46 PHYSIOLOGY OF FARM ANIMALS [CH. 



The bile contains the waste products of the liver which result 

 from its other activities. Nevertheless when associated with 

 pancreatic jviice it is of considerable importance in the process 

 of fat digestion; moreover it promotes intestinal peristalsis, its 

 absence leading to constipation. Bile is an alkaline fluid of a 

 slimy consistence and with a bitter taste ; in herbivorous animals 

 it is yellow-green or dark green, in the pig reddish-brown, and in 

 carnivorous animals golden red. The differences in colour 

 depend on the bile pigments of which bili-rubin and bili-verdin 

 are the chief. Its composition is approximately as follows : 



Water, 85 parts. 



Bile salts, 10 parts (glycocholate and taurocholate of soda). 



Fats, lecithin and cholesterin, 1 part. 



Mucus and pigments, 3 parts, 



Inorganic salts, 1 part. 



The secretion of bile occurs under a pressure lower than that 

 of the blood, and in this respect is different from the salivary 

 secretion. There is no evidence that nerve fibres are concerned 

 with bile secretion which seems to be controlled by the com- 

 position of the blood carried to the liver and by the activity of 

 the liver cells. There is some evidence that the secretion is 

 stimulated by a substance called secretin which is liberated from 

 the wall of the duodenum and is of the nature of a hormone or 

 chemical excitant. This substance, however, acts chiefly upon 

 the cells of the pancreas as will be described below. 



The bile is either conveyed directly by the bile duct into the 

 duodenum or by the cystic duct into the gall bladder, in which 

 it is retained until required ; it is then poured out into the 

 duodenum by the bile duct. The latter opens a short distance 

 from the pyloric end of the stomach. In some animals there is 

 no gall bladder. This was pointed out long ago by Aristotle 

 who records its absence in the horse, ass, deer and roe. It is 

 also absent in the elephant, tapir and rhinoceros, and is im- 

 perfectly develo])ed in the camel. In the ox, sheep, pig, and all 

 carnivora it is present. In man it is present with a few individual 

 exceptions, as noted by Aristotle, whose observations on this 

 subject show a remarkable accuracy. It has been suggested 

 that the gall bladder is absent in the horse because this animal 

 in the natural state is constantly eating, and consequent^ the 

 continual outpouring of bile into the intestine is beneficial, 



