452 HUMAN BIOLOGY 



the extract of the thyroid gland of an animal; but this substi- 

 tution treatment must be continued indefinitely as the 

 individual's own thyroid cannot supply the hormones. 

 Excessive and probably also altered secretion of the thyroid 

 gland causes a condition (exophthalmic goiter or Graves' 

 disease) which is the opposite of myxedema and is character- 

 ized by extreme nervousness, protrusion of the eyes, enlarge- 

 ment of the thyroid gland (goiter), sweating, palpitation 

 and rapid action of the heart; this disease is greatly benefited 

 by removal of part or almost the whole of the gland, and is, 

 at any rate temporarily, improved by the administration of 

 iodine. The thyroid gland is concerned with the metaboHsm 

 (or changes connected with the presence) of iodine in the 

 body, just as the small parathyroid glands in its immediate 

 neighborhood regulate the metaboHsm of calcium. In certain 

 regions, such as the basin of the Great Lakes and the valley 

 of the Mississippi in North America, parts of Switzerland, 

 and some valleys of the Himalayas, enlargement (goiter) of 

 the thyroid gland is endemic; this has long been connected 

 with the water supply. According to McCarrison simple 

 goiter is due to a number of causes, viz. deficiencies and 

 excesses in food, polluted water, gastrointestinal infection, 

 insanitary conditions of fife, and deficiency of iodine. This 

 simple goiter, which is not accompanied by the symptoms of 

 Graves' disease, occurs much more frequently in young girls 

 than in males, and according to D. Marine is due to a lack 

 of iodine; he has found that it can be prevented by the 

 administration of small doses of iodine twice a year, a 

 striking demonstration of scientific preventive medicine. 

 The pituitary gland at the base of the brain exerts a well- 

 marked influence on physical growth; deficiency of its 

 internal secretion leads to a form of obesity, with, in children, 

 arrest of development so that the changes of puberty do not 

 appear. Overactivity of the anterior lobe of the pituitary 

 causes excessive growth which in early fife is responsible 

 for giants, and in older people, whose bones can no longer 

 grow in length, for a characteristic increase in size of the 

 extremities known as acromegaly. An extract of the posterior 

 lobe of the pituitary has been found to prevent for some 

 hours the excessive excretion of urine which so disturbs the 



