Discrete organs, such as the pituitary and the thyroid, are constituents of a large and intrinsic system of ductless glands, commonly referred to as endocrine glands. However, several non-discrete organs also possess endocrine tissues and function like endocrine glands, and can therefore be referred to as endocrine glands. Collectively, these two groups of discrete and non-discrete organs make up the endocrine system. Endocrine tissue is generally represented by aggregates of secretory epithelial cells that deliver their hormones directly into the blood stream, bypassing any sort of exocrine characterized, ductile framework. The true, discrete organs of the endocrine system include the following glands: The hypothalamus, pituitaries, pineal, thyroid and para-thyroid and the adrenals. The non-discrete endocrine organs, those which possess endocrine tissues, include the following: The gastrointestinal and respiratory tracts, pancreas, heart, kidneys, testes, ovaries, placenta, and the thymus. The pineal gland is located deep within the brain, thus can be considered discrete or non-discrete. Additionally, and for practical reasons, several of the non-discrete organs with endocrine tissues are simply referred to as endocrine organs, e.g. the endocrine pancreas, or the endocrine heart. The endocrine glands release of hormones, which is finely regulated and coordinated, provides homeostatic stability throughout the body. This stabilization, in turn, ultimately enhances the numerous mammalian functions that must occur every day of our lives. The epithelial cells of endocrine tissues are highly active; generally, these cells present prominently stained nuclei and cytoplasmic organelles. The mitochondria, Golgi bodies, secretory granules and endoplasmic reticulum are the main contributors to the cells prominent cytoplasmic staining character. Two general types of hormones are produced and secreted from endocrine glands, amino acids and steroids. The amino acid derivatives include peptides and proteins, which are both polar, water soluble molecules, whereas the steroid hormones are primarily cholesterol based, making them lipid soluble. In turn, proteins and peptides must employ a second messenger system to initiate a response whereas steroids don’t. Steroid hormones pass through the plasma membrane relatively easy, thus eliminating the need for a second messenger intracellular activator, such as cAMP (cyclic adenine monophosphate), Ca2+ or cGMP (cyclic guanosine monophosphate). Because of this, steroid hormones generate an effect which is slower acting, longer lasting and less cellular specific than peptide or protein based hormones. As previously mentioned, amino acid derivatives require specific membrane receptors in order to bind; this in turn activates the second messenger and eventually initiates a response. The effects generated by protein/peptide hormones occurs much quicker, is shorter lived and more tissue specific than those of steroidal bases. Other types of hormones use neurotransmitters to deliver their message or initiate a response. This mechanism is confined to the cells of the nervous system and preceded by an initial stimulation, or neurotransmitter release, from the targeted tissue via its representative hormone. In either of the cases described above, a general rule exists; that is, in order for stimulation to occur, the hormone must bind to its specific receptor or target structure. Notably, blocking a specific receptor decreases or terminates that hormones particular effect. This knowledge of the endocrine system is the basis for numerous pharmaceuticals, particularly those carried out via the autonomic nervous system. The remainder of this document will describe and illustrate the discrete endocrine organs of the endocrine system. Specifically, the anatomy, histology and function (hormone activity) of each discrete endocrine organ will be addressed. Though, particular emphasis will be directed toward the histological aspects of the system. The hypothalamus, located in the forebrain between the pituitary and the thalamus, represents the endocrine systems analogous control center. Histologically, the hypothalamus is best represented in conjunction with the pituitaries, and therefore, the two will be complimentarily addressed. The pituitary gland, also referred to as the hypophysis, can be found just beneath the hypothalamus, as hinted previously. The hypothalamus and the pituitaries are both composed of nervous tissue and divided into anterior, intermediate and posterior regions. In reference to the pituitary, these regions correspond to the pars distalis, pars intermedia and the pars nervosa, respectively. In the hypothalamus, these areas correspond to the supraoptic, tuberal and mammillary regions, respectively. The anterior pituitary (pars distalis) and the posterior pituitary (pars nervosa) are divided by the Vestigium of Rathke’s po