38. It therefore appears, that every acute excitement of mucous surfaces determines, in the corresponding glands, a remarkable augmentation of action. But how can this excitement, which does not take place immediately upon the glands, have so great an influence over them? For, as we have said, these glands are always subjacent to the membrane, and are consequently separated by it from the irritating bodies. It appears that the above fact belongs to a general modification of the glandular sensibility, which is susceptible of being put into action by every irritation upon the extremities of the excretory ducts, which will be proved by the following considerations: (1) The presence of food in the mouth produces a more abundant flow of saliva. (2) The catheter fixed in the bladder, and irritating the ureters, or their vicinity, increases the flow of urine. (3) The introduction of a bougie, but half way up the urethra, will often be sufficient to occasion the bladder to contract with a power equal to force the urine through the passage, and so to overcome an obstruction in the canal. (4) The irritation of the glans, and of the extremity of the urethra, sub coitu, determines the contraction of the vesiculæ seminales, and augments the secretory action of the testes. (5) The action of an irritating fluid on the tunica conjunctiva occasions an abundant flow of tears. (6) In making experiments upon the state of the abdominal viscera during digestion, and under the influence of hunger, I have observed, that whilst the food is only in the stomach there is very little flow of bile; but it increases when the aliment passes into the duodenum, so that then there is a considerable quantity in the intestines. During hunger the gall bladder is distended, and but little bile flows into the intestines. At the end of digestion, and even when that process is half finished, the gall bladder contains but half of its full quantity; yet it might be expected to empty itself more easily during abstinence, for then the bile it contains is of a deep green colour, very bitter, very acrid, and likely to irritate the organ which encloses it. On the contrary, during, or immediately after digestion, it is more clear, mild, and less irritating; there must, therefore, be, during digestion, another stimulus: now this stimulus is the aliment passing over the mouth of the ductus communis choledochus[A].