FORMATION OF GLYCOGEN IN THE LIVER. 411 



subsequent transformation into what is called liver-sugar, it is not perfectly 

 adapted to the purposes of nutrition. In many cases of diabetes, a possible 

 explanation of the glycosuria is that the carbohydrates pass unchanged into 

 the vena cava and do not undergo the changes which take place normally 

 in the liver, at the same time being received into the general circulation sud- 

 denly and in large quantity, instead of gradually, as when they are changed 

 into glycogen and afterward into liver-sugar. When an excess of sugar finds 

 its way into the blood, it is probable that the liver, under normal conditions, 

 retains it for a time in the form of glycogen. 



The sugar which is discharged into the venous system by the hepatic 

 veins is usually lost in the passage of the blood through the lungs. The ques- 

 tion of the final destination of sugar will be taken up again in connection 

 with the physiology of nutrition. 



Conditions which influence the Quantity of Sugar in the Blood. It is 

 probable that disturbances of the circulation in the liver are the most impor- 

 tant conditions influencing the discharge of sugar by the 

 hepatic veins, and these operate mainly through the nervous 

 system. 



The most remarkable experiment upon the influence of 

 the nervous system on the liver is the one in which artificial 

 diabetes is produced by irritation of the floor of the fourth 

 ventricle (Bernard). This operation is not difficult. The 

 instrument used is a delicate stilet, with a flat, cutting ex- 

 tremity, and a small, projecting point about ^ of an inch 

 (1 mm.) long. In performing the operation upon a rabbit, 

 the head of the animal is firmly held in the left hand, and 

 .the skull is penetrated in the median line, just behind the 

 superior occipital protuberance. This can easily be done by 

 a few lateral movements of the instrument. Once within 

 the cranium, the instrument is passed obliquely downward 

 and forward, so as to cross an imaginary line drawn be- 

 tween the two auditory canals, until its point reaches the 

 basilar process of the occipital bone. The point then pene- 

 trates the medulla oblongata, between the roots of the audi- 

 tory nerves and the pneumogastrics, and by its projection 

 it serves to protect the nervous centre from more serious 

 injury from the cutting edge. The instrument is then care- 

 fully withdrawn and the operation is completed. This ex- 

 periment is almost painless, and it is not desirable to ad- 

 minister an anaesthetic, as this, in itself, would disturb the 

 glycogenic process. The urine may be drawn before the op- Fio 137 

 eration, by pressing the lower part of the abdomen, taking care 



not to allow the bladder to pass up above the point of press- fourth ventricle 

 ure, and it will be found turbid, alkaline and without sugar. 

 In one or two hours after the operation, the urine will have become clear and 

 acid, and it will react readily with any of the copper-tests. When this opera- 



