THE LIVER AND PANCREAS 261 



the effect of stimulating this nerve is to cause a fall in abdominal 

 blood-pressure. The impulses passing to the diabetic centre 

 may, it has been suggested, originate in the contracting skeletal 

 muscles by the compression of the muscle spindles. The heart 

 muscle contains more glycogen than skeletal muscle ; when the 

 amount of glycogen in the latter has fallen to one-tenth or even 

 one-thirtieth of the normal the heart muscle still maintains its 

 due proportion. The fibres from the heart to the diabetic centre 

 are conveyed in the vagus, and it is easy to see that on this theory 

 the heart, which is the most active muscle in the body, may 

 regulate the production of the material which furnishes it with 

 energy. 



In distinction to the above, which may be termed the ' nervous 

 theory of sugar liberation,' we have another, the chemical, based 

 upon the knowledge which exists of the glycosuria, which results 

 from depriving an animal of its pancreas. In some way or other 

 which is not known, the pancreatic tissue is intimately mixed up 

 with the sugar question, and this can be explained on the sup- 

 position of an internal secretion, which prevents the blood from 

 becoming overloaded with sugar, either by regulating the amount 

 which is liberated from the liver, or by stimulating the sugar- 

 splitting action of the tissue-cells. It would on this basis be 

 reasonable to suppose that the pancreatic extract should yield 

 a glycolytic substance, but no such has been found. That the 

 visible pancreatic secretion takes no share in the process is evident 

 from what has been previously stated — viz., that if only a frag- 

 ment of pancreas is left behind in the body of a depancreated 

 animal no glycosuria results. 



An extract of pancreas, it has been stated above, has no 

 glycolytic power ; further, an extract of muscle has no sugar- 

 destroying power, but if the extracts be mixed glycolysis results. 

 From this it has been supposed that the pancreatic extract 

 activates a sugar-destroying enzyme present in muscle, which 

 enables the latter, under physiological conditions, to oxidise 

 sugar in the body, and obtain from it heat and energy. The 

 activating substance furnished by the pancreatic extract is not 

 an enzyme, for if the extract be boiled the substance is not 

 destroyed ; it is therefore assumed that the internal secretion of 

 the pancreas is a hormone (see p. 230). There are others who 

 consider that the function of the internal secretion is not to 

 furnish the tissues with the power to metabolise sugar, but rather 

 that it regulates its output from the liver, and that when this 

 regulation fails diabetes results, owing to the excess of sugar in 

 the blood. The balance of evidence, however, suggests that 

 the difficulty lies with the tissues — especially the muscular — 

 being unable to oxidise the sugar brought to them. 



