DIGESTION IN THE MOUTH 675 



into the lumen, so giving rise to secretion. This discharge of a fluid with 

 a smaller molecular concentration than the cell or surrounding blood plasma 

 must lead to an increased concentration in the remaining parts of the cell. 

 The increased concentration would naturally induce a flow of water from 

 lymph into cell, and the consequent concentration of the lymph would in the 

 same way cause a flow of water from blood to lymph. This pull of water by 

 the cell from the blood is still further increased in another way. The act 

 of secretion, involving as it does the expenditure of energy, can be carried out 

 only at the expense of chemical changes in the cell. These chemical changes, 

 as in all other metabolic processes of the body, will result in the formation of 

 a number of small molecules from the great colloid molecules of the proto- 

 plasm. The products of metabolism, or metabolites, will therefore accumulate 

 in the cell, pass into the lymph, and increase the concentration of the latter. 

 The increased concentration will call forth an increased transudation of 

 fluid, e.g. water, from the blood-vessels, and the transudation thus evoked 

 will be greater than that necessary to provide the water of the saliva, and 

 will therefore produce a distension of the lymphatic spaces of the gland and 

 an increased discharge of lymph along its efferent lymphatics. As a second- 

 ary result of the activity, perhaps in consequence of the removal of the 

 products of the resting metabolism of the gland, there is increased growth of 

 protoplasm, increased activity of the nucleus, and therefore a tendency to 

 increased assimilatory changes and a preparation of the cell for further 

 secretory changes either immediately or hereafter. 



In the gland, however, as in muscle, when we attempt to form a concep- 

 tion of the mechanism of the chemical machine in the living cell, we are 

 brought up against insuperable difficulties. One might perhaps conceive of 

 the secretory granules being bounded by a membrane impermeable to inter- 

 mediate metabolites and salts, but permeable to carbon dioxide. If the 

 first effect of stimulation of the secretory nerves were to produce an explosive 

 disintegration of the complex molecules making up the granules, we should 

 have a sudden multiplication of molecules within the granules. This would 

 cause a large rise of the osmotic pressure in these granules and the consequent 

 absorption of water from the surrounding protoplasm. This process, how- 

 ever, could only result in the production of a fluid in the granules having 

 the same osmotic pressure as the surrounding medium, whereas we know that 

 saliva has a molecular concentration which is only one half of that of the 

 blood or lymph. We should therefore have to make a second assumption, 

 namely, that, before the extrusion of the solution from the granules, there is 

 a further breakdown of the metabolites by a process of oxidation, with the 

 production of carbon dioxide which diffuses into the surrounding protoplasm. 

 We have, however, no evidence of either of these processes or for any of 

 these assumptions, and I have only adduced them in order to show how far 

 we are still from the actual comprehension of the events occurring in every 

 living cell, and underlying its conditions of rest and activity. 



