770 PHYSIOLOGY 



The chief value of the large intestine in carnivora and in civilised man 

 would seem to be as an excretory organ, since it plays an important part in 

 the excretion of lime, magnesium, iron, and phosphates. Lime salts are 

 excreted partly with the faeces, partly in the urine. The path taken by the 

 lime under different conditions varies with the character of the other con- 

 stituents of the food. If phosphates are present in large quantities, the 

 greater part of the lime will be excreted by the large intestine and escape 

 with the faeces as insoluble calcium phosphate. If acids be administered, 

 such as hydrochloric acid, the amount of lime in the urine will increase, that 

 in the faeces will diminish. Thus in herbivora normally only about 3 to 6 

 per cent, of the lime is excreted with the urine, whereas in carnivora with 

 an acid urine the proportion leaving the body by this channel rises to 

 27 per cent. The excretion of magnesium is determined by very similar 

 conditions. Its phosphates are somewhat more soluble than those of lime. 

 In man about 50 per cent, of the magnesium leaving the body is contained in 

 the urine, whereas the amount of lime in the faeces is ten to twenty times 

 as much as that contained in the urine. It must be remembered that the 

 whole of this difference is not due to excretion of lime into the gut, since a 

 certain proportion of this substance may be precipitated as an insoluble 

 phosphate or carbonate in the upper part of the small intestine and pass 

 through the gut without undergoing absorption. 



The absorption of iron takes place in the duodenum and upper part of 

 the jejunum. Only 1 or 2 milligrammes appear in the urine, all the rest 

 being excreted in the large gut and appearing in the faeces, chiefly as sulphide 

 of iron. 



Of the acid radicals phosphates may pass out either with the urine or with 

 the faeces, the exact path taken being determined by the relative amount of 

 calcium and alkaline metals present in the food. If there 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 sub- 

 cutaneously, the faeces 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 *his part of the gut in cases of poisoning by mercury. 



