BLOOD-PRESSURE AND BLOOD-VELOCITY. 517 



them to a common level, that is, to the level of the heart at the cos- 

 tal angle (p. 507). 



Accessory Factors Aiding the Circulation. The force of the 

 heart beat is the main factor concerned in the movement of the 

 blood, but certain other muscular movements aid more or less in 

 maintaining the circulation as it actually exists in the living animal, 

 particularly in their effect upon the flow of blood in the veins. 

 The most important of these accessory factors are the respiratory 

 movements and the contractions of the muscles of the limbs and 

 viscera. At each inspiratory movement the pressure relations are 

 altered in the thorax and abdomen, and reverse changes occur dur- 

 ing expiration. These effects influence the flow of blood to the 

 heart, and alter the velocity and pressure of the blood in a way that 

 is described in the section on Respiration under the title of The 

 Respiratory Waves of Blood-pressure. In brief, it may be said 

 that the main effect of the respiratory movements is to force or to 

 suck blood from the large veins of the abdomen and neck into the 

 large thoracic veins, and, therefore, into the right side of the heart. 

 Keith* has called attention to the fact that the system of large 

 veins in the thorax and abdomen, namely, the superior and in- 

 ferior venae cavse, the innominate, iliac, hepatic, and renal veins 

 constitute what he calls a venous cistern, whose capacity may be 

 reckoned as about 400 to 500 c.c. This cistern is shut off below 

 from the veins of the lower extremity by the valves in the femoral 

 veins at their entrance into the pelvis; it is shut off from the veins 

 of the upper extremity by valves in the subclavian veins, and from 

 the veins of the neck and head by the jugular valves. When an 

 inspiration is made, the lowered pressure in the thoracic cavity 

 aspirates blood from the veins in the neck and upper extremities 

 into the superior cava, and on the return to the expiratory position 

 blood cannot be forced back owing to the jugular and subclavian 

 valves. In the same way the lessened intrathoracic pressure during 

 inspiration must tend to aspirate blood from the abdominal portion 

 of the inferior vena cava into the thoracic portion, and this move- 

 ment of blood into the thorax is probably aided by the rise in 

 pressure in the abdomen caused by the descent of the diaphragm, 

 since an increase of pressure in the abdomen would be prevented 

 from driving blood toward the legs by the presence of the femoral 

 valves. The play of the respiratory movements must, therefore, 

 constitute a constant factor tending to empty the venous cistern 

 into the right heart, and in this way promoting the circulation on 

 the venous side. Contractions of the skeletal muscles must also 

 influence the blood-flow. The thickening of the fibers in con- 

 traction squeezes upon the capillaries and small vessels and tends 

 * Keith, "Journal of Anatomy and Physiology," 42, 1, 1908. 



