258 



PHYSIOLOGY. 



coming from the auricle, with consequent lengthening of the latent 

 period of the ventricular muscle. At this stage of compression of the 

 bundle of His it is usual for the time between the auricular and 

 ventricular systoles to lengthen until finally the ventricle fails to be 

 fired off by the impulses from the auricle, so that we have a loss of a 

 beat of the ventricles to every eight, or ten, auricular systoles. If 

 the clamp is tightened more, then we have a failure of the ventricle to 

 contract with every other auricular beat; here the rhythm is 2 to 1. 

 Further compression by the clamp causes, for every three or four 



SEC, 



Fig. 77. A Case of Heart Block. 



The uppermost line represents the time in seconds, the next line the apex 

 beat, and the third the jugular pulse, showing the transmitted carotid wave, c. 

 The third auricular contraction is not followed by any contraction of the ven- 

 tricle. The diagrammatic figure below shows the time of the auricular systole 

 by the upper verticals, the time of the ventricular systole by the lower verticals, 

 and the a-c period by the obliquity of the lines connecting these. Numerals in 

 this diagram represent fractions of a second. Notice the shortening of the a-c 

 period after the heart-block, and how this lengthens the succeeding ventricular 

 period. In the venous pulse, a represents the auricular contraction and c, the 

 "carotid pulse" (ventricular contraction), or, more correctly, the closing of the 

 tricuspid valve. The a-c interval normally does not exceed a fifth of a second, 

 but where conduction is poor in the bundle of His, it may be prolonged to two- 

 fifths or more. (HEWLETT.) 



auricular beats, a single ventricular contraction, a rhythm of 3 to 1 or 

 4 to 1. These cases of partial heart-block are to be explained by the 

 fact that after each contraction of the ventricle it experiences a fall in 

 irritability. But by its rest, there is an increase in irritability and then 

 the ventricle is fired off by the auricular impulses. A reduction in the 

 efficiency of the wave of excitation from the auricles may ensue from 

 the compression of the bundle of His ; hence the ventricles will not be 

 fired off until they have increased their irritability by rest to respond to 

 the weakened excitation from the auricles. When a complete heart- 



