The differentiation of function of organs is not as sharp as was formerly supposed. Evidence of their interdependence is rapidly accumulating. The activity of various organs is known to result in the formation of by-products termed “internal secretions,” which influence the activity of other organs, or even of the body as a whole. The spleen enlarges after meals. This may be merely connected with the engorgement of the abdominal viscera which occurs during active digestion, or it may indicate, as some physiologists hold, that an internal secretion of the spleen aids the pancreas in preparing its ferments. The spleen enlarges greatly in ague and in some other diseases of microbial origin. This has been regarded as evidence that it takes some part in protecting the body against microbes. But whatever may be the accessory functions which it exercises, they are not of material importance to the organism as a whole, seeing that removal of the spleen causes no permanent inconvenience either to men or animals. Its blood-destroying functions are taken on by accessory spleens, if there be any, and by lymphatic glands. The marrow of bone also becomes redder and more active. Under certain circumstances, red corpuscles, or fragments of red corpuscles, are to be seen within liver-cells; but it is uncertain whether blood-destruction is a standing function of the liver.
CHAPTER V
INTERNAL SECRETIONS
Thyroid Gland.—On either side of the windpipe, rather below the thyroid cartilage (Adam’s apple), lies a somewhat conical mass of tissue. The two masses are connected by an isthmus; lobes and isthmus make up the thyroid gland. The whole weighs about an ounce. In health it is so soft that only the finger of an anatomist could detect it through the skin and the thin flat muscles which connect the hyoid bone and the thyroid cartilage with the breast-bone. It makes no visible prominence on the front of the neck. The thyroid gland is, however, liable to enlargement, especially amongst the people who live in certain districts. In the Valais, “goitre,” as it is termed, is so frequent that anyone walking up the Rhone Valley is sure to meet a number of persons—for the most part women—whose swollen necks overhang their collar-bones, like half-filled sacks. Goitre is even more common in the Valle d’Aosta, on the Italian side of the Alps. In England this condition, comparatively rare, is known as “Derbyshire” or “Huntingdonshire” neck.
In the majority of cases the tumour in the neck develops slowly, and does not reach its full dimensions until after middle life. Goitre in this form, although inconvenient, causes no serious discomfort. But when it appears in early life, it is associated with an extraordinary complex of malformations and ill-performed functions. The condition into which a goitrous child sinks is known as cretinism. With the exception of the skull-case, its skeleton does not attain to its proper proportions; and, since the soft parts do not equally submit to arrest of growth, the dwarf is heavy and ungainly, with large jowl and protuberant abdomen. The appearance of distortion is extraordinarily heightened by hypertrophy of the skin and the subcutaneous connective tissue. Ears, eyelids, nose, lips, fingers, are thick and heavy. The hair and nails are coarse. The skin is folded, wrinkled, rough.
The bodily ungainliness of a cretin has its counterpart in the deformity of his mind. He is an idiot whose deficiency is chiefly marked by apathy.
Cretinism exhibits itself in varying degrees. The description that we have just given would not be accurate for all. For the sake of brevity, we have chosen a case which might be that of a goitrous cretin of a certain type, or that of a cretin whose thyroid gland, in lieu of showing what looks like overgrowth, has failed to properly develop. Nothing is more remarkable with regard to this organ than the fact that the condition associated with its overgrowth and the effects of its atrophy, or inadequate growth, are the same. A consideration of the function of the gland will suggest an explanation of this seeming paradox.
The inconvenience caused by goitre induced surgeons, about twenty-five years ago, to remove the tumour in simple uncomplicated cases. Owing to the accessibility of the gland, the operation is both safe and easy; but its removal was found to be followed by symptoms of a very serious nature, especially overgrowth and œdema of subcutaneous tissue, muscular twitchings and convulsions, mental dulness. About the same date, physicians recognized that the disease myxœdema—so called because the œdema is not watery, as in dropsy, but firm and jelly-like—is due to deficiency of the thyroid gland.
No other organ of the body has so weird an influence upon the well-being of the whole. No other organ has an equally mysterious ancestral history. Assuredly the thyroid gland was not always such as we see it now. In prevertebrate animals it must have been quite different, both in structure and in function. From fishes upwards, however, its structure is always the same. It is composed of spherical vesicles or globes. Every globe is lined by a single layer of cubical epithelial cells. Its cavity is filled with a homogeneous semi-solid substance known as “colloid.” The globes are associated into groups or lobules. They are in contact with large wide lymphatic vessels. The organ has a lavish supply of blood. It is also well supplied with nerves. Colloid is the secretion of the epithelial cells which line the globes. As these globes have no openings, the secretion must be passed by osmosis into the lymphatic vessels. There is abundant reason for believing that by this route the products of the gland reach the blood, and are distributed by the blood to all the tissues of the body. And here it is important to notice that associated with the thyroid gland are certain very small masses of tissue termed “parathyroids.” There may be four of these—two on the course of the large arteries which supply the thyroid gland from above, two related with the almost equally large arteries which supply it from below; but the number varies. The parathyroids do not contain vesicles. They are solid masses of epithelial cells, traversed by bloodvessels and lymphatics. Yet, like the epithelial cells of the vesicles, they secrete colloid. Granules of this substance are to be seen within their cells. We cannot pass over the parathyroids without this reference, since, small though they are, they seem to be quite as important as the thyroid gland itself, judging from the effects which follow their removal.