120 APPLIED PHYSIOLOGY 



pressure on the inner aspect of its wall during diastole. 

 Now, the degree of resistance offered to such dilatation 

 appears to vary considerably in different individuals. 

 In other words, the * tone ' of the heart seems to be by 

 no means constant. That is probably why one man 

 gets cardiac dilatation as the result of a degree of 

 exertion which is borne with impunity by another. The 

 toxins of some diseases, too, such as diphtheria and 

 influenza, appear to lower heart tone, and predispose to 

 dilatation ; and the same is true of some poisons, such 

 as muscarine. Considerable variations in the tone of 

 the heart may, and probably do, occur unobserved. 

 Thus a degree of relaxation sufficient to double the 

 output would only mean an increase in diameter of the 

 heart of 2 centimetres an increase inappreciable by 

 percussion.* 



On the other hand, the most valuable property of 

 digitalis as a drug appears to be its power of increasing 

 the * tone ' of the heart muscle and so of opposing dila- 

 tation. The possible relation of such tone to nervous 

 influence will be discussed later. 



Tonicity resembles in a minor degree an active con- 

 traction of the heart, and we shall see later (p. 137) that 

 contraction suspends the other functions of the heart 

 muscle whilst it is taking place, and for some time after- 

 wards. Similarly, when tone is increased, conductivity 

 and excitability are diminished, and when tone is 

 diminished these are increased. t Now, diminution of 



* See Hensen, Deutsch. Arcli. f. Klin. Med., 1900, Ixvii. 436. 

 t See a very suggestive paper by Gossage, Med. Chir. Trans., 

 1906, vol. xc. 



