DIGESTION IN THE MOUTH 



669 



FIG. 326. Tracing of volume of sub-maxillary 

 gland, showing effect of stimulation of the 

 chorda after administration of 10 mg. atro- 

 pine. The blood-pressure (lowest line) was 

 unaltered by the stiumlation. ( BUNCH.) 



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- 

 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 



FIG. 327. Tracing of volume of sub-maxillary 

 gland showing decrease on excitation of 

 chorda. ( BUNCH.) 



