INFLUENCE OF THE NERVES ON SECRETION. 755 



termines an increase in the amount of its secretion, as is illustrated 

 by the increased redness of the gastric mucous membrane observed in 

 the case of the Canadian voyageur, Alexis St. Martin, during the ac- 

 tive secretion of the gastric juice, and by the increased vascular tur- 

 gescence of the mammary gland during lactation. As in ordinary 

 nutrition, however, the secretive demand, implied by an increased 

 secretive act, precedes the actual flow of additional blood to a given 

 gland. 



The influence of quality in the blood, is perhaps greater in regard 

 to the excreting than to the secreting glands, as might, indeed, be ex- 

 pected. The presence of a greater or less quantity of the special 

 materials to be separated and eliminated from the blood, must have a 

 direct effect. An excess of urea or uric acid in the blood, whatever 

 may be its cause, determines an increased elimination of those pro- 

 ducts from the kidneys, and an increased consumption of water, aug- 

 ments the quantity of the renal excretion. A temporary increase of 

 carbonic acid in the blood, owing to the rapid oxidation of combustible 

 substances, is followed by an increased evolution of that gas from the 

 lungs ; whilst the drinking of water augments the pulmonary exhala- 

 tion and the cutaneous transpiration. The various secretions are also 

 modified in quantity, by the amount of fluid absorbed from the stom- 

 ach ; and the relative amount of their characteristic ingredients, is 

 dependent on the existence of certain proportions of particular blood 

 constituents from which they are derived, as seen in the production of 

 the fatty acids of the bile, and of the sugar, and the peculiar fatty 

 acids of the milk. 



The effects of stimuli are chiefly to be noticed as influencing the 

 quantity of a given secretion, as in the case of a flow of tears, induced 

 by a foreign body irritating the conjunctiva, or of saliva, from the 

 action of vinegar, mustard, or salt. Stimuli act probably through the 

 intermediation of the vasi-motor nerves, either directly, or else by re- 

 flex action, through other nerves and nervous centres with which the 

 vasi-motor nerves are connected. The general effect of such stimuli is 

 to dilate the small arteries of the gland ; a corresponding increase then 

 occurs in the flow of blood to it, and is the proximate cause of an in- 

 creased secretion or excretion. This increase may, as in the case of 

 the saliva, augment the quantity, but not improve the quality of the 

 secretion, which becomes more watery than usual. 



There are many facts which show an intimate relation between the 

 nervous system, and the secreting activity of the several glands. Thus, 

 the emotions often determine increased secretion, as, e. g., from the 

 lachrymal glands, the skin, the alimentary mucous membrane, and the 

 kidneys. The sight, or even the idea of food, will excite the flow of 

 saliva. Extreme passion or grief has been known to modify, or even 

 render poisonous, the mammary secretion. Direct experiments also 

 show most remarkable effects produced upon the secretive process, 

 through the nervous system, as illustrated in regard to the salivary 

 glands (pp. 264, 522), the gastric glands (p. 526), and the liver (p. 

 748). 



All glands are provided with sympathetic nerves, and many, if not 



