216 EXPERIMENTAL PHYSIOLOGY. 



to bring the, string back to its original position. The switch A is 

 next turned to 1/10 and finally to 0, any movement of the string 

 from zero being compensated for each time. The skin current is 

 now fully compensated for and the cardiac deflections are more 

 pronounced. 



In order that all electrocardiograms may be compared one with 

 another, the sensitivity of the string must be standardized so that 

 a given strength of current will produce a certain magnitude of 

 deflection. The standard used is that of Einthoven a deflection 

 of 1 centimetre for a millivolt of current. The switch A is turned 

 to the left from to 1. One millivolt derived from the standard 

 cell is now passing through the string and causes it to be deflected 

 to an extent which is dependent upon the sensitivity of the string. 

 With the aid of the scale on the front of the camera box the magni- 

 tude of the deflection is measured. If it is less than 1 c.m. the 

 string is not sensitive enough and should be slackened. This is 

 accomplished by turning the mill-headed screw above the fibre 

 case to the right, i.e., in the direction of the arrow marked thereon. 

 If the deflection is more than 1 c.m. the string is two sensitive and 

 should be tightened by turning the milled head to the left. 



The deflections of the string are now standardized and a record 

 may be taken. The plate is exposed by removing its sliding cover, 

 the camera slit is opened and the carrier released. As the plate 

 descends behind the lens of the camera the string movements are 

 recorded. The time in 1/50 of seconds is indicated by upright 

 lines marked upon the plate by means of a revolving wheel placed 

 between the source of light and the string. Horizontal lines repre- 

 senting 1/10 millivolt are marked upon the plate by means of 

 lines engraved upon the surface of the camera lens. A record 

 should be taken from each of the three leads. 



Each student will be provided with a record which he should 

 study with regard to the following points 



1. Identify the various waves upon the tracings denoting each 

 by its appropriate letter. 



2. Measure the length of the P-R interval. What may an 

 examination of this interval tell you concerning the heart's action? 



3. Paste the electrocardiogram in your note-book and draw 

 beneath it a jugular pulse tracing and an intraventricular pressure 



