THE FLOW OF BLOOD IN THE VEINS 223 



this is expressed as the length of a tube through which just as much blood 

 would flow in unit time as flows through the animal body. According to this 

 calculation the resistance is equal to that of a cylindrical tube with the 

 diameter of the aorta and with a length of 296 m. It need scarcely be re- 

 marked that this value relates only to a special case, and that it is adduced 

 here only for the purpose of giving an approximate idea of the amount of 

 resistance in the vascular system. 



7. THE FLOW OF BLOOD IN THE VEINS 



A. PRESSURE AND VELOCITY 



The cubic distention of a vein to internal pressures increasing by equal 

 increments is exactly like its longitudinal distention to loads increasing by 

 equal increments i. e., it becomes less the higher the total pressure becomes 

 (Fig. 92). The veins, therefore, behave differently in this respect from the 



FIG. 92. The cubic enlargement of the inferior vena cava of the cat, under uniformly increasing 



internal pressure, after Roy. 



arteries (cf. page 201). The resistance of the veins to rupture by internal 

 pressure, is, under normal conditions, very great, just as it is in the arteries. 



The essential physiological purpose of the veins is to return the blood to 

 the heart. The force which drives the blood forward in the veins is the force 

 of the heart itself. But the friction in the small arteries and in the capillaries, 

 has by this time consumed the major part of the heart's driving power, con- 

 sequently the total energy with which the blood flows in the veins is but a 

 fractional part of the energy which it possessed as it left the heart. The 

 greater part of that energy has been transformed into heat during the passage 

 of the blood through the arteries and capillaries. 



The lateral pressure in the veins is, therefore, much smaller than in the 

 arteries. In the central veins of the thorax the blood pressure is negative 

 because of the aspirating action of the thorax. In more peripheral veins it 

 is positive and is higher the farther the vein from the heart, e. g., in the 

 right jugular of the sheep, 0.2 mm. Hg., in the external facial 3.0, internal 

 facial 5.2, brachial 4.1, in a branch of the latter 9.0, in the crural 11.4 mm. 

 Hg. ( Jacobson) ; in the superior vena cava of the dog close to its entrance 

 into the right auricle 3.0 mm. Hg., in the distal portion of the same vein 

 -1.4, in the right external jugular -0.1, left external jugular 0.5, in the 

 right brachial 3.9, in the left facial 5.1, left femoral 5.4, left saphena 7.4 mm. 

 Hg. (Burton-Opitz). Observations on the sheep and on the dog, as will be 

 seen, agree on the whole very closely. 



After opening the thorax and thereby obliterating the negative intra- 



