72G USES OF GLYCOGEN. [BOOK n. 



by older and more recent observers that the blood of the hepatic 

 vein under normal conditions is richer in sugar than the blood of 

 the portal vein or indeed of any other part of the vascular system ; 

 and this has been regarded as an indication that the liver is 

 always engaged in discharging a certain quantity of sugar into 

 the hepatic veins. On the other hand others maintain that the 

 blood in the hepatic vein, if care be taken to keep the animal in a 

 perfectly normal condition, contains no more sugar than does the 

 blood of the right auricle or of the portal vein, and indeed that 

 the liver itself, if examined before any post-mortem changes 

 have had time to develope themselves, is absolutely free from 

 sugar. 



Normal hepatic blood may be obtained by means of an ingenious 

 catheterisation. This consists in introducing through the jugular vein, 

 into the superior, and so into the inferior vena cava, a long catheter, 

 constructed in sucli a manner that the vena cava can at pleasure be 

 plugged below the embouchement of the hepatic veins, and blood so 

 drawn exclusively from the latter; or vice versa. 



Now the quantitative determination of sugar in blood by any 

 of the methods as yet suggested is open to many sources of error. 

 And when the quantity of blood which is continually Howing 

 through the liver is taken under consideration, it, is obvious that 

 an amount of sugar, which in the specimen of blood taken for 

 examination fell within the limits of error of observation, might 

 when multiplied by the whole quantity of blood, and by the 

 number of times the blood passed through the liver in a certain 

 time, reach dimensions quite sufficient to account for the conversion 

 into sugar of the whole of the glycogen present in the liver at any 

 given time. Hence we may safely conclude that the comparative 

 analysis of hepatic and portal blood, if they do not of themselves 

 prove that the liver is either continually or at intervals converting 

 some of its glycogen into sugar and discharging this sugar into the 

 general system, are at least not sufficiently trustworthy to disprove 

 the possibility of such a discharge of sugar being one of the normal 

 functions of the liver. Indeed it may be doubted whether any 

 great trust can be laid on experiments of this kind. We may add 

 that similar experiments have led one observer to deny wholly the 

 connection between the sugar which may be found in the hepatic 

 vein and the glycogen of the hepatic cells. 



Refusing then to admit the validity of these experiments we 

 may regard the view that glycogen is simply a stage in the 

 formation of fat as not proved ; and indeed we shall presently see 

 reason to believe that fat is formed elsewhere. 



Another view, one which has already been suggested while we 

 were dealing with the manner of formation of glycogen, makes use 

 of the formation of fat for the purposes of analogy only. Seeing 

 that adipose tissue serves as a storehouse of fat which is not 



