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.