THE LIVER AND PANCREAS 243 



acids are the best of cholagogues, and stimulate the production 

 of bile. 



Cholesterin finds its way to the liver for the purpose of being 

 excreted. It is a substance found in many of the tissues of the 

 body, but especially in the white matter of the nervous system. 

 It is insoluble in water, but soluble in a solution of bile acids. 

 When cholesterin finds its way into the bile, it is eliminated with 

 the faeces. Cholesterin is found in very regular quantities in the 

 body, and forms one of the principal constituents of certain gall- 

 stones, and also, it may be added, of tumours in the lateral 

 ventricle of the brain of the horse. 



Lecithin is another waste product brought to the liver for 

 excretion. It is of unknown physiological significance, but it 

 has been suggested that it may serve to activate the lipase of 

 the pancreatic secretion. 



Secretion of Bile. — Bile is secreted under a very low pressure, 

 which is the reverse of what occurs in the saliva ; low as the 

 pressure is— 13 mm. (0-58 inch) of mercury — it is higher than that 

 of the blood in the portal vein. If the pressure in the bile-duct 

 be raised, the bile is reabsorbed, being taken up by the lymphatics 

 of the liver, and so conveyed to the blood-stream. It is probable 

 that the majority of cases of jaundice are due to obstructive 

 causes, though exceptions to this rule occur. The secretion of 

 bile is a continuous one ; whether the animal be in full digestion 

 or fasting, the flow is not intermittent, as in the case of the saliva. 

 Though continuous, it is not uniform ; it reaches its maximum 

 in the dog between the second and fourth hours after a meal ; 

 this is followed by a fall, and again about the seventh hour by 

 a rise. A similar curve is given by the pancreatic secretion, 

 which suggests how closely these two fluids are co-operative in 

 digestion, while it can be shown that a specific substance, 

 secretin, which stimulates the production of pancreatic juice, 

 also hastens the secretion of bile. Acids injected into the duo- 

 denum increase the flow of bile, even after all nervous connections 

 are severed. This is due to the acid chyme converting the pro- 

 secretin into secretin, and this, as above stated, has a specific 

 action on the liver cells. 



The secretion of bile is increased by any agent which destroys 

 the red blood-cells. A solution of haemoglobin injected into the 

 blood produces the same effect. The administration of bile 

 increases its production ; this is due to the bile acids having a 

 specific action on the liver cells, and acting as cholagogues. 



In those animals possessing a gall-bladder this receptacle is 

 filled with bile during abstinence, or, if it be empty, it is filled 

 even during digestion. The reflux of bile from the biliary duct 

 to the gall-bladder is caused by a sphincter-like contraction of 



