430 A TEXTBOOK OF PHYSIOLOGY 



condition which results from the spasm of respiration induced by the 

 vagus stimulation. The chief efferent fibres ultimately reach the 

 splanchnic nerve. When the splanchnics are cut, puncture of the 

 medulla induces no glycosuria. Stimulation of the uncut splanchnic 

 nerves brings about glycosuria (Fig. 217). 



To sum up, in the medulla is situated a centre which regulates 

 the glycogen metabolism of the liver. The vagus nerve possibly 

 contains afferent fibre:* to this centre; the splanchnic nerve conveys 

 efferent fibres from it to the liver. It is possible that this centre is 

 directly affected by the sugar content of the blood supplied to it, 

 and that some drugs which induce glycosuria act directly upon the 

 centre, exciting in much the same manner as a puncture does. 



(2) The Chemical Influence. Adrenalin plays a part in the regula- 

 tion of the glycogenic function. It has been known for a considerable 

 \irne that injection of adrenalin into the blood-stream causes glyco- 

 suria. This is now attributed to the action of adrenalin upon the 

 endings of the splanchnic nerves in the liver. Adrenalin has the 

 property of stimulating in the body all the functions which the sympa- 

 thetic nerves excite. It acts upon the so-called " receptive sub- 

 stance," which occurs between the sympathetic nerve fibre and the 

 effector tissue. 



We may provisionally conclude that the liver performs its glycogenic 

 function (1) under the influence of nervous impulses brought from 

 the glycogenic centre by the splanchnic nerves ; (2) under the influence 

 of adrenalin brought by the blood. 



By some authorities it is held that the internal secretion of the 

 pancreas also plays a part in regulating the glycogenic function of 

 the liver. The evidence on this point is contradictory in nature. 



The Influences under which Sugar is Broken Down. Various 

 agencies have been or are held to influence the splitting of dextrose 

 in the body. At one time, considerable weight was attached to the 

 supposed presence of a glycolytic ferment in the blood. Recent 

 evidence indicates that the presence of such an enzyme in the blood 

 is more than doubtful. At the present time, great importance is 

 given by many authorities to the action of an internal secretion from 

 the pancreas. This internal secretion is believed to be derived from the 

 " islets of Langerhans " in the pancreas. Circulating in the blood, it 

 enables the tissues, particularly the muscles, to break down dextrose. 

 The tissue fluid obtained from a mixture of pancreas and muscle can 

 break down dextrose; extract of pancreas by itself has no such 

 action; that of muscle but slight action on dextrose. The active 

 substance obtained from the pancreas is not an enzyme. It is 

 soluble in water and alcohol, not destroyed by heat, and is believed 

 to activate the zymogen of a glycolytic enzyme present in muscle, or 

 possibly in the leucocytes. 



The above statement has not been accepted by all workers. 



Pancreatic glycosuria is discussed more fully elsewhere. From 

 the evidence, we tentatively conclude that in the normal body an 

 internal secretion derived from the pancreas plays a part in the 



