264 DIGESTION. 



after absorption, the carbon and hydrogen of the bilin com- 

 bining with oxygen, are excreted as carbonic acid and water. 

 The destination of the bile is, on this theory, essentially the 

 same in both foetal and extra-uterine life; only, in the former, 

 it is directly excreted, in the latter for the most part indirectly, 

 being, before final ejection, modified in its absorption from the 

 intestines, and mingled with the blood. 



The change from the direct to the indirect mode of excre- 

 tion of the bile may, with much probability, be connected with 

 a purpose in relation to the development of heat. The tem- 

 perature of the foetus is maintained by that of the parent, and 

 needs no source of heat within the body of the foetus itself; 

 but, in extra-uterine life, there is (as one may say) a waste of 

 material for heat when any excretion is discharged unoxid- 

 ized ; the carbon and hydrogen of the bilin, therefore, instead 

 of being ejected in the faeces, are reabsorbed, in order that 

 they may be combined with oxygen, and that in the combina- 

 tion, heat may be generated. 



From the peculiar manner in which the liver is supplied 

 with much of the blood that flows through it, it is probable, 

 as Dr. Budd suggest, that this organ is excretory, not only 

 for such hydro-carbonaceous matters as may need expulsion 

 from any portion of the blood, but that it serves for the direct 

 purification of the stream which, arriving by the portal vein, 

 has just gathered up various substances in its course through 

 the digestive organs substances which may need to be ex- 

 pelled, almost immediately after their absorption. For it is 

 easily conceivable that many things may be taken up during 

 digestion, which not only are unfit for purposes of nutrition, but 

 which would be positively injurious if allowed to mingle with 

 the general mass of the blood. The liver, therefore, may be 

 supposed placed in the only road by which such matters can 

 pass into the general current, jealously to guard against their 

 further progress, and turn them back again into an excretory 

 channel. The frequency with which metallic poisons are 

 either excreted by the liver or intercepted and retained, often 

 for a considerable time, in its own substance, may be adduced 

 as evidence for the probable truth of this supposition. 



Though one chief purpose of the secretion of bile may thus 

 appear to be the purification of the blood by ultimate excre- 

 tion, yet there are many reasons for believing that, while it is 

 in the intestines, it performs an important part in the process 

 of digestion. In nearly all animals, for example, the bile is 

 discharged, not through an excretory duct communicating 

 with the external surface or with a simple reservoir, as most 

 secretions are, but is made to pass into the intestinal canal, so 



