Endocrine System

Introduction

There are secretory glands found in various parts of the body which have no ducts of their own, so that their secretions called hormones or internal scretions are carried by diffusion into blood and lymphatic vessels, these glands are known as endocrine glands, as opposed to those glands which liberate their scretions into ducts and are called exocrine glands. Alls endocrine glands are derived from embryonic epithelial tissues.

Though the endocrine glands are widely separated they are an integrated group linked together functionally by nervous and circulatory systems, so that collectivly they constitute and endocrine system which co-ordinates and dominates various activities by its hormones, bu the endocrine and nervous sytems are not sharply separated from each other, and some of their activities overlap. The thyroid, parathyroid, adrenal glands, and pituitary glandare solely endocrine in function.

Some glands have a dual role, being both exocrine and endocrine glands, among those are the pancreas, stomach duodenum, and gonads.

Thyroid Gland

Thyroid gland arises in an embryo as a diverticulum of the floor of the pharyns which proliferaes to form follicles and loses its connection with the pharynx. In lower vertebrates it arises midventrally from between the second and fourth visceral clefts, but in higher forms it develops between the first and second clefts. The thyroid has small follicles of cubical cells, the lumen of follicles is filled with an iodine-righ colloid, the follicles are enclosed in a capsule of connective tissue.