480 CIRCULATION OF BLOOD AND LYMPH. 



plete in regard to the arterial side of the circulation than for the 

 venous side, and it is important that the latter should be more 

 thoroughly studied. In animals the venous pressures may be 

 determined directly by connecting the vein with a manometer. 

 In man the same facts may be obtained for the superficial veins 

 by determining the pressure necessary to obliterate the vein. 

 A simpler although less exact method (Gaertner), applicable to the 

 veins of the arm and hand, is to note the level, as regards the heart, 

 .at which the vein collapses and disappears from view when the 

 arm is elevated. By means of a special instrument Sewall f has 

 shown that in muscular exercise the pressure in the veins is in- 

 creased. External cold lowers venous pressure, while heat increases 

 it, and during digestion venous pressure in some cases is lowered 

 and in others raised, depending on the nature of the adjustment of 

 the heart to the changes in the splanchnic system. 



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. 

 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 during 

 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. Contractions of the skeletal 

 muscles must also influence the blood-flow. The thickening of the 

 fibers in contraction squeezes upon the capillaries and small vessels 

 and tends to empty them. On account of the valves in the veins 

 the blood is forced mainly toward the venous side of the heart; 

 so that rhythmical contractions of the muscles may accelerate the 

 circulation. The contractions of the smooth muscles, especially 

 in the stomach and intestines during digestion, have a similar effect. 

 The musculature of the spleen also is supposed to aid the circulation 

 through that organ by its rhythmical contractions. 



The Conditions of Pressure and Velocity in the Pulmonary 

 Circulation. The general plan of the smaller circulation from 

 right ventricle to left auricle is the same as in the major or systemic 

 circulation, and the same general principles hold. The right 

 ventricle pumps its blood into the pulmonary artery, and, on ac- 

 count of the peripheral resistance in the lung capillaries, the side 

 pressure in the artery is higher than in the capillaries, and higher in 

 these than in the pulmonary veins. The velocity of movement is 

 * Sewall, "Journal of the American Medical Association, " 1906, xlvii., 1279. 



