PART II. 



THE PROPERTIES OF PROTOPLASM 



CHAPTER XII. 



PROPERTIES CONFERRED BY THE DIFFUSIBLE 

 CONSTITUENTS. 



THE OSMOTIC PRESSURE OF THE TISSUE FLUIDS. 



The diffusible constituents of living matter and of the media which 

 bathe it, play a leading part in determining the movements and dis- 

 tribution of the most abundant constituent of living cells, namely 

 Water. Water is a very essential constituent of protoplasm, for a 

 variety of reasons. In the first place it is a solvent for the majority 

 of the protoplasmic constituents, and thus permits their mobility and 

 promotes, by reduction of internal friction and cohesion, the free 

 and rapid interplay of chemical reactions which characterizes the 

 unstable equilibria of life. Then, again, water is the most efficient 

 ionizing solvent, and thus permits electrical forces to come into play, 

 and that notable increase in chemical reactivity which accompanies 

 the ionization of dissolved substances. The low internal friction of 

 water permits the changes of form, and rapid displacements of sub- 

 stance which render the mobility of living matter possible. The high 

 surface-tension of water is essential in the conservation of the boun- 

 daries of the cell, and their restoration after displacements due to 

 motion, and this, in turn, conserves the minute internal structures 

 of the cell. The high specific heat of water enables it to absorb a 

 great deal of heat without increasing very greatly in temperature and, 

 conversely, to part with stored-up heat without falling very much in 

 temperature. Sharp inequalities of temperature which might other- 

 wise arise in living tissues are thus smoothed out by the "buffer 

 action" of the prevailing solvent. 



It is of interest to consider the percentages of water which are 

 contained in the various tissues of the animal body. The following 

 are illustrative analyses cited after Hammarsten: 



