218 ELEMENTARY PHYSIOLOGY less. 



so as to leave a large central space ; this space in each 

 alveolus is tilled with a clear, viscid, often semi-solid 

 fluid. The viscidity of this fluid is due to the presence 

 in it of a substance which in some respects is like the 

 mucin of mucus. This material is known as the colloid 

 substance of the thyroid and is remarkable among the 

 various constituents of the body as containing the 

 chemical element iodine. 



The thyroid gland contributes an internal secretion to 

 the blood the importance of which may be judged from 

 the following circumstances. Children in which the thy- 

 roid is deficient present a very painful picture. They 

 develop neither physically nor mentally, at fifteen or 

 twenty years of age they have not advanced further than 

 a normal child of five years old, their stature is as small, 

 their appearance somewhat deformed, and they lack 

 any mental development. They are known as "cretins." 

 If such are made to eat thyroid glands, or extracts of 

 these glands, they at once develop rapidly and in the 

 course of a few years they become normal persons, and 

 remain such so long as they persist with the diet. 

 Perhaps even more remarkable is the fact that, when 

 pieces of thyroid from normal persons are grafted under 

 the skin of cretins, if the grafts establish themselves, the 

 patients develop rapidly into normal individuals. 



Disease of the thyroid glands often leads to disorders, 

 strikingly manifest in the skin, but also involving other 

 organs and tissues, especially perhaps the nervous system 

 and thus leading to nervous troubles. Occasionally the 

 degenerations of the tissues take on the form of a 

 change into a mucin-like substance. These troubles m'h.y 

 be largely mitigated by administering an extract of the 

 fresh gland or by eatiiig the fresh gland-substance. Goitre 

 is an enlargement of the thyroid ; at one time surgeons 

 removed the gland, but the removal of the gland was 

 found to be followed by the symptoms which we have 

 described. 



