AND BLOOD PRESSURE 151 



required to disprove the filtration hypothesis because the con- 

 ditions are the same both in the resting and the excited gland, 

 in as far as the whole of any lobule of the gland must be at the 

 same pressure, and therefore no filtration pressure exists in the 

 one state more than in the other. 



The gland is composed of a connective tissue framework, hold- 

 ing together tubules full of secreting cells, whose protoplasm con- 

 tains some 80 per cent, of water, the tissue spaces surrounding 

 the tubules are full of lymph, the capillaries of blood, and the 

 tubules of saliva. The wet films of protoplasm which form the 

 walls of the tubules, the capillaries, and the lymphatics, may act 

 as colloidal surfaces separating fluids of different chemical con- 

 stitution, but cannot possibly act as rigid sieve-like structures. 

 There cannot be a difference of hydrostatic pressure on either side 

 of the films, as is required by the mechanical theory. Molecular 

 not molar forces are here at play. The salivary cells are endowed 

 by their colloidal structure with the power of linking up or setting 

 free crystalloids brought to them in solution, and are thus the seat 

 of the play of complex forces of surface and osmotic energy, and 

 at the same time are the seat of chemical reaction, selective in 

 character and depending on the ferments they contain the keys 

 which fit the locks of chemical constitution. These living cells 

 control the passage of fluid in one or other direction, and the 

 mystery of the secretory process requires the same solution as in 

 the case of the unicellular organism, and this solution at present 

 is just as far from attainment in the one case as in the other. 



The comparative study of the structure of the nephridial 

 tissues shows how far away from the truth are the mechanical 

 theories of renal secretion which have held their ground in physio- 

 logical text-books for the last fifty years. The writer has con- 

 densed the following passages from the work of Dahlgren and 

 Kepner. 



In the unicellular animals the excretory organ is formed by 

 contracting vacuoles which form channels leading from the endo- 

 plasm to the exterior of the cell. The fluid that the vacuoles 

 throw out by their rhythmic expansion and contraction is drained 

 from the cell and is charged with uric acid. The contractile 

 vacuoles of ParamoBcium are two in number and permanent 

 features of the cell. Their inner surface dips into the endoplasm 

 and their outer surface opens through the ectoplasm to the exterior. 



