62 EXPERIMENTAL PHYSIOLOGY 



Action of Cations on Cardiac Muscle. The ions found 

 in the blood play a very important role in the maintenance of the 

 normal heart beat. Remove one of these essential ions and the 

 heart finally stops beating. 



The SODIUM IONS not only play a large part in the maintenance 

 of the normal osmotic pressure but when present alone, they pro- 

 duce relaxation of the heart. 



POTASSIUM IONS are present in much smaller quantity than 

 sodium and are not so essential. They tend to produce relaxation 

 and when increased above normal the heart rate is reduced, then 

 finally the heart stops in extreme diastole. 



CALCIUM IONS are absolutely essential. Excess of calcium 

 produces calcium rigor, that is the heart finally stops in systole. 

 Calcium is supposed to promote contraction, while sodium and 

 potassium aid in relaxation. 



Temperature Effects. Although the mammalian heart is so 

 protected in an organism with a relatively constant temperature, 

 it is influenced by thermal changes if subjected to them. In fact 

 it acts like the heart of a frog or turtle, except that it does not 

 withstand the effects of a lowered temperature long. The rate 

 increases with increase of temperature up to a certain point. 

 Below 17 C. the mammalian heart does not beat, likewise above 

 44 C. contraction soon ceases. The heart of cold-blooded animals 

 responds at lower temperatures. 



NERVOUS CONTROL OF THE HEART BEAT. 



Inhibition. It can be shown by stimulation that the vagus 

 inhibits the activity of the heart. Weak stimuli may merely 

 decrease the magnitude of the beat. Stronger stimuli slow or even 

 stop the contractions. Even with stimulation intense enough to 

 stop the heart, after a time the beats begin to break through or 

 escape. In the frog, because the vagus and sympathetic fibres run 

 together in the same trunk, both are stimulated. The stimulus 

 must be chosen so that the vagus is affected more than the accele- 

 rator or else there will be either no change or an increase in rate. 

 This is done by gradually increasing the stimulation until vagal 

 effects predominate. 



