450 TEXT-BOOK OF PHYSIOLOGY. 



In opposition to this view, Dr. Pavy, after years of accurate experimenta- 

 tion, states that the blood on the cardiac side of the liver never under nor- 

 mal circumstances contains a larger percentage of sugar than is to be found in 

 any part of the circulation, except in the portal vein. He states that glycogen 

 is never reconverted into sugar, and denies that the liver produces sugar, to 

 be discharged into the blood; the function of the liver is merely to arrest 

 the passage of sugar, and so to shield the general circulation from an 

 excess; the sugar which arises in the liver after death is a post-mortem 

 product and not an illustration of what takes place during life. Dr. Pavy, 

 having apparently demonstrated the glucosid constitution of protein mate- 

 rial in general, accounts for the presence of glycogen in muscles and other 

 tissues on the assumption that during the cleavage of the protein molecule 

 the carbohydrate element is set free and temporarily stored as glycogen. 

 He thus accounts for the production of sugar in the body, even in the absence 

 of all sugar and starch from the food. Pavy believes that the glycogen pro- 

 duced in the liver is utilized in the formation of fat and the synthesis of 

 complex proteins necessary to the construction of the tissues. 



The Influence of the Nerve System. The results of various experi- 

 mental investigations indicate that the production of sugar from the glycogen 

 in the liver is influenced by the activities of the nerve system. It was dis- 

 covered by Bernard that puncture of the floor of the fourth ventricle, at a 

 point between the acoustic and vagus nerves, near the middle line, is followed 

 within an hour or two by the appearance of sugar in the urine, which lasts 

 for from five to six hours in the rabbit and from two to three or even seven in 

 the dog. For this reason Bernard gave to this area the name of "diabetic 

 area." 



Coincident with the appearance of sugar in the urine (glycosuria) there 

 is an increase in the percentage of sugar in the blood (hyperglycemia) . The 

 liver at the same time contains a higher percentage of sugar than normally. 

 Apparently the initial step in this series of phenomena is an increased con- 

 version of glycogen into sugar. This supposition receives support from the 

 fact that the degree of the hyperglycemia, and the subsequent glycosuria, 

 will depend on the amount of glycogen previously in the liver. If the animal 

 has been well fed on carbohydrates, the resulting glycosuria will be pro- 

 nounced; if, on the contrary, it has been allowed to fast for several days, 

 the glycosuria will be slight. 



Assuming that the nerve-cells which constitute the diabetic area influence 

 the conversion of glycogen into sugar, the question arises as to whether the 

 puncture destroys the nerve-cells, or whether it stimulates them to increased 

 activity. The results of experiment lead to the latter supposition. Thus 

 if the vagus nerve is divided in the neck and its central end stimulated there 

 is developed a glycosuria. Stimulation of other sensor nerves has a similar 

 effect. As stimulation of the vagus has the same effect as the puncture, the 

 inference is that the center is normally excited to physiologic activity by 

 impulses reflected from some surface or organ in the peripheral distribution 

 of this nerve. 



If the nerve-cells in the diabetic area regulate the production of sugar in 

 the liver, the further question arises as to the pathway through which the 

 nerve impulses emanating from them reach the liver, whether by way of the 



