io6 PROTOPLASMIC ACTION AND NERVOUS ACTION 



the different phases; and the physical resemblances 

 between protoplasm and emulsions are apparently 

 referable to the presence of these surface-films. Both 

 systems are examples of what may be called film- 

 pervaded or film-partitioned systems. By the formation 

 of these films a certain structure is imparted to the whole 

 system; this structure is largely the expression of 

 surface-forces, in which chemical, electrical, and mechan- 

 ical (surface-tension) factors all enter. 



It should be noted that according to this conception 

 of protoplasmic structure no essential distinction is to 

 be drawn between the intracellular surface-films or 

 membranes (alveolar membranes, vacuole membranes, 

 nuclear membranes) and the surface-films inclosing 

 entire cells (plasma membranes). In plant cells it can 

 be shown experimentally that vacuole membranes and 

 plasma membranes are similar in osmotic properties, 

 and Hamburger has shown the same for the nuclear 

 membranes and plasma membranes of animal cells.' 



In forming a general conception of the physico- 

 chemical characteristics of protoplasmic membranes, 

 the properties of colloidal gels, especially in their relation 

 to diffusion-processes, may be taken as a starting-point. 

 In a gel, i.e., a solid mixture of colloidal material (such as 

 gelatine) and water, diffusion is hindered only slightly if 

 the concentration of the colloid is low. When the gel 

 is made denser — as the proportion of water is decreased — 

 diffusion becomes slower and more restricted. At a 

 sufficiently high density certain solutes, especially 

 colloids, can no longer diffuse through the gel, although 

 water and cr^-stalloids may still pass. If the density 



' Hamburger, Osmotischer Driick u. Io7ienlehre, III, 8 fit. 



