The subject of endocrinology is more difficult to define in a meaningful way. The concept has becoming been accepted for many years that an endocrine gland produces a chemical substance (hormone that is carried by the circulation to a target organ at some considerable distance from the original endocrine gland. As an example of some difficulty of classification under this definition, the adrenal medulla produces epinephrine, which is camied by the blood to distant target organs. 
Sympathetic nerves, however, produce a similar substance, that e immediate vicinity of the nerve to classify onesubstance as a hormone and not the other, because they have so many similarities, and yet one fits the definition and the other does not. On the other hand, if the definition is broadened to include all chemically integrative products, many substances such as carbon dioxide, that are not usually considered to be hormones would fit the definition. So, endocrinology is defined as the science dealing with the chemical integration or the so-called chemical control of the organism. It is a sow system as compared to nervous integration or nervous control.

The endocrine and peripheral nervous system are the two parts of the same coordinating and integrating (administrating) mechanism operating through the central nervous system. Response by the nervous system is rather localized and more rapid through the faster neural impulses and by the release of a local transmitter (neurohumor). Endocrine system, on the contrary, responds through the release of chemicals (hormones directly in the circulation, the influence of which is rather prolonged and may be on distinct target organs.

Previously, the informal secretions of ductless glands were known as the autocoids. Those that stimulate were called hormones and those that depress or inhibit the activities chalones. But since the same chemical secretion many show opposite effects in two places, the term hormone is used for all.

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