756 PHYSIOLOGY 



under the influence of the nucleus, perhaps into the nucleus itself, 

 and certainly into the undifferentiated cytoplasm. In this cytoplasm 

 a further change occurs, leading to its transformation into granules. 

 When activity is excited by the stimulation of secretory nerves, 

 the primary change appears to involve simply the granules. These 

 structures must absorb water, apparently against osmotic pressure. 

 Those nearest the lumen swell up, become converted into spheres 

 containing water and salts in smaller proportion than exists in the 

 lymph bathing the cells (and presumably in the protoplasm sur- 

 rounding the granules), and in this swollen form are discharged or 

 ruptured on the periphery of the cell 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 protoplasm. 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 con- 

 centration 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 lym- 

 phatics. As a secondary result of the activity, perhaps in con- 

 sequence 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 assimi- 

 latory 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 conception 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 intermediate metabolites 

 and salts, but permeable to carbon dioxide. If the first effect of 

 stimulation of the secretory nerves were to produce an explosive 



