GLANDS, GENERAL PHYSIOLOGY OF SECRETION. 261 



plain why the flow of tears may continue indefinitely, the 

 lachrymal secretion being entirely a transudation and con- 

 taining no specific element of the lachrymal glands them- 

 selves'. It will also explain why by thinking of palatable 

 foods, etc., the mouth begins to water. It is a stimulation 

 of the cerebro-spinal nerve, the result of which is an im- 

 mediate flow of the transudatory part of the saliva. It must 

 not be imagined, however, that the flow of tears or the flow 

 of this watery saliva is a mere filtration from the blood. It 

 is an actual picking up of these materials from the blood by 

 the gland cells themselves. If it were a mere filtration 

 there is no reason why the tears should not flow at an un- 

 varying rate all the time. 



2. The sympathetic nerves. In addition to the cerebro- 

 spinal nerve to the submaxillary gland this gland is supplied 

 with branches from the sympathetic system. When these 

 nerves are stimulated the gland begins to secrete saliva, 

 also, but the saliva is now of an entirely different kind. 

 Instead of being watery it now becomes exceedingly viscid 

 and ropy, and contains a much greater proportion of the 

 specific elements of the secretion. Continued stimulation 

 of the sympathetic nerve may cause the saliva to cease 

 flowing altogether ; but this is because the secretion has be- 

 come so thick and concentrated that it is not able to force 

 its way through the delicate tubes. Evidently the sympa- 

 thetic nerve has to do with the production of the specific 

 elements themselves, and in no integral way whatever is it 

 concerned with the transudation elements. It seems to 

 govern the production of those things which the gland must 

 make for itself and store up. If, now, after the sympa- 

 thetic nerve has been stimulated for some time and the 

 saliva has thus been made thick and ropy, the cerebro- 

 spinal nerve be also stimulated, there is at once a copious 

 flow of saliva. Under the influence of the latter nerve large 

 quantities of water and mineral salts are actively picked up 

 by the gland from the blood and passed through into the 

 tubules, washing out the specific elements formed. 



