182 FOUNDATIONS OF BIOLOGY 



mones as examples of the general method of chemical regu- 

 lation which their study, ENDOCRINOLOGY, is revealing. 



Certain hormones are secreted by organs whose sole func- 

 tion is their production, such as the various endocrine glands 

 which pour their secretion directly into the blood stream. 

 Others are elaborated by special cells imbedded in organs, 

 such as the pancreas and reproductive organs, of which they 

 physiologically form no part. As a concrete example of an 

 endocrine gland we may select the THYROID which, as has 

 been seen, arises as an outpocketing of the digestive tract in 

 the neck region and finally loses all connection with its point 

 of origin and becomes a ductless gland. (Fig. 88.) 



The general effect of the thyroid hormone on metabolism 

 is a regulation of the rate of oxidation in the body. An excess 

 of the substance results in such vigorous fuel consumption 

 that no surplus remains in the body to be stored as fat ; while 

 a deficiency in the glandular secretion results in a tendency 

 toward fat formation. Accordingly the administration of 

 thyroid extract is often an efficient means of reducing flesh 

 by increasing the oxidative processes of the body. A defi- 

 ciency of the hormone during adult life frequently results in 

 a type of mental deterioration called MYXEDEMA. Children 

 in whom the development of the thyroid is suppressed be- 

 come dwarfish idiots known as CRETINS, while overdevelop- 

 ment of the gland induces increased nervous activity and 

 mental disorders. Feeding with thyroid material sometimes 

 prevents the development of cretinism and cures myxedema, 

 while a surgical removal of part of the gland may cure the 

 nervous instability and other symptoms due to an excessive 

 amount of the hormone. Goitre is a pathological enlarge- 

 ment of the thyroid. 



Finally, as a further indication of the nicety of the recip- 

 rocal adjustments within the organism, it may be mentioned 



