CH. XXI.] THE ELECTRO -CARDIOGK AM 243 



heart and the peripheral resistance may present a gradual ascent or 

 descent ; it occupies about 18 second. Almost immediately after the 

 relaxation begins the intraventricular pressure falls below the 

 aortic, so that the aortic valves close near the upper part of the 

 descent at E. 



The actual amount of pressure in the heart is measured by a 

 mercurial manometer, which is connected to the heart by a tube con- 

 taining a valve. This was first used by Goltz and Gaule. If the valve 

 permits fluid to go only from the heart, the manometer will indicate 

 the maximum pressure ever attained during the cycle. If it is 

 turned the other way, it will indicate the minimum pressure. The 

 following are some of the measurements taken from the dog's heart 

 in terms of millimetres of mercury : 



Maximum Minimum 



pressure. pressure. 



Left ventricle . . .140 ram. - 30 to 40 mm. 



Right ventricle . . .60 mm. - 15 mm. 



Right auricle . . . .20 mm. - 7 to 8 mm. 



By a negative ( ) pressure one means a pressure less than that of 

 the atmosphere, so that the mercury is sucked up in the limb of the 

 manometer towards the heart. 



Another valuable instrument introduced by Hiirthle is called the differential 

 manometer. In this instrument, two cannulae are brought into connection with 

 tambours (A and u) which work on points of a lever at equal distances from and on 



B A 



FIG. '224. Diagram of Hiirthle's differential Manometer. 



opposite sides of its fulcrum (r). The lever sets in motion a writing style (s). This 

 instrument enables us to determine the relations of the pressure changes in any 

 two cavities. For instance, suppose A is connected to the left ventricle, and B to 

 the aorta ; when the pressure in the ventricle is greater than that in the aorta, the 

 writing style will be raised ; when the pressure in the aorta is greater than that in 

 the ventricle, the style will fall ; when the two pressures are equal, it will be in the 

 zero position. 



The Electro-Cardiogram. 



The muscular tissue of the heart gives rise on action to an 

 electrical disturbance which is in all essential features the same 

 as the diphasic variation we have already studied in Chapter XI. 

 in connection with voluntary muscle. The excised beating heart 

 of a frog can be readily connected either to a galvanometer or 



