The adrenals, or suprarenals, are compound endocrine glands peculiar to vertebrates. In man they are two small but important structures, weighing only eight or nine grams each, which fit “like cocked hats” over the anteromesial ends of the kidneys (Fig. 419). They are enclosed in delicate but firm capsules of connective tissue that separate them from the kidneys to which they are closely adherent but of which they are physiologically independent. They are relatively the most richly vascularized organs of the body, since five or six times their intrinsic weight of blood passes through them per minute. They are somewhat smaller in the female than in the male, and frequently in the same individual the one on the right is smaller than that on the left.
Although discovered long ago by Eustachius, who has so many other anatomical discoveries to his credit, their significance and importance as endocrine glands has only recently been made clear by Cannon and others. The first hormone to be isolated in pure form, adrenalin, or epinephrin, was obtained from these glands. This substance, which has also been chemically synthesized, is known to chemists as ortho-dioxy-phenyl-ethanol-methylamine.
In structure the adrenals consist of an outer cortex and an inner medulla. These two parts are not only morphologically and embryonically distinct, but are also chemically and physiologically different.
In origin there is a close relationship between the inner medullary cells of the adrenal glands, which produce the adrenalin hormone, and the sympathetic nervous apparatus. When migrating cells of the central nervous system become detached during development and move out to establish the sympathetic ganglia, they are of two kinds, although indistinguishable without recourse to differential staining. Some of them, sympathoblasts, become involuntary neurons of the sympathetic ganglia, while others, chromaffinoblasts although of common origin with the sympathoblasts, are transformed into so-called chromaffin cells, since they show a special affinity for chromic-acid salts, taking on a distinctive brownish color in the presence of the latter.
These chromaffin cells form glandular endocrine masses, and the extent to which they may be stained by chromium compounds is proportional to the amount of adrenalin which they are producing at the time.
It is the chromaffin derivatives of the coeliac plexus in the autonomic nervous system which give rise to the medulla of the adrenal glands. Chromaffin tissue, however, is not confined to the adrenals, but may be associated with any of the sympathetic plexuses or ganglia, forming paraganglia, or chromaffin bodies, of various sorts. These are frequently associated with concentrations of capillaries, as for example in the carotid glands at the junction of the internal and external carotid arteries, and the so-called aortic bodies of Zukerkandl, two or three in number, which are near the aorta in the human fetus and during early childhood.
In fishes the chrome-staining adrenalin-secreting tissue is not concentrated into a medulla within a cortex, but is a diffuse double row of groups of cells associated with the trunk-line chains of sympathetic ganglia. During the phylogenetic ascent of the vertebrate series, the neuronic elements of this partnership increase in number and importance, while the chromaffin cells diminish.
In addition to the chromaffin bodies in fishes there are, lying between the straplike kidneys, two elongated masses of tissue, derived from the coelomic epithelium, and named from their position the interrenals (Fig. 420). These are destined to form the cortical part of the future adrenals.
A transitory evolutionary stage is presented in amphibians, in that the original close connection between the chromaffin cells and the sympathetic ganglia is partially lost, and the interrenal tissue comes into close contact with and begins to envelop the chromaffin bodies. This new association becomes still more intimate in reptiles and birds, but it is only in mammals that a definite chromaffin medulla becomes established inside an interrenal cortex. The lower the species within the mammalian line, the greater is the relative amount of the cortical component.
There are various theories as to the function of the chromaffin system. It is known that when adrenalin is injected into an animal there is a sudden, decided, but comparatively brief rise in blood pressure; the heart beat is steadied, becoming slower and more powerful; there is a quick increase of emergency sugar-fuel (glycogen) poured into the blood stream; and the muscles, particularly the involuntary muscles that are supplied by sympathetic nerves, are thrown at once into their most efficient state of tonus. Thus, in emergencies, the temporary “strength of desperation” is furnished by the adrenalin, and fatigue is for the time banished.
Although minute quantities of this hormone are essential to the normal process of metabolism, it is not supplied continuously to the blood in any great quantity, but only as occasion demands to meet some transient physiological crisis.
A separate hormone, cortin, is developed from the adrenal cortex, either singly, or as a group of hormones having a common base. It plays an important role in carbohydrate metabolism and aids in the utilization of certain vitamins which are indispensable components of foods. According to Swingle, cortin has to do with maintaining the acid-base equilibrium within the body.
At least one pathological condition, Addison’s disease, is definitely correlated either with deficiencies or lesions of the adrenals, especially those arising from tuberculosis. This disease, which was first described a century ago by Addison from Guy’s Hospital in London, is characterized on the part of the patient by muscular weakness, low blood pressure, digestive disturbance, and the appearance of peculiar bronze patches upon the skin. Although the course of the disease may be somewhat modified by the administration of cortin, a fatal termination seems to be the usual outcome.
The adrenals undergo precocious growth, reaching their largest relative size in man about the third fetal month, when they are as large as the neighboring kidneys.
The Islands of Langerhans
The pancreas is another gland compounded of two kinds of tissues. The pancreatic cells proper are grouped about a system of drainage ducts, and the substances elaborated by them are important digestive enzymes, which escape through these ducts into the small intestine, as described in Chapter XI.
Throughout the vertebrates there appear in isolated groups between the cells of this glandular system distinct interalveolar cells arranged as anastomosing cords alternating with sinusoids and forming rounded masses. They have no outlet by way of ducts for the substances that they produce, and consequently resort to the endocrine method of disposal through the blood system. Such interstitial masses of endocrine tissue are known as the islands of Langerhans.
It has been ascertained that insulin (insula, island), the hormone which they produce, affects carbohydrate metabolism by regulating the use of sugar within the body. Disturbances in this function lead to the pancreatic disease of diabetes, which is characterized by an excess of unoxidized sugar in the blood and urine. This condition is being effectively treated by the use of insulin, which was first successfully isolated by Banting and Best in 1922.
In fishes the islands of Langerhans are condensed into large superficial lumps that may be easily severed from the remaining pancreatic tissue, but in higher forms the characteristic grouping into small insular masses occurs.
It is estimated that in the pancreas of the guinea pig there may be as many as 25,000 “islands,” while in man their number may exceed a million. They are rather more abundant in young animals than later in life. In man they make their appearance first in embryos about 54 mm. long.