DIGESTION OP FOOD. 315 



when gone wrong — passed from under the dominion of the 

 nerve-centers. Secretion is a part of the natural life-processes 

 of gland-cells — we may say a series in the long chain of pro- 

 cesses which are indispensable for the health of these cells. 

 They must be either secreting cells, or have no place in the nat- 

 ural order of things. It is to be especially noted that the secre- 

 tion of saliva continues when the pressure in the ducts of the 

 gland is greater than that of the blood in its vessels or even 

 of the carotid; so that it seems possible that over-importance 

 has been attached to blood-pressure in secretory processes gen- 

 erally. 



It may, then, be safely assumed that formation of saliva re- 

 sults in consequence of the natural activity of certain cells, the 

 processes of which are correlated and harmonized by the nerv- 

 ous system ; their activity being accompanied by an abundant 

 supply of blood. The actual outpouring of saliva depends usu- 

 ally on the establishment of a nervous reflex arc. The other 

 glands have been less carefully studied, but the parotid is 

 known to have a double nervous supply from the cerebro- 

 spinal and the sympathetic systems. 



It would appear that, as the vaso-motor changes run paral- 

 lel with the secretory ones, the vaso-motor and the proper 

 secretory centers act in concert, as we have seen holds of the 

 former and the respiratory center. But it is to our own mind 

 very doubtful whether the doctrine of so sharp a demarkation 

 of independent centers, prominently recognized in the physi- 

 ology of the day, will be that ultimately accepted. 



Secretion by the Stomach.— The mucous membrane of St. 

 Martin's stomach was observed (through an accidental fistulous 

 opening) to be pale in the intervals of digestion, but flushed 

 when secreting, which resembled sweating, so far as the flow 

 of the fluid is concerned. When the man was irritated, the 

 gastric membrane became pale, and secretion was lessened or 

 arrested, and it is a common experience that emotions may 

 help, hinder, or even render aberrant the digestive processes. 



While the evidence is thus clear that gastric secretion is 

 regulated by the nervous system, the way in which this is ac- 

 complished is very obscure. We know little of either the cen- 

 ters or nerves concerned, and what we do know helps but 

 doubtfully to an understanding of the matter, if, indeed, it does 

 not actually confuse and puzzle. 



Digestion can proceed in a fashion after section of the nerves 



