182 MANUAL OF PHYSIOLOGY. 



The secretion of bile is also influenced by the amount of blood 

 flowing through the organ. Ligature of the hepatic artery causes 

 cessation of the secretion, and ultimately death, from malnutri- 

 tion of the tissue of the liver. 



These variations in the rate of secretion may depend on direct 

 nervous influence, but no special secretory nerve mechanism has 

 been discovered for the liver, and it is quite possible that the 

 changes in the activity of the gland which accompany the differ- 

 ent periods of digestion may be accounted for by changes in the 

 intestinal blood supply, which give rise to corresponding differ- 

 ences in the amount of blood flowing through the portal vein. 

 If the vena porta be ligatured, an effect corresponding to the 

 magnitude of the vessel is produced, the secretion is arrested, and 

 the animal dies ; but it has been said that the secretion continues 

 in the peripheral part of the lobules. If both the portal vein 

 and hepatic artery are ligatured, the secretion at once stops. 



The force with which the bile is secreted is very small. That 

 is to say, the pressure in the ducts never exceeds that of the blood 

 as occurs in the salivary glands ; but, on the contrary, when a 

 pressure of about 16 mm. (.63 in.) mercury is attained, the evac- 

 uation of the bile ceases, and with a little increase of opposing 

 force the fluid in the manometer retreats and finds its way into 

 the blood. The low pressure which can be reached in the gall 

 ducts does not imply any want of secretory power on the part of 

 the liver cells, but merely that there exists a great facility of 

 communication between the duct radicles and the blood vessels, 

 most probably through the medium of the lymphatics. This is 

 made obvious by experiment, by which it can be shown that with 

 a comparatively low pressure (200 mm. = nearly 8 in. of water 

 for a guinea pig) any fluid can be forced into the circulation 

 from the bile ducts. 



It is observable, also, in the stoppage of the bile ducts in the 

 human subject, when some at least of the bile constituents con- 

 tinue to be formed, and pass into the blood, where their presence 

 is demonstrated by the yellow color characteristic of jaundice. 

 The ready evacuation of the bile is then a matter of great import- 

 ance for health, the least check to its free exit causing the secre- 



