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 coördinated 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.
Chapter II
No Sharp Line Of Demarkation Between Health And Disease.—The Functional Nutritive And Formative Activities Of Cells.—Destruction And Repair Constant Processes In Living Matter.—Injuries To The Body.—The Effect Of Heat.—The Action Of Poisons.—The Lesions Of Disease.—Repair.—The Laws Governing Repair.—Relation Of Repair To Complexity Of Structure And Age.—The Reserve Force Of The Body.— Compensatory Processes In The Body.—Old Age.—The Diminution Of Resistance To The Effect Of The Environment A Prominent Factor In Old Age.—Death.—How Brought About.—Changes In The Body After Death.— The Recognition Of Death.
There is no sharp line separating health from disease; changes in the tissues of the same nature, or closely akin to those which are found in disease, are constantly occurring in a state of health. The importance of parasites in causing disease has led to the conception of disease as almost synonymous with parasitism; but it must be remembered that the presence of parasites living at the expense of the body is perfectly consistent with a state of health. Degeneration, decay and parasitism only become disease factors when the conditions produced by them interfere with the life which is the normal or usual for the individual concerned.
All the changes which take place in the cells are of great importance in conditions of both health and disease, for life consists in coördinated cell activity. The activities of the cells can be divided into those which are nutritive, those which are functional and those which are formative. In the functional activity the cell gives off energy, this loss being made good by the receipt of new energy in the form of nutritive material with which the cell renews itself. In certain cells an exact balance seems to be maintained, but in those cells whose activity is periodic function takes place at the expense of the cell substance, the loss being restored by nutrition during the period of repose. This is shown particularly well in the case of the nerve cells (Fig. 13). Both the functional and nutritive activity can be greatly stimulated, but they must balance; otherwise the condition is that of disease.