doing. The nerves, reaching all parts of the body, are sensitive to changes 

 and in turn bring about changes. Again, the blood, reaching all parts of the 

 body, is altered chemically by slight changes in any set of organs, and so 

 brings about important changes in the activity of protoplasm in all parts 

 of the body. In this way emotions influence our thinking, our actions, and 

 the behavior of the internal organs. On the other hand, both our thinking 

 and the action of the skeletal muscles can modify our emotions. 



In Brief 



Since the time of Hippocrates, people have associated temperament and 

 illness with the fluids, or "humors", of the body. 



In the higher animals the rate at which the chemical processes in the 

 living protoplasm go on is influenced by the amounts or proportions of 

 certain specific substances in the body fluids. 



The ductless glands are special organs that produce and discharge spe- 

 cific substances directly into the blood. Distinct from one another, they are 

 closely related in a system of interactions. 



Everything that modifies the normal action of any of the internal organs 

 at once brings about an increase or decrease in the secretion of one or more 

 of the ductless glands. 



The internal secretions of the various ductless glands, called hormones or 

 endocrines, are rapidly distributed by the blood and act in amazingly small 

 quantities to stimulate action in various organs, including other ductless 

 glands. 



The endocrines of all the mammals are very similar, so that it is possible 

 to use animal extracts in making up human deficiencies. 



Some of the endocrine glands act throughout life, others for only a rela- 

 tively short period; some produce but a single known hormone, others pro- 

 duce several hormones; some of the hormones secreted have but a single 

 known effect, others have multiple effects. 



Hormones modify the basic protoplasmic activities: some affect growth 

 and development, some sensitiveness to external conditions, some the use 

 of energy in movement or other activities. 



In higher animals, emotions seem to accompany the processes that have 

 to do with preserving the organism or the species. 



Whatever happens to the emotions influences the whole body, probably 

 through the chemical effects of the substances from the ductless glands; the 

 experiences and activities of the whole body in turn modify the ductless 

 glands and the emotions. 



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