226 HUMAN BIOLOGY 



the problem met of delivering a continuous supply of this 

 important material to the active cells? 



The problem is more complex than at first appears. 

 Ordinarily the concentration of circulating glucose is lOO 

 miUigrams in lOO cubic centimeters of blood (commonly 

 expressed as lOO mg. per cent). If the concentration rises 

 to about 1 80 mg., the glucose will be lost by escape through 

 the kidneys; if it falls to about 45 mg., convulsions are 

 Hkely to occur, which may be followed by coma and death. 

 The sugar supply, consequently, must not only be con- 

 tinuous, but cannot vary beyond certain hmits without 

 danger of Joss or serious disturbance. 



The problem of constancy of sugar in the blood is solved 

 by storage. When food containing much starch and sugar 

 is eaten, the glucose which results from digestion is stored 

 locally in the muscles and for general use in the cells of the 

 liver, as glycogen. The general reserve in the liver can be 

 built up, however, only by collaboration of the pancreas. 

 In that organ are groups or "islands" of cells that elaborate 

 a peculiar substance which they discharge into the blood 

 stream as an internal secretion. When it is lacking, sugar is 

 not stored in the liver, is not normally used by the tissues, 

 and may accumulate in the blood until it has a concen- 

 tration there of 300 mg. per cent or higher. If under these 

 circumstances an extract of the island cells (insulin) is 

 injected into a vein, the blood sugar is promptly reduced 

 both by storage and by use. There is evidence that normally 

 an increase of blood sugar causes impulses to be discharged 

 through the vagus nerves to the island cells; they then secrete 

 natural insulin into the circulating blood, and this sub- 

 stance carried about the body induces both the building-up 

 of the glycogen reserves and the more efficient utilization 

 of glucose. 



The stored glucose is needed when extra combustion or 

 special muscular activity is demanded in the organism, for 

 example, on exposure to cold or in severe and prolonged 

 physical struggle. Under these circumstances the glycogen 

 in the liver cells is changed to soluble glucose, is discharged 

 into the blood, and is distributed to all parts of tbe body. 

 The mechanism by which glucose is thus increased in the 



