140 



Mr. H. E. Roaf. The Vapour- Pressure 



osmometer, it unites with the protein to form an ionising salt. Of this salt 

 the protein ion cannot pass through the parchment paper membrane, and the 

 acidic ion is held back by the opposite electrical charge on the protein. The 

 pressure inside the osmometer consists of the sum of the protein and acidic 

 ions and the free acid ; the pressure outside the osmometer consists of the 

 free acid alone. Therefore the excess pressure inside is due to the protein and 

 acidic ions (14, 17, 18, 20). 



If in calculating the rate of contraction we assume that the lactic acid is 

 liberated in the isotropous substance, and that it must diffuse into the 

 anisotropous substance, we are using the most unfavourable conditions for the 



Diagram illustrating to scale the osmotic con- 

 traction of two segments of four fibrils of 

 frog's sartorius muscle. Dim band 0"9 fi 

 long ; light band 0'9 ft long and fibrils - 3 fi 

 diameter. A, relaxed ; B, contracted condi- 

 tion. S, sarcolemma ; shaded portion aniso- 

 tropic substance ; unshaded portion isotropic 

 substance. 



osmotic hypothesis. The anisotropous substance is considered to be an 

 ellipsoid, but calculations assuming that the anisotropous substance is 

 cylindrical give practically the same results. 



In calculating the extent and rate of contraction the results will depend 

 upon the dimensions of the structures concerned. Measurements made by 

 Dr. F. O'B. Ellison on frog's sartorius show that there the length of the dim 

 band is 0"9 fi, the length of the light band is - 9 fi, and the diameter of the 

 fibril is approximately 0'3 fi. As the diameter of the fibril and relative 

 lengths of the dim and light bands may vary in different muscles, the 

 calculations are made so as to show what variations these differences may 

 allow in the extent and rate of contraction. In the Table (p. 146), therefore, 



