208 



THE CIRCULATION OF THE BLOOD 



production. The inorganic salts seem to be peculiarly closely related 

 to the development and character of the cardiac rhythm, figures 171, 172, 

 and 173. Both the cold-blooded heart and the mammalian heart respond 

 very quickly to the influence of these salts. The details of this influence 

 have been discussed on page 199. It is somewhat surprising, however, 

 that the highly organized mammalian heart will contract rhythmically 

 for hours on purely inorganic nutrient fluid, provided only that the oxygen 

 be supplied in sufficient quantity and under high enough tension. The 

 isolated mammalian heart also responds sharply to a change in the salt 

 content of the perfusing solution. For example, addition of potassium 

 chloride to a Locke solution slows or even suppresses the rate, as is shown 

 in figure 178. 



Irregularities of Cardiac Rhythm. There are a number of cardiac 

 irregularities in rhythm that are due to variations in irritability, con- 

 ductivity, or other of the normal properties of the differentiated tissue of 

 the heart itself. The phenomena of this type of most common occurrence 

 are heart block, extra ventricular systoles, and auricular fibrillation. 



Heart Block. It occasionally happens that the heart rate becomes 

 very slow, 30 or 40 a minute, and the rate does not vary much from this 



FIG. 178(1. An electrocardiogram, lead II, and auricular and ventricular muscle 

 tracings, A and V taken simultaneously from the dog's heart. Two extra ventricular 

 contractions are artificially produced by stimulation of the right ventricle. They 

 show the characteristic right ventricular dominance. The first is taller because the P 

 of the natural stimulus and the R of the artificial coincide. Time, fifths and twenty- 

 fifths of a second. (Lewis.) 



low level. On careful examination it is found that the rate is ventricular 

 only and that the auricle is contracting much faster. The latter may con- 

 tract in multiples of the ventricular rate, 2-1, 3-1, or 4-1 rhythm in 

 which case there is said to be partial block. Only every second, third, 

 etc., contraction reaches the ventricle. Or the two rates may be wholly 

 independent as in total block. 



