Chemical Activators The endocrine glands are stimulated to secrete 

 by nerve impulses, primarily by those from the autonomic nervous system. 

 They are also influenced by chemical changes in the blood and by hormones 

 from other glands. The hormones, the specific products of the endocrine 

 glands, have been called "chemical messengers". In their rapid distribution 

 they act like nerve impulses, which arouse action at points remote from a 

 stimulus: something happens here and sets offs some action there. 



Since the blood keeps the fluids of the body constantly stirred up, these 

 chemical messengers take part in all that happens in the body. They pro- 

 duce effects in all parts of the body, and events in the various organs con- 

 stantly influence the kinds and amounts of hormones secreted. Like the 

 nervous system, the endocrine system keeps all the parts in constant com- 

 munication. We consider the endocrine system as "older", or more primi- 

 tive, than the nervous system; for among simpler organisms that have no 

 nervous structures, the protoplasm is sensitive to chemical stimulation, and 

 it also responds to stimuli with chemical changes. Moreover, the hormones 

 operate in higher vertebrates, like ourselves, for example, without producing 

 sensations and without seeming to stir the "newer" parts of the nervous 

 system to consciousness. 



What Do Hormones Do? 



Temporary Service The pineal gland and the thymus (chest sweet- 

 bread at the butcher's) seem in all mammals to be active only during the 

 early period of life. That is, the structures normally shrink away before 

 sexual maturity is reached. The relation of the pineal structure to life proc- 

 esses is still very uncertain; but when the organ is injured, the results suggest 

 a specific hormone which influences sexual development. 



Hormones and Growth Some hormones accelerate growth, either of 

 the whole organism or of special parts. When the thymus, for example, is 

 injured or removed, as through disease, the organism is stunted. When it is 

 overactive, the body grows very rapidly. In certain experiments J. F. Guder- 

 natsch, of the Cornell medical school, fed some tadpoles on thymus glands 

 obtained from calves, and others on thyroid material. The first lot of tad- 

 poles grew to a large size, but remained tadpoles. The second lot, however, 

 quickly passed through the stages of development without increasing much 

 in size (see illustration opposite). 



In human beings the thymus reaches its greatest relative size during the 

 second year. Where, for any reason, the thymus persists past puberty, the 

 person remains childish in many ways; that is, he fails to mature physically, 

 intellectually and emotionally. 



One of the several hormones found in the front lobe of the pituitary also 



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