ISOTOPES — ATEN AND HEYN 227 



tion here an investigation into the rate of absorption of insulin which 

 is injected periodically under the skin of sufferers from diabetes. It is 

 often desirable to restrict the number of injections and it is therefore 

 favorable if the insulin is retained for a relatively long time near the 

 point of injection, or stored there as it were, and only slowly taken 

 up in the circulation. By building a radioactive atom (radioactive 

 iodine) into the molecule of three kinds of insulin, viz., "ordinary" 

 insulin, globine insulin, and protamine-zinc insulin, and measuring 

 from time to time the decrease of radioactivity at the point of injection, 

 it has been possible to determine that the rate of absorption of the 

 three kinds of insulin in the body decreases in the order given above. 



Exchange processes. — Although also in chemistry and technology 

 numerous processes play a part where an exchange of identical particles 

 occurs — we mention only autodiffusion, e. g., the diffusion of lead 

 atoms in lead — physiology is the most prominent field for such ex- 

 changes. One of the most striking examples is the continuous ex- 

 change of the building materials of the body. This has been studied 

 in detail with phosphorus in the form of various compounds, with the 

 help of radioactive phosphorus, which lends itself so well for such 

 experiments. Particularly Hevesy has done a great deal of these in- 

 vestigations. It has been established that phosphorus does not remain 

 permanently bound in any constituent of the body. The exchange 

 takes place most rapidly between the blood and various organs: of 

 the phosphate ions present in the blood at a given moment after 2 hours, 

 only 2 percent are still present, the rest having been exchanged. In 

 the liver and kidneys, too, there is a rapid renewal, but also in the bones 

 and even in the brain the phosphorus is in course of time renewed, 

 though at a much slower rate. The parts of the body that take least 

 part in the continual exchange are the teeth: after 250 days only 1 

 percent of the pliosphorus in the dental enamel is renewed. 



In a certain case it is not so much a matter of exchange as one of 

 selective assimilation of substances by certain constituents of the body, 

 namely where the exchange leads, as it were, to a credit balance for 

 that part of the body. A striking example is the assimilation of iodine 

 by the thyroid gland. With the help of a radioactive iodine isotope it 

 has been determined that, out of an extra amount of iodine admin- 

 istered in the food, after 1 or 2 days a healthy person has stored up in 

 the thyroid gland about 3 percent, whereas a sufferer from goitre stores 

 up 30 percent or more. (See fig. 1.) In certain cases of cancer of the 

 thyroid gland the radioactive iodine was found to be accumulated not 

 in the cancer tissue but in the healthy tissue. (Thus we again arrive 

 at the problems referred to under "localization.") Somewhat similar 

 phenomena are found in the case of the assimilation of radioactive 

 strontium in the blood and in the bones. It has been possible not only 



