The ductless glands include a group of organs of very varied functions. They have no special ducts to convey their secretions either into the digestive tract or elsewhere. They do, however, form substances which pass directly into the blood stream or into the lymph channels. These substances belong to the class of bodies known as hormones, which are also formed by organs such as the pancreas which discharge their secretions externally. The presence of these hormones in the body is in many cases essential to health and even to life, and the activity of the ductless glands is correlated with and regulates the functions of distant organs, the only link being the blood by which the hormone is carried from its place of origin to its place of action, each hormone exercising a specific function in exciting the activity of a particular organ or tissue. Those which have been studied are of comparatively simple nature, dialysable, readily soluble in water and not destroyed by boiling. '

The chief ductless organs employed in modern medicine are the thyroid and parathyroid glands, the suprarenal glands, the pituitary body, the spleen and the thymus.