292 THE HUMAN BODY. 



processes taking place in the cell are different from those at 

 other periods; and we have next to consider how this change 

 in the mode of life of the cells is brought about. 



Influence of the Nervous System upon Secretion. 

 When the gland is active it is fuller of blood than when at 

 rest: its arteries are dilated and its capillaries gorged so that 

 it gets a brighter pink color; this extra blood-supply might 

 be the primary cause of the altered metabolism. Again, the 

 activity of the pancreas is under the influence of the nervous 

 system, as proved not only by the reflex secretion called forth 

 when food enters the stomach, but also by the fact that 

 electrical stimulation of the medulla oblongata will cause the 

 gland to secrete. The nervous system may, however, only 

 act through the nerves governing the calibre of the gland 

 arteries, and so but indirectly on the secreting cells; while 

 on the other hand it is possible that nerve-fibres act directly 

 upon the gland-cells and, controlling their nutritive pro- 

 cesses, govern the production of the trypsin. To decide be- 

 tween the relative importance of these possible agencies we 

 must pass to the consideration of other glands; since the 

 question can only be decided by experiment upon the lower 

 animals, and the position of the pancreas and the difficulty 

 of getting at its nerves without such severe operations as 

 upset the physiological condition of the animal furnish ob- 

 stacles to its study which have not yet been overcome. 



In certain other glands, however, we find conclusive evi- 

 dence of a direct action of nerve-fibres upon the secreting 

 elements. When the sciatic nerve of a cat is stimulated 

 electrically, the balls of its feet sweat. Under ordinary cir- 

 cumstances they become at the same time red and full of 

 blood; but that this congestion is a factor of subsidiary im- 

 portance as regards secretion is proved by the facts that stim- 

 ulation of the nerve is still able to excite the gland-cells and 

 cause sweating in a limb which has been amputated ten or 

 fifteen minutes (and in which therefore no circulatory changes 

 can occur) and also by the cold sweats, with a pallid skin, of 

 phthisis and the death-agony. It is, however, with reference 

 to the submaxillary and parotid salivary glands that our in- 

 formation is most precise. 



When the mouth is empty and the jaws at rest the sali- 

 vary secretion is comparatively little : but a sapid substance 

 placed on the tongue will -cause a copious flow. The phe- 



