190 MODERN SCIENCE READER 



the liquid media of the body, effect a correlation between 

 the activities of the organ of origin and the organ on 

 which they exert their specific effect. As regards the 

 nature of the action of the hormones on the organ affected 

 we know too little to. make any safe generalization. In the 

 case of the secretin it seems most probable that the hormone 

 arouses the pancreatic cells to an act of secretion, and there- 

 fore it has in this instance the value of a chemical stimulus. 

 But in other cases the effect of the hormone may be rather 

 of the nature of an activation. This at least would seem 

 to be true for the hormone, of unknown nature, given off by 

 the pancreas and concerned in the glycolysis of sugar in the 

 organism. The effect of the hormone adrenalin upon the 

 musculature innervated by the sympathetic system may 

 also be of the nature of an activation rather than of a 

 chemical stimulation. 



The substances of known composition which may be re- 

 garded as playing the role of hormones are few in number, 

 three or four at most, as follows: First, the carbon dioxide 

 formed in the tissues, particularly in muscle during con- 

 traction. It seems agreed now that the carbon dioxide 

 acts as the normal stimulus to the respiratory center. 

 When produced in the working muscles in such quantities 

 as to raise perceptibly the carbon dioxide tension in the 

 alveoli of the lungs and the blood of the pulmonary veins, 

 the respiratory center is excited to greater activity and 

 the excess above the normal contents is thereby removed; 

 second, the adrenalin of the adrenal glands which in some 

 way, directly or indirectly, makes possible the full func- 

 tional activity of the involuntary musculature of the body . 

 third, the hydrochloric acid produced in the stomach which 

 stimulates the formation of secretion in the duodenal 

 epithelium; and fourth, possibly the iodothyrin of the 

 thyroid gland with its dynamogenic effect upon the neuro- 

 muscular apparatus of the body. In addition there are a 

 number of hormones of unknown composition which have 

 been either proved or assumed to exist, and which are held 



