THE BLOOD-PRESSURE IN THE VEINS. 169 



resistance to the blood-current is greatest in the small arteries and in the capillary 

 system, the blood especially in long capillaries must be subject to different 

 degrees of pressure at the beginning and at the end of such capillaries. In the 

 middle of the capillary system the pressure may not be much less than one-half 

 the pressure prevailing in the main arterial trunks. The capillary pressure exhibits 

 many variations in different parts of the body. Thus, in the erect position, the 

 pressure in the capillaries both of the intestine and of the glomeruli of the kidneys, 

 as well as in those of the lower extremities, will be greater than in those of other 

 regions of the body; in the former case on account of the two-fold resistance 

 offered by the duplicate arrangement of the capillaries; in the latter case, from 

 purely hydrostatic influences. 



THE BLOOD-PRESSURE IN THE VEINS. 



In the large venous trunks near the heart (innominate, subclavian, 

 and common jugular veins) the blood is under a negative pressure, 

 which is on the average equivalent approximately to o.i mm. of mercury. 

 This enables the lymph-stream to empty itself freely into the large venous 

 trunks. 



As the distance from the heart increases, the lateral pressure in the 

 venous trunks gradually increases. In the external facial vein of the 

 sheep it is +0.3 mm., in the brachial 4.1 mm., in branches of the brachial 

 9 mm.; in the femoral 11.4 mm. The following conditions influence the 

 pressure in the veins : 



1. All factors that tend to diminish the difference in pressure exist- 

 ing between the arterial and the venous system, which maintains the 

 circulation of the blood, necessarily increases the pressure in the veins, 

 and conversely. 



2. General plethora increases the pressure in the veins, while anemia 

 diminishes it. 



3. A special influence on the tension in the large trunks situated near 

 the heart is exerted by the respiration; for during each inspiration the 

 pressure diminishes and the blood rushes toward the thoracic cavity; 

 while with each expiration the pressure increases and the blood stag- 

 nates. This effect is intensified in proportion to the depth of the res- 

 pirations, and when the respiratory passages are closed it must be par- 

 ticularly great. 



4. The slight stagnation of the blood in the venae cavae that accom- 

 panies every contraction of the right auricle has already been discussed 

 in the section devoted to the movements of the heart The respiratory, 

 as well also as the cardiac, fluctuations can sometimes be detected in 

 the common jugular vein of healthy individuals. 



5. Changes in the position of the limbs or of the body through 

 hydrostatic influences modify the pressure in the veins in various ways. 

 The highest pressure is found in the veins of the lower extremities, and 

 they are accordingly most abundantly supplied with muscle-tissue. 

 When the muscles and valves in these veins become insufficient, dila- 

 tation is likely to develop (varices). 



THE BLOOD-PRESSURE IN THE PULMONARY ARTERY. 



Method. Direct estimation of the pressure in the pulmonary artery was made 

 in 1850 by C. Ludwig and Beutner, who opened the left pleural cavity and con- 

 nected the tube of a manometer directly with the left pulmonary artery, artificial 

 respiration being resorted to. In this way the lesser circulation of the left lung 

 was interrupted completely in cats and rabbits and almost completely in dogs. In 

 addition to this disturbance, the normal flow of the venous blood into the right 



