io6 



THE MECHANISM OF THE CIRCULATION. 



Valsalva's experiment forced expiration with the mouth and nose shut 

 produces the same effect on the pulse as compression of the veins of the arm. 

 Ey the rise of intra-thoracic and abdominal pressure, the outlet of the veins 

 of the limbs and head and neck is blocked, and the pressure in the venae 

 comites radiales rises to the mean arterial tension. Thus the pad of the 



sphygmograph is ele- 

 vated, the line of the 

 tracing is raised, and 

 the bottom of each 

 pulse curve is cut off 

 (Fig. 71, tracing 1). 



If the brachial 

 artery be occluded at 

 the height of the ex- 

 piratory effort, the line 

 of the tracing does not 

 fall so long as the 

 venous outlets are 

 blocked. It is, on the 

 other hand, lowered so 

 soon as the veins are 

 emptied by one or two 

 deep inspirations (Fig. 

 71, tracing 1, CCG). 



During the expira- 

 tory rise of intra- 

 thoracic pressure in 

 Valsalva's experiment, 

 the blood in the abdo- 

 minal veins is at first 

 driven on into the 

 right heart, and the 

 outlet through the ab- 

 dominal capillaries is at 

 the same time dimin- 



A, Forced expiration ; ished. Owing to these 



B, brachial artery occluded ; C C C, inspirations ; D, com- effects the arterial ten- 

 pression of brachial artery withdrawn. ,. , , 



Tracing 2. Valsalva's experiment. A, Forced expiration of Slon rlses to a sll S nt 

 severe intensity ; B, cessation of expiratory effort followed extent. In the next 

 by inspiration." stage, by the rise of 



Tracing 3. Sphygmograph resting on the femoral artery and intrathoracic pressure, 

 vein of a dog. A. Expiratory spasm ; B. cessation of ,, , 

 expiratory spurn ; C, compression of the femoral vein. the lungs are emptied 



Tracing 4. Passive compression of the thorax, while the of blood, the filling of 

 breath is held. A, Compression applied ; B, compression the right heart is op- 

 withdrawn. pose d t he blood is 

 Tracing 5. Passive compression of the thorax while the , i ,-, 

 breath is allowed to escape. A, Compression applied; con g es 

 B, compression withdrawn. Hill, Barnard, and Sequeira. pheral veins, and the 



pressure in these rises 



to the mean arterial tension. The arterial pressure may then begin to gradually 

 fall, but before any considerable fall has taken place the expiratory effort 

 ceases from exhaustion of the subject of the experiment, and a deep gasping 

 inspiration is taken. During this inspiration the whole of the blood delivered 

 by the right heart is swallowed up in the roomy reservoirs of the expanded 

 lungs ; thus several beats of the left ventricle become abortive (Fig. 71, 

 tracing 2). The radial pulse is then felt with the finger to stop, the face 

 blanches, and the subject turns faint owing to acute cerebral anaemia. 



TRACING 5 



FIG. 71. 

 Tracing 1. Valsalva's experiment. 



