816 PHYSIOLOGY 



is an excess of calcium most of the phosphates will leave with the 

 faeces. 



The large intestine is the main channel of excretion for certain 

 substances which cannot be regarded as normal constituents of the 

 food, e.g. the heavy metals, such as bismuth and mercury. If 

 bismuth be administered subcutaneously the fseces will be found to 

 contain this substance, and the wall of the large intestine will be 

 stained black from a deposit of sulphide of bismuth. This deposit 

 stops short at the ileocolic valve. The excretion of mercury by the 

 wall of the large intestine may account for the frequent presence of 

 ulceration of this part of the gut in cases of poisoning by mercury. 



