DIGESTION 345 



pancreatic juice and of saliva : the pressure of the latter 

 in the submaxillary duct may, as we have seen, greatly 

 exceed the arterial blood-pressure, without reabsorption and 

 consequent oedema of the gland occurring ; but the secretory 

 pressure of the pancreatic cells is very low, not more than a 

 tenth of that of the salivary glands. GBdema begins before a 

 manometer in the duct shows a pressure of 20 mm. of mercury. 



(4) The Influence of Nerves on the Secretion of Bile. 

 Although bile is secreted constantly, it only passes at 

 intervals into the intestine. For the liver in many animals, 

 unlike every other gland except the kidney, has in connection 

 with it a reservoir, the gall-bladder, in which its secretion 

 accumulates, and from which it is only expelled occasionally. 

 We have therefore to distinguish the bile-secreting from the 

 bile-expelling mechanism. Of the direct influence of nerves 

 on either we have scarcely any knowledge, scarcely even 

 any guess which is worth mentioning here. It is true the 

 secretion of bile may be distinctly affected by the section 

 and stimulation of nerves which control the blood-supply of 

 the stomach, intestines, and spleen, for the quantity of blood 

 passing by the portal vein through the liver depends upon 

 the quantity passing through these organs, and the rate of 

 secretion is closely related to the blood-supply. In this way 

 stimulation of the medulla oblongata, the spinal cord, and 

 the splanchnic nerves stops or slows the secretion of bile by 

 constricting the abdominal vessels ; and the same effect can 

 be reflexly produced by the excitation of afferent nerves. 



The muscular fibres of the gall-bladder and the larger 

 bile-ducts are thrown into contraction by stimulation of the 

 spinal cord. It is possible that this takes place naturally in 

 response to reflex impulses from the mucous membrane of 

 the duodenum, for the application of dilute acid to the 

 mouth of the bile-duct causes a sudden flow of bile, and the 

 acid contents of the stomach, when projected through the 

 pylorus into the intestine, have a similar effect. ' 



The pressure under which the bile is secreted is remarkably 

 small, the maximum being no more than 15 mm. of mercury. 

 But small as this is, it is higher than the pressure of the 

 portal blood, and therefore the liver ranges itself with the 



