Endocrine glands are glands of the endocrine system that secrete their products, hormones, directly into the blood rather than through a duct.
The major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus and adrenal glands. The hypothalamus and pituitary gland are neuroendocrine organs.
There are certain organs which are very similar to secreting glands, but differ from them in one essential particular, viz., they do not possess any ducts by which their secretion is discharged. These organs are known as ductless glands. They are capable of internal secretion—that is to say, of forming, from materials brought to them in the blood, substances which have a certain influence upon the nutritive and other changes going on in the body. This secretion is carried into the blood stream, either directly by the veins or indirectly through the medium of the lymphatics.
These glands include the thyroid, the parathyroids and the thymus; the pituitary body and the pineal body; the chromaphil and cortical systems to which belong thesuprarenals, the paraganglia and aortic glands, the glomus caroticum and perhaps theglomus coccygeum. The spleen is usually included in this list and sometimes the lymph and hemolymph nodes described with the lymphatic system. Other glands as the liver, pancreas and sexual glands give off internal secretions, as do the gastric and intestinal mucous membranes.