5 i6 TEXT-BOOK OF PHYSIOLOGY 



de-hydra tion, of the sugar of the blood. By reason of this fact it may be said 

 that the muscle also possesses a starch-forming or a glycogenic or an amylo- 

 genetic function. If it is a fact that of the sugar absorbed only from 12 to 

 20 per cent, is temporarily arrested by the liver, the remainder passing on 

 into the blood of the general circulation, it is readily conceivable that the 

 storage of the sugar under the form of glycogen by the muscle-cells is neces- 

 sary not only for the activity of the muscle itself, but as a means of preventing 

 an abnormal percentage of sugar in the circulating blood. It is generally 

 admitted that though the glycogen is the source of the energy expended by 

 the muscle, it cannot be disrupted and oxidized as such, but that it must 

 first be transformed into sugar (glucose); and for this purpose the assump- 

 tion is made that a special enzyme is present and active. The muscle is 

 therefore said to possess or exhibit a sugar-forming or a glycogenetic func- 

 tion. The muscle-cells are thus like the liver cells characterized by the two 

 processes amylogenesis and glycogenesis. During the periods of prolonged 

 activity of the muscles the percentage of glycogen rapidly diminishes, a fact 

 that leads to the inference that it is the source in large part of the energy 

 expended by the muscle. During the period of rest the percentage oi 

 glycogen rapidly increases until the normal is regained. 



The Influence of the Nerve System. The physiologic mechanism by 

 which glycogen is changed to glucose (glycogenolysis) in amounts just suffi- 

 cient to supply the needs of the tissues without perceptibly increasing the 

 amount of sugar in the blood is obscure and but imperfectly known. Thai 

 the nerve system is in some way concerned in the regulation of glycogenolysis 

 is apparent from observation of the effects of injuries, major emotional states 

 as well as of the results of experimental procedures, but whether the regula- 

 tion is direct, i.e., through its action on the liver-cells, or indirect, i.e., through 

 its action on the glands of internal secretion, e.g., adrenals and hypophysis, 

 is a subject of investigation and discussion. It was discovered 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 tc 

 six hours in the rabbit and from two to three or even seven in the dog. Foi 

 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 (glycogenolysis). This supposition receives 

 support from the fact that the degree of the hyperglycemia, and the subse- 

 quent glycosuria, will depend on the amount of glycogen previously in the 

 liver. If the animal has been well fed on carbohydrates, the resulting gly- 

 cosuria will be pronounced; 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, influ- 

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

 the puncture destroys the nerve-cells, or whether it stimulates them in some 

 way to increased activity. The results of experiment lead to the latter sup- 

 position. Thus stimulation of sensor nerves in different regions of the body, 

 injuries to the central nerve system, major emotional states, etc., which have 



