f>72 METABOLISM 



possible ways by which this could be done: (1) by means of a nervous 

 reflex, or (-2) by changes in the composition of the blood, either with 



regard to the percentage of sugar itself or because of the appearance in 

 it of decomposition products of glucose or of some special exciting 

 agent or hormone. 



In order to ascertain the relative importance of these methods of 

 correlation between the places of supply and demand of glucose in the 

 normal animal, it is necessary to investigate the conditions under which 

 an excessive discharge of glucose occurs cither because of overstimulation 

 of the nervous control, or because of the presence of exciting substances 

 (hormones) in the blood. The glycogenic function can be excited through 

 the nervous system in a variety of ways so as to cause hyperglycemia 

 and glycosuria. This constitutes one form of experimental diabetes. In 

 laboratory animals mechanical irritation of the medulla oblongata and 

 stimulation of the great splanchnic nerves act in this way. Similar stimula- 

 tion may also occur under certain conditions in man. Excitation as a result 

 of changes in the composition of the blood can be produced experimen- 

 tally by certain drugs (phlorhiziri), or by the removal of certain of the 

 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 Ave may draw from these results is that 

 the splanchnic nerve contains efferent fibers controlling the 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 aboul in the aeration of the 



