THE VITAL PHENOMENA OF CELLS 31 



so that they may be transferred to the blood. The respiratory system supplies 

 the blood with the oxygen necessary for combustion in the body. 



The decomposition products arising from combustion must not remain in 

 the lymph, because, if they did, they would finally poison the cells. They must 

 therefore be removed by passage first into the blood and thence out of the body 

 through the excretory organs. 



In order that these necessary functions shall be directed to the proper end 

 of maintaining life, they are all subordinated to the influence of the nervous 

 system whose important object it is to control the organs and to regulate their 

 functions. In addition to this, in the warm-blooded animals a constant body 

 temperature i. e., a constant temperature of the lymph is maintained through 

 the influence of the nervous system. 



1. Cells completely surrounded by membranes can take up only gaseous 

 and dissolved substances. The processes concerned in the absorption of gases 

 by the elementary organisms are but little known, and the phenomena accom- 

 panying these processes in the higher animals are fully discussed in Chapter 

 IX. Our knowledge has progressed somewhat further concerning the absorp- 

 tion of fluids and compounds in solution, and since the phenomena of osmosis 

 figure prominently in this connection, it seems best to discuss them here 

 somewhat in detail. 



Osmosis. When a layer of pure water is carefully stratified upon a solu- 

 tion e. g., sugar in water the layers do not remain separate. The sugar begins 

 at once to rise in spite of the force of gravity and to diffuse into the water; and 

 the movement ceases only when the sugar is distributed uniformly throughout 

 the whole volume of water. The same thing occurs if the water and the sugar 

 solution are separated by a partition which is equally permeable for both. The 

 dissolved substance passes from the place of higher concentration to the place 

 of lower concentration just as if no separating membrane were present. 



Quite a different order of things prevails if between the water and the 

 solution a partition is interposed which allows the water but not the dissolved 

 substance to pass th/rough. Such " semipermeable " walls are obtained by soak- 

 ing a porous clay cell in a solution of copper sulphate, carefully pouring this 

 out and filling -the cell with a solution of potassium ferrocyanide. There is then 

 formed within and upon the clay wall a coherent layer of copper ferrocyanide 

 through which water can be filtered; but if one attempt to filter through it a 

 sugar solution, a much higher pressure is required, and what finally comes 

 through is not the sugar solution at all but pure water. 



If a cell prepared in this way be filled with a sugar solution and be closed 

 by means of a stopper through which connection is made with a manometer, 

 and the cell then be placed in pure water, an increase in pressure inside the 

 cell is noticed, which finally rises to a definite maximum value. This value 

 represents the osmotic pressure of the fluid inside the cell, and is equal to the 

 gas pressure which would be exerted by the same quantity of sugar in the form 

 of a vapor inclosed within the same space at the same temperature. For a one- 

 per-cent solution of cane sugar at 13.7 C. the osmotic pressure amounts to 

 0.691 atmospheres. A four-per-cent sugar solution raises the pressure to 2.74 

 atmospheres. 



Since there is no membrane which is semipermeable for all substances, it is 

 necessary to resort to indirect methods of determining the osmotic pressure of 

 some solutions. One such method which has found wide application both in 

 physiology and medicine is based upon the fact that in a watery solution the 



