Often called the ductless glands, the nine distinct glands of the human endocrine system secrete hormones or "chemical messengers" directly into the blood stream, without the ducts found in the exocrine system. The endocrine system also includes numerous specialized tissues, including stomach cells that secrete hormones. The hormones, secreted in minute amounts and controlled by feedback mechanisms, travel throughout the body to target organs, often remote from the secreting gland. They cause no changes in tissues they pass through but cause significant changes in the target organs that have receptors for the hormones. Working in concert with the nervous system, the hormones released by the endocrine system regulate growth, ovulation, milk production, sexual development, and many other physiological and behavioral functions. Thus, the endocrine system is the body's chemical coordinating system, initiating and deactivating bodily changes as needed. The hormones secreted by each gland or tissue...