Modern Manual of Endocrinology

Endocrine or ductless glands secrete hormones into the extra cellular space surrounding them. These hormones enter the circulatory system and are distributed to the target sites. 'Endocrinology' is the study of the ductless glands or tissues and their hormonal products. Hormones are considered to be synthesized within specific endocrine organs and then secreted into the blood stream to act on specific target sites some distance away to evoke a physiological response.