THE VASCULAR MECHANISM. 



I 9 7 



II 



The advantages of this method are, first, that the movements of 

 the membrane are directly proportional to the variations in pressure, 

 and by the elimination of a lever are recorded without inertia, and, 

 secondly, that when the membrane is thrown into vibrations these are so 

 rapid (250 per second) that they cannot 

 possibly be mistaken for oscillations pro- 

 duced by changes in the endocardiac 

 pressure, and are so rapidly damped that 

 they practically do not occur when the 

 pressure is recorded. A similar cannula 

 may be passed into an artery so as to 

 record the changes in arterial pressure. 



Fig.|69 represents a record, obtained 

 by this method, of the pressure changes 

 in the left auricle and ventricle and 

 in the aorta during a cardiac cycle. 

 The middle curve representing intra- 

 ventricular pressure shows at 1 a slight 

 elevation (not always present) due to 

 the auricular systole. This is followed 

 almost immediately by the systole of 

 the ventricle, which begins at 2 and 

 occupies the period between 2 and 3 ; 

 it usually lasts from 0*25 to 0'3 second. 

 At first the curve rises very steeply ; 

 the auriculo- ventricular valves close 

 at the point a, and from a to b the 

 ventricle is a closed cavity. At b the 

 intra-ventricular pressure becomes 

 higher than that in the aorta, the 

 semilunar valves open, and blood flows 

 from the ventricle into the aorta during 

 the whole period from b to 3, when the 

 ventricle ceases to contract. The 

 notch in the ascending part of the 

 tracing immediately after the point b is due to the fact that, when the 

 semilunar valves open, the escape of blood from the ventricle is momen- 

 tarily impeded by the inertia of the column of blood in the aorta. 



The portion of the tracing from b to 3 lasts about (H8 second, and 

 is generally known as the systolic plateau ; it resembles a plateau, 

 however, only when the arterial pressure is low, and its usual shape 

 would be more accurately described as the systolic arch. 



FIG. 63. Piper's manometer, 

 planation in the text. 



