activities, 

 Professor Gaskell was led to regard all life as an alterna- 

 tion of two processes, one of them a running down or dis- 



