714 



PHYSIOLOGY 



FIG. 338. Tracing of volume of submaxillary 

 gland, showing effect of stimulation of the 

 chorda after administration of 10 mg. atro- 

 pine. The blood pressure (lowest line) was 

 unaltered by the stimulation. (BUNCH.) ( j 



of the secretion in the corresponding manometer until it attains a height 



which may be double that of the mercury in the manometer connected with 



the carotid artery, and therefore must be still greater than the pressure in the 



capillaries of the gland. This ex- 



periment, which is easy to repeat, 



showed the impossibility of the 



act of secretion being in any way 



determined by a process of filtra- 



tion. We have now further evi- 



dence that work is done in the 



production of the salivary secre- 



tion, evidence which was not 



available when Ludwig first car- 



ried out the experiment just de- 



scribed. When a fluid containing 



salts in solution is filtered through 



a porous membrane, the filtrate has 



the same content in salts as the 



original fluid. We can effect a 



separation of dissolved salts from 



a fluid by filtration under pressure 



through some membrane which is 



impermeable to the salts, e.g. a 



membrane of copper ferrocyanide 



a so-called semipermeable mem- 



brane. Under these circumstances 



a very large pressure is necessary 



in order to cause the filtration of 



any fluid at all, a pressure which 



is equal to the osmotic pressure 



exerted by the substances in solu- 



tion. Thus if we were filtering 



a 1 per cent, solution of NaCl 



through a semipermeable mem- 



brane, we should have to exert a 



pressure of about Seven atmO-FiQ. 339. 



spheres in order to obtain a filtrate 

 free from sodium chloride. To 

 obtain a filtrate containing half the amount of sodium chloride, if such 

 were possible, would therefore need a pressure of about three and a half 

 atmospheres. On comparing the osmotic pressures of saliva and blood 

 respectively and for this purpose we can employ the depression of freez- 

 ing-point as our index we find that the molecular concentration of saliva, 

 and therefore its osmotic pressure, is always very much less than that of 

 the blood plasma, and may vary between half and three-quarters of the 

 latter. Supposing the membrane separating the lumen of the duct from the 



Tracing of volume of submaxillary 



n excitation of 



