THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS 767 



Autacoids have further been subdivided by Schafer into two classes 

 according to whether they excite metabolic processes or depress them. 

 Examples of excitatory autacoids, also designated as hormones, are the 

 epinephrine produced by the adrenal glands, which excites the termina- 

 tions of the sympathetic nervous system, and pituitrin produced by the 

 posterior lobe of the pituitary gland, which excites plain muscular fiber. 

 Inhibiting autacoids, also called chalones, are not so commonly known, but 

 are illustrated by the substance contained in extract of the placenta, 

 which tends to prevent the secretion of milk. 



Autacoids may have either an immediate or a delayed action ; the effect 

 which they produce may be like that with which we are familiar as the 

 result of stimulation of the nerve supply of a gland, being illustrated 

 again by the effect of epinephrine, or they may act so slowly that it is 

 only after a considerable period of time during which they have been 

 in the organism in excess, that any apparent effect is produced. The 

 slowly acting autacoids have been called morpho genetic, and they are 

 well illustrated in the internal secretions of the anterior lobe of the 

 pituitary and of the generative glands secretions which affect growth. 



Regarding the chemical nature of autacoids, certain facts stand out prominently. 

 Being very largely the products of glands, it might be imagined that they would be 

 enzymic in nature, for enzymes are now known to be the most important active agents in 

 bioplasm as well as the active agents in many of the external secretions, like those of 

 the salivary, gastric and intestinal glands. Autaeoids, however, are not enzymes. They 

 are far simpler in chemical structure, and are not destroyed by heat in the presence of 

 water. They are represented by a comparatively small molecule, and are therefore 

 dialyzable. This latter fact justifies the hope that it may be possible to prepare them 

 or their simpler salts in crystalline form a hope which has already been realized in 

 the case of thyroxin and epinephrine. Progress has likewise been made in isolating 

 the active principles of the anterior and posterior lobes of the pituitary gland. To 

 sum up, then, we may say that an autacoid is a specific organic substance, formed 

 by the cells of one organ and secreted into the circulating fluid, which carries it to other 

 organs, upon which it produces effects similar to those of drugs. 



Methods of Investigation 



To investigate the function of an autacoid, careful studies are made of 

 the effects produced (1) by excision of the gland which furnishes the 

 autacoid and (2) by administering intravenously or subcutaneously or 

 >rally extracts prepared from the gland. Frequently, also light is thrown 

 on the function of the autacoid by observing the effect which fol- 

 lows prolonged feeding with the endocrine organ that manufactures it 

 and by observing the pathological changes in the various endocrine organs 

 in diseased conditions. Embryological and histological studies are also of 

 the greatest importance. A difficulty in investigating the function of 

 an endocrine organ lies in the fact that the secretion of no one gland 



