704 METABOLISM 



ductless glands or the injection of extracts prepared from them, such 

 as epinephrine. 



Nerve Control and the Nervous Forms of Experimental Diabetes. 



The simplest experimental condition which illustrates the relationship 

 between the nervous system and the blood sugar is electrical stimulation 

 of the great splanchnic nerve in animals in which, by previous feeding 

 with carbohydrates, a large amount of glycogen has been deposited in 

 the liver. By examination of the blood as it is discharged into the vena 

 cava from the hepatic veins, the increase in blood sugar is very evident 

 in from five to ten minutes after the first application of the stimulus; 

 but it is not until later that a general hyperglycemia becomes estab- 

 lished. The conclusion which we may draw from these results is that 

 the splanchnic nerve contains efferent fibers controlling the s rate at 

 which glycogen becomes converted to glucose in the liver. The center 

 from which these fibers originate is situated somewhere in the medulla 

 oblongata, for the irritation that is set up by puncturing this portion of 

 the nervous system with a needle yields results similar to those which 

 follow splanchnic stimulation. This "glycogenic" or diabetic center, as 

 it has been called, must be provided with afferent impulses. Such im- 

 pulses have indeed been described in the vagus nerves, but their dem- 

 onstration is by no means an easy matter on account of the disturbance 

 in the respiratory movements coincidently produced by the stimulation. 

 The changes that such disturbances bring about in the aeration of the 

 blood may in themselves be responsible for the hyperglycemia (see page 

 348). It can at least be said that when the respiratory disturbances are 

 guarded against, as by intratracheal insufflation of oxygen, vagal hyper- 

 glycemia is much less marked, if not entirely absent. But this question 

 awaits more thorough investigation. 



The increased glycogenolysis which results from stimulation of the 

 efferent fibers in the splanchnic nerves may depend either on^a direct 

 control exercised over the glycogenic functions of the hepatic cells, or 

 on the discharge into the blood of some hormone which excites the 

 glycogenolytic process. It must furthermore not be lost sight of that 

 the glycogenolysis may be secondary to local asphyxial conditions in 

 the liver cells resulting from vasoconstriction. From their anatomic 

 position, the adrenals are to be thought of as the source of the hormone, 

 and evidence that splanchnic hyperglycemia is due to hypersecretion 

 from these glands has seemed to be furnished by the fact that after they 

 are extirpated splanchnic stimulation no longer produces hyperglycemia, 

 neither, indeed, does puncture of the medulla. There is also no doubt 

 that the nervous system, acting by way of the splanchnic nerves, does 

 exercise a control over the discharge of the internal secretion of the 



