The endocrine glands are ductless and their production passes directly into the blood-stream, circulates all over the body and acts at places far from where it originated. These glands are scattered throughout the body like islands. Nevertheless, they are unified into a finely coordinate system, function in a marvellously harmonious fashion and control and coordinate the activities of the body.
The main endocrines are: the pineal, pituitary, thyroid, parathyroid, thymus, adrenals, islets of langerhans and gonads.
All these glands are comparatively small. They have access to very rich vascular supply. The products of these glands are organic chemical compounds which are effective in very small quantities.