PHYSIOLOGICAL 295 



A disease of adults due to thyroid deficiency or degeneration is 

 known as myxoedema. It is marked by thickening and coarsining 

 of the skin, loss of hair, in many cases subcutaneous fattening, 

 reduced metabolism, low temperature, diminished sexual activity, 

 dullness of sense and mind. As with cretinism, restoration to health 

 may be secured by adding thyroid gland or its extracts to the diet, 

 but intermission of the treatment may be followed by a return of 

 the symptoms. 



In the huge enlargement of the thyroid gland, which is called 

 goitre, there is often, though by no means always, an abundance 

 of secretion, as might be expected from a big gland. But as the 

 symptoms are in the majority of cases like those associated with 

 thyroid deficiency, the probability is that the specific hormone is 

 absent or scanty. 



An exaggeration of the specific thyroid secretion (hyper-thyroid- 

 ism) is associated with exophthalmic goitre, so called from the 

 protrusion of the eyeballs. It is marked by increased metabolism 

 (especially nitrogenous), irregular pulse, throbbing, excitability, 

 restlessness. The disease occurs most frequently in femsdes. A con- 

 sideration of diseased conditions associated with exaggerated or 

 deficient thyroid activity throws light on the normal function of 

 the gland; and it has this wider biological interest, that sundry 

 peculiarities of animals, such as the coarseness of the elephant's 

 hide, the whale's blubber, the relative hairlessness of the rhinoceros, 

 the prominent eyeballs of the mouse, besides more general features 

 like dwarfness and nervousness, may very possibly be associated 

 with variations in their thj^roids. In other words, it is worth con- 

 sidering the possibility of the variation and racial normalising of 

 hypo- and hyper-thyroidism in various types of animals. The 

 general theory of somatic variations being due to changes in 

 endocrinal secretion is considered under Evolution. 



The specific autacoid of the thyroid has been isolated, crystal- 

 lised, and analysed by Kendall (1919), who calls it thyroxin. It 

 is an iodine-bearing derivative of a protein (tyrosin), and its 

 chemical structure is expressed in its chemical name tetraiodo- 

 hydroxyphenoxy-phenyl-amino-propionic acid! There may well be 

 room for variability in such a complex body as this. 



The everyday use of this thyroid hormone is probably to increase 

 the excitability of nerve-cells; but it appears to exert a regulating 

 influence on many parts and processes. The autacoid of the para- 

 th37roid is believed by some to be chalonic, putting a check on 

 nervous activity, indirectly by controlling the amount of calcium 

 in the fluids of the body. Thus some regard thyroxin as a general 

 catalyst which increases the rate of the fundamental metabolic 

 processes. Similarly, some would say that the parathyroids, so 

 intimately mingled with the thyroid, are not strictly endocrinal. 



