162 THE BODY AT WORK 



it is in excess, and passes out to the blood when it is deficient, 

 and an excretory organ which eliminates the refuse of hsemo- 

 globin. The iron derived from haemoglobin it stores, and 

 returns to the blood. 



Another function of the liver has been referred to already. 

 It is the organ, and, as far as we know, the only organ, in 

 which urea is made in mammals, and uric acid in birds. If 

 the liver of a freshly killed animal be excised and a stream 

 of blood passed through it, the blood which leaves the organ 

 contains urea. If a salt of ammonia, even the carbonate, 

 be added to the blood, it is converted by the liver into urea. 

 When a bird's liver is made the subject of the same experi- 

 ment, uric acid appears instead of urea. The liver can convert 

 many nitrogenous substances into urea, but it seems probable 

 that, normally, the salt with which it has chiefly to deal is 

 lactate of ammonia (c/. p. 146). 



A few words must be added with regard to the functions of 

 the liver during prenatal life, obscure though these functions 

 are. The liver develops very early, and attains a relatively 

 enormous size. At the third month it weighs as much as the 

 whole of the rest of the body (c/. p. 34). Yet it cannot, one must 

 suppose, have to do much of the work which falls to its share 

 in postnatal life. Food is reaching the embryo in a constant 

 stream, and not as the result of intermittent meals. The 

 embryo has no need to store glycogen ; nor does its liver, on 

 analysis, yield much of this substance. In the embryo glyco- 

 gen is widely distributed throughout the tissues, not specially 

 accumulated in the liver. No digestion is occurring in the 

 alimentary canal. Bile is not needed to aid the hydrolysis 

 and absorption of fats. A small quantity of cholesterin and 

 less lecithin is being eliminated, but not much bile is needed to 

 facilitate this process. 



A process which is proceeding at a great rate in the embryo, 

 in various situations, is the formation of red blood-corpuscles. 

 In this the liver takes part. But its duty in regard to blood- 

 formation is not sufficiently onerous to account for its size. 

 The formation of blood-corpuscles in the liver is observed with 

 difficulty in microscopic sections. It is therefore impossible to 

 speak with certainty as to the extent to which it is going on, 

 but it may be safely asserted that this function by itself cannot 



