METHODS OF ELECTRICAL STIMULATION 21 



pressure of the contents is from four to six 

 atmospheres. When the cell is placed in water, 

 the water penetrates both envelopes and the cell 

 swells so far as the resistant outer envelope 

 may permit. In a concentrated salt solution, 

 the osmotic pressure pushes the inner envelope 

 away from the outer envelope, contracting the 

 volume of the cell contents (plasmolysis), and 

 water leaves the cell until the concentration of 

 the cell contents equals that of the surrounding 

 solution. Solutions whose concentration is such 

 that cells immersed in them are not deformed 

 must possess the same osmotic pressure. Such 

 solutions are termed isotonic or isosmotic. 



Estimation of Osmotic Pressure in the Blood 

 Serum. 1. The method of de Fries. 1 Make a 

 section along the midrib of the violet side of 

 the leaf of Tradescantia discolor and two other 

 sections one on each side, parallel to and about 

 2 mm. from the first. Now make transverse 

 sections across the three vertical sections about 

 5 mm. apart, and a final very thin section paral- 

 lel to the surface. Place some of these thin tan- 

 gential sections in the serum diluted with 20 per 

 cent of water, and others in salt solutions of 

 different concentrations (0.60, 0.65, 0.70, 0.75, 0.80 



1 DE VRIES: Zeitschrift fur physikalische Chemie, 1888, ii, 

 p. 419. 



