CIRCULATION IN THE VEINS. 329 



to the elastic tension of the arteries, and are therefore more 

 marked when this is at its highest. They are also caused to 

 some extent by oscillation of the sphygmograph. Their causes 

 are not thoroughly understood. 



The dicrotic wave is, on the other hand, a very constant and 

 valuable feature of the sphygmogram. It is probably caused by 

 a second wave, which is produced by the closure of the aortic 

 valve, although on this point opinions are at variance. The 

 arteries are already filled when the left ventricle throws in its 

 contents, causing the expansion of the aorta and putting its elastic 

 coat upon the stretch ; when the systole is at an end the elastic 

 recoil upon the blood drives this fluid both forward and backward ; 

 the backward flow closes the aortic valve, and, being suddenly 

 brought to a standstill, a wave of blood is produced which is 

 propagated along the arterial system closely following the pri- 

 mary wave and causing, the dicrotic wave. This is sometimes so 

 marked that it can be felt by the finger, constituting the dicrotic 

 pulse; thus each pulsation of the heart produces 2 pulse-beats. 

 The sphygmograph shows this condition much better than the 

 finger. 



The Plethysmograph (Fig. 174). This is an instrument 

 for recording the volume-pulse i. e. y the increase in the volume of 

 an artery caused by the pulse-wave. It consists of a chamber into 

 which the organ to be experimented upon is inserted and filled 

 with fluid, the opening being closed with a rubber baud. At the 

 other end is a rubber tube communicating with a vessel, in which 

 is also fluid, and on its surface a float with a writing-point attached, 

 which is so arranged as to record on a drum. If the arm is 

 placed in the chamber in the manner illustrated, at every contrac- 

 tion of the ventricle the volume of blood in the arteries will be 

 increased, and a movement be set up in the fluid which will cause 

 the float to rise ; when the diastole occurs the float will sink. 

 The record made is called a plethysmogram (Fig. 175). 



CIRCULATION IN THE VEINS. 



The forces which propel the blood through the arteries and 

 capillaries i. e., the contractile force of the ventricles and the 

 elastic force of the arteries, collectively called the vis a tergo are 

 sufficient to carry the blood back to the heart through the veins ; 

 for, as has been stated, the pressure in the aorta is equal to a 

 column of mercury 200 mm. in height, while in the veins it is at 

 most only 5 mm., and sometimes actually negative, so that there 

 is a difference in pressure of 195 mm. of mercury. This visa tergo 

 is, however, aided by two other forces : (1) compression of the 

 veins and (2) aspiration of the thorax. 



Compression of the Veins. It will be remembered that 

 in the veins there are, at different points along their course, valves 



