THE HEART 



tonicity is one of the commonest events in cardiac 

 disease, and, probably in consequence of this, increased 

 excitability and conductivity of the heart muscle are 

 common occurrences in disease too, and are mainly 

 responsible for some of the chief forms of irregularity 

 of the heart's action. 



The Nervous Control of the Heart. 



The maintenance of the heart's action is a matter of 

 such transcendent importance to the organism that, as 

 we have seen, it has been made to depend upon the 

 intrinsic qualities of the heart muscle. On the other 

 hand, the dependence of every organ upon the heart for 

 a due supply of blood, and the necessity for frequent 

 variations in the rate and force of the blood-stream and 

 the height of the general blood-pressure, make it essential 

 that the heart should be ' connected up ' with all other 

 parts of the body by nervous communications. As a 

 matter of fact, this is very carefully provided for, and the 

 heart is accordingly played upon by more reflex influences 

 than almost any other organ in the body. 



These nervous influences act upon the various physio- 

 logical properties of the heart muscles, either intensifying 

 or lessening them. Thus they may : 



1. Accelerate or lessen the rate of stimulus production, 

 and so increase or lessen the rate of the beats. 



2. Accelerate or lessen the rate of conduction of the 

 contraction, and so accelerate or retard the beats. 



3. Increase or diminish the excitability of the muscle, 

 and so affect the strength of the stimulus required to 

 produce a beat. 



