These two parts differ in function and origin, while their association is probably an accident.
Though adrenal gland was discovered by Eustachius but its endocrine function was demonstrated by W. B. Cannon.
In the lower category vertebrates, the two parts corresponding to the cortex and medulla occur separately as inter-renal bodies and chromaffin bodies respectively.
Chromaffin bodies, because of their affinity for chromate, are termed as chromaffin bodies.
Chromaffin bodies are supposed to be homologous with the medulla of higher forms, while the inter-renal bodies are said to be homologous with the cortex.
The adrenals of the mammals (man) are two small yellowish glands, each lying above or near the kidney.
They are richly vascularized and they are smaller in the female, while larger in the male. Each adrenal gland weighs about 5 gm. Histologically each gland is composed of two distinct parts: an outer cortex and an inner medulla.
The two parts differ in function and development. The cortex is originated from tissue from which the reproductive glands (gonads) are developed, while the medulla is derived from the embryonic tissue which also gives rise to sympathetic nervous system. We shall discuss the two parts of adrenal gland and their secretions separately.
The two parts of adrenal gland of mammals, though closely associated structurally with each other, are functionally quite distinct, these two parts are found separately in the lower vertebrates.
The adrenal cortex of lower vertebrates has its homologue in the inter- renal tissue located in more or less discrete clumps surrounding the renal arteries as they enter the kidneys.
In the cyclostomes the inter- renal bodies are found as small irregular structures along the blood vessels located above the kidneys.
The chromaffin cells are either absent or found as small rounded bodies at the dorsal aorta and its branches.
In the hagfish (Myxine) the inter-renal bodies are completely Jacking. In fishes particularly the dog fishes the inter-renal bodies representing the cortex, lie between the kidneys and the chromaffin bodies or the suprarenals, representing the medulla, consist of small masses on the sympathetic chains.
In teleosts, amphibians and reptiles the chromaffin bodies are dispersed widely as islets in or among what are known as inter-renal glands.
In amphibian the two elements are intermingled very closely forming a single compound endocrine gland, the adrenal of an orange or yellow colour located on the ventral side of the kidneys. In reptiles the two elements are more intimately connected.
In the birds the two elements are intermingled but nevertheless are contained in a single pair of encapsulated elands of yellowish colour found on either side of the post caval vein just anterior to the kidneys and close to the gonads.