Several of the organs of the body, in addition to the formation of secretions which are discharged on the surfaces by means of their ducts, produce also substances which pass directly into the blood or lymph, and have an influence in stimulating or otherwise regulating the activity of other organs.  There are also certain organs of glandular structure which are called the ductless glands; these are not connected with the surface and all their secretion passes into the blood.  It is a part of recent knowledge that the substances produced in these glands are of great importance for the body, some of them even essential for the maintenance of life.  In front of the neck is such an organ, the thyroid gland (Fig. 8, 14).  Imperfect development or absence of this organ, or an inactive condition of it, produces in the child arrested growth and deficient mental development known as cretinism, and in the adult the same condition gives rise to mental deterioration, swelling of the skin, due to a greater content of water, and loss of hair.  This deficiency in the production of thyroid secretion can be made good and the symptoms removed by feeding the patient with similar glands removed from animals.  The very complex disease known as exophthalmic goitre, and shown by irregular and rapid action of the heart, protruding eyeballs and a variety of mental symptoms, is also associated with this gland, and occasioned not by a deficiency but by an excess or perversion of its secretion.

Adjoining the thyroid there are four small glands, the parathyroids, each about the size of a split pea.  The removal of these glands in animals produces a condition resembling acute poisoning accompanied by spasmodic contraction of the muscles.  A small glandular organ at the base of the brain, the pituitary body, produces a secretion, one of the most marked properties of which is a control of growth, particularly that of the bones.  Most cases of giantism, combined as they are with imperfect mentality, are due to disease of this gland.  There are glands near the kidney which regulate the pressure of the blood in the arteries by causing contraction of their muscular walls.  The sexual characteristics in the male and female are due to an internal secretion produced by the respective sexual glands which affects growth, body development and mentality.

So is the body constituted.  A series of surfaces, all connected, of enormous size, which enclose a large number of organs and tissues, the activities of which differ, but all are cooerdinated to serve the purposes of the organism as a whole.  We should think of the body not as an assemblage of more or less independent entities, but as a single organism in which all parts are firmly knit together both in structure and in function, as are the components of a single cell.

FOOTNOTES:  [1] They do, however, take place, since within comparatively few years whole species have completely disappeared; for example, the great auk and the passenger pigeon.  In these cases it is not known what part disease played in the destruction.