CHEMICAL CORRELATION 559 



proposed to give a special name to the class— viz. " hormones" 

 from opfidto = " I arouse or excite." A few examples of the 

 most striking of these hormontic reactions may suffice to draw 

 attention to the importance of this class of reactions, and to the 

 promise, afforded by research in this direction, of further power 

 in the hands of the physician. 



The simplest example of a chemical correlation is seen in the 

 mechanism by means of which the contracting skeletal muscle 

 determines for itself an adequate supply of oxygen. Many years 

 ago Miescher propounded the idea that the activity of the respira- 

 tory centre was determined by the tension of the C0 2 in the 

 blood-plasma, and this in its turn would depend on the tension 

 of the CO, in the alveoli of the lungs. This idea has been fully 

 confirmed lately by Haldane and Priestley, and is borne out by 

 the results obtained by Zuntz and his school. The effect of 

 increased muscular activity, within physiological limits, is to 

 augment the output of C0 2 by the muscles, and therefore to 

 raise the tension of this gas in the blood. The immediate 

 result is an increased activity of the respiratory centre. The 

 respiratory movements are deepened and quickened, until the 

 increased ventilation is just sufficient to reduce the C0 2 tension 

 in the blood to its normal amount. If the muscular activity be 

 carried to excess, so that the oxygen supply is insufficient to 

 meet the total requirements of the muscles, there is a discharge 

 into the blood of acid substances, such as lactic acid. These 

 also will have the effect of raising the C0 2 tension in the blood 

 and still more in the respiratory centre, so that the effect on 

 the respiratory movements is even more marked than before. 

 In this case the hormone is one of the commonest products of 

 the metabolic activity of protoplasm. The chemical correlation, 

 the fitting of the activity of the respiratory centre to the needs 

 of the muscular system, is rendered possible by the development 

 in the respiratory centre of a special sensibility to carbon 

 dioxide. It is probable that the other hormones, whose action 

 I propose to deal with, are also, in origin, products of the 

 ordinary metabolic activity of some tissue, and that the evolution 

 of the chemical correlation has been effected, not by the 

 production of a special substance to act as a chemical 

 messenger, but by the acquisition of a specific sensibility on 

 the part of some other functionally related tissue. 



It is in the alimentary tract that we meet with the most 



