226 PHYSIOLOGICAL CHEMISTRY 



by certain glands and other tissues. Since these products are 

 delivered into the blood stream they are called internal secre- 

 tions. We already have considered the internal secretion of the 

 adrenals, adrenaline, in its effect upon the control of liver 

 glycogen, the internal secretion of the pancreas and its impor- 

 tance for the utilization of sugar by the muscles, the hormones 

 secreted by the stomach and intestinal walls, and their impor- 

 tance in inducing the flow of digestive juices. To this list many 

 more examples might be added. Thus the thyroid gland is 

 known to have a profound influence on general metabolism. If 

 the gland atrophies, or is removed in a young individual, 

 growth is retarded and dwarfism results. Mental development 

 also is checked. The condition is known as cretinism, and it may 

 be improved by feeding thyroid. In adults, removal or atrophy 

 of the gland results in thickening of the skin, and coarseness 

 of the hair, the temperature is below normal and general depres- 

 sion of metabolism is observed. If the thyroid is too active or 

 too large, goitre results, with a general quickening of metab- 

 olism and increased nervous irritability. These and other facts 

 are interpreted to mean that the thyroid pours out a substance 

 into the blood which has a profound effect on general metab- 

 olism, and influences the metabolism of the bones, the nerves 

 and the skin. This substance undoubtedly contains iodine. It 

 is being studied by Kendall. The parathyroids undoubtedly also 

 have important functions, but they are not so well understood. 

 Careful removal of these organs causes symptoms of poisoning, 

 but the cause is still obscure. 



In addition to the action of adrenaline above mentioned, this 

 secretion of the suprarenal glands increases vasomotor tone and 

 has other less understood functions. Even if adrenaline is 

 injected into an animal from which the suprarenals have been 

 removed, the animal dies, but the cause of its death is unknown. 



The sexual glands also markedly influence metabolism. Re- 

 moval of the testes in immature males, a process known as cas- 

 tration, results in the failure of the secondary sexual character- 



