190 



THE INTERNAL nNVIRONMFNT OF THE BODY 



Part III 



will contract after the nerves arc cut away. They have also shown that its 

 contraction is stimulated by a hormone, cholecystokinin, secreted by the 

 lining of the duodenum. Products of the digestion of fats in the intestine 

 seem to stimulate the production of this hormone much more than those 

 of proteins and carbohydrates. Bile contains organic salts. Some of these 

 are absorbed by the lining of the intestine, taken up by the blood and returned 

 to the liver. These bile salts and secretin, a hormone from the intestine, both 



ARTERIES 



VEINS 



LYMPH NODE 



LYMPHATICS 



NERVES 



SMALL INTESTINE 



Fig. 11.18. The mesentery formed by a double layer of the peritoneum supports 

 the intestine; between its layers are the blood vessels, nerves and lymphatics that 

 supply the intestine. (Courtesy, Haggard: Science of Health and Disease. New 

 York, Harper & Bros., 1927.) 



stimulate further bile-making in the liver. As digestion in the intestine is 

 completed, the production of secretin and cholecystokinin is reduced. With 

 this reduction the sphincter muscle at the exit end of the bile duct tightens. 

 This prevents the escape of bile which once more fills the bile duct and backs 

 into the gall bladder. 



A gall bladder may be regularly present in one species of animal and 

 regularly absent in another nearly related one with a similar diet. It is lack- 

 ing in the white rat, horse, pocket gopher, and pigeon, but present in the 

 Norway rat, mouse, cow, striped gopher, chicken and duck, also in cats 

 and dogs. Experimental study of these animals has shown that the ones 

 without gall bladders have a relatively larger production of bile than the 

 others. The human gall bladder is often removed because of inflammation 

 and the formation of gallstones by accumulations largely of cholesterol, the 



