238 The Vertebrate Organ Systems 



enzymes; seemingly insulin's role may be that of modifying the en- 

 zyme systems. This, however, remains as a subject for further specula- 

 tion and experimentation. 



The Composition of Insulin. — The first usable extracts of the 

 islets of Langerhans were prepared in 1922. An enzyme-free extract 

 was prepared by utilizing the pancreases of embryonic animals. In 

 the embryo, the islets develop before the enzyme-secreting portion, 

 making this process possible. At present, insulin is produced chiefly 

 from the pancreases of cattle, although it can be extracted from those 

 of numerous other animals. The empirical formula of insulin is 

 CisHflBOuNuS. By extensive testing, it has been shown that insulin 

 is an extremely complex molecule, apparently composed of amino acids. 



The Effects of Hypoactivity. — Diabetes mellitus, the disease re- 

 sulting from hypoactivity of the islets of Langerhans, has been re- 

 corded in history since the time of the Romans. The actual cause of 

 this disease was not known until 1889, however. This knowledge was 

 a fortuitous accident which occurred during the study by two workers 

 on the efifect of removing the pancreases from dogs. The man who 

 was in charge of the kennels where these dogs were quartered noted 

 that flies gathered around the voided urine. Study of this urine re- 

 vealed that there was sugar present. The urine of normal animals 

 shows no detectable amounts of sugar ; thus the scientists soon realized 

 that they had finally discovered a clue as to the cause of the well- 

 known diseaese, diabetes mellitus. 



In an individual having a defective functioning of the islets, sugars 

 canijot be stored in the normal manner, and the liver glycogen stores 

 tend to become depleted. These sugars are lost through the urine. 

 Normally, sugar is not lost in this manner, as the kidney has certain 

 thresholds below which materials are retained in the blood. If the 

 concentration of a substance becomes too high, it is excreted ; thus with 

 the rise in the level of the blood sugar, the kidney excretes the excess. 



As the carbohydrates are lost so rapidly through the urine, the 

 stores of fats and proteins are called upon as sources of energy. Nor- 

 mally, fats are metabolized into fatty acids and glycerol, but in the 

 case of extreme insulin lack, these are not properly metabolized. In- 

 stead, ketone or acetone bodies become abundant and may even be 

 detected on the breath of such an individual. In extreme cases, the 

 bases of the body are depleted and acidosis results. At this point, the 



