METABOLISM 525 



the hepatic plexus whereby arriving nerve impulses become more efficient. 

 These facts do not decide the question, however, as to the character of the 

 nerves involved, that is, whether they are purely vaso-motor or secretor 

 in character. The evidence for the existence of glycogenolytic nerves is 

 not decisive. 



The strictly physiologic factors that maintain and regulate the activity 

 of the cells in the medulla, and which in turn cause by way of the splanch- 

 nics a discharge of sugar from the liver, and in amounts just sufficient 

 to replace the sugar oxidized, are but imperfectly known. Whether they 

 are to be found in variations in the composition of the blood or in the play 

 of nerve impulses transmitted from special regions of the body cannot with 

 any degree of definiteness be stated. 



The glycogen of the muscles is released and converted by intracellular 

 enzymes into glucose during muscle activity, presumably when the supply 

 of sugar delivered by the blood stream is insufficient, whereupon it under- 

 goes oxidation and contributes to the production of heat and supplies 

 energy for muscle work. During rest the glycogen reaccumulates, by the 

 conversion of glucose arriving in the blood stream, and which has been 

 released by the liver. 



The Oxidation of Sugar.- As previously stated the final stage in the 

 metabolism of sugar is its oxidation to carbon dioxid and water with the 

 liberation of its contained energy, largely in the form of heat. This is 

 accomplished in the tissues generally, but to the greatest extent by far in 

 the muscle tissue. The intermediate stages are, however, but imper- 

 fectly known, though the first stage is believed to be lactic acid. The 

 successive oxidations due to the presence of enzymes in the muscle tissue 

 are embraced under the term glycolysis. The glycolytic enzymes, how- 

 ever, appear to be incapable in themselves of initiating the changes. For 

 if glucose is added to an expressed juice of muscle tissue and the mixture is 

 kept at a suitable temperature, oxidation does not take place, but if an 

 expressed juice of pancreas is added to the muscle tissue juice, oxidation of 

 sugar is promptly brought about. 



This experiment was based on the fact that destructive disease of the 

 pancreas prevented the oxidation of sugar. It was demonstrated ex- 

 perimentally that after removal of the pancreas and the survival of the 

 animal, glycosuria is promptly established followed by a series of symptoms 

 which gradually increase in severity and lead to the death of the animal in 

 from two to four weeks. In addition to the presence of sugar, acetone, 

 aceto-acetic, /3-oxybutyric acid, and an excess of urea have been found in 

 the urine. The quantity of sugar excreted and the gravity of the attendant 

 symptoms may be much diminished, if not entirely prevented by allowing 

 a portion of the gland to remain even though its capacity for the produc- 

 tion of pancreatic juice is entirely abolished. Transplantation of various 

 portions of the pancreas into the subcutaneous tissue, in the walls of the 

 abdomen, will also prevent the glycosuria. The explanations which have 

 been offered as to the manner in which the pancreas prevents and its 

 removal gives rise to the excretion of sugar are in part hypothetical. 



It has been claimed by many investigators that the pancreas secretes a 

 specific material partaking of the nature of a hormone which is discharged 

 into the blood and by it distributed to various tissues, especially the muscles 



