INFLUENCE OF THE DUCTLESS GLANDS ON METABOLISM 



483 



inert in this sense, though little physiological consideration is given to this 

 class of material in this regard. On the other hand, many end products of 

 the metabolism of certain tissue, the ductless glands in particular, are 

 thrown back into the blood stream to influence the reactions of other 

 tissues, just as do blood constituents from outside sources. The tissues 

 and organs have become adapted to reaction in the presence of chemicals 

 of this type and in many instances the relation is so firmly established as to 

 become an interdependent one. Adaptations have taken place as between 

 the tissue and its special environment of material derived from its neigh- 



FIG. 311. Part of a Section of the Human Thyroid, a, Fibrous capsule; b, thyroid 

 vesicles filled with, e, colloid substances; c, supporting fibrous tissue; d, short columnar cells 

 lining vesicles;/, arteries; g, veins filled with blood; h, lymphatic vessel filled with colloid 

 substance. (S. K. Alcock.) 



boring sources. Such substances are called internal secretions, and the 

 active principle, either isolated and identified or assumed, goes by the 

 name hormone (Starling). A hormone may be defined, therefore, as a 

 chemical substance produced by one tissue or organ to which some other 

 portion of the body has become biologically adapted to such an extent 

 that its normal function can proceed only under the influence of the 

 substance. Hormones are more or less specific in character. A hormone 

 for one tissue acts particularly on that tissue to the practical exclusion 

 of all others. 



