n6 THE MECHANISM OF THE CIRCULATION. 



is laid on a glass plate, and compressed by a thin and transparent 

 membrane, which forms the base of a glass capsule. The latter is filled 

 with water, and connected with a pressure bottle and manometer. 



They found that a pressure of 100 to 150 mm. H 2 is sufficient to 

 stop the circulation in the capillaries and veins of the frog's web ; to 

 expel the blood from the arteriole 200 to 350 mm. is necessary. From 

 time to time the pressure varies 20 to 30 mm. H>0. On producing 

 splanchnic paralysis, by striking the abdomen, the pressure sank to zero, 

 and then rose again in the veins to 70 or 100 mm., owing to venous 

 congestion. Temporary amemia of the web caused dilatation of the vessels, 

 and this produced in its turn a higher capillary pressure. 



v. Kries found that, in the sitting posture, the position of the arm 

 influenced the capillary pressure in the fingers very markedly. Thus 

 the pressure varied with the distance from the vertex of the head in 

 the following way : — 



The pressure is seen not to increase by the full hydrostatic 

 effect, and v. Kries failed to explain this fact. It is probably due 

 to a compensation for the effect of gravity, which is brought about by the 

 vasomotor mechanism. In the ear, v. Kries found the mean capillary 

 pressure to be 20 mm. Hg; in the gums of the rabbit 33 mm. Hg; 

 or one -third of the aortic pressure. 



If the finger be ligatured, so as to block the venous exits, the 

 capillary pressure rises to the statical arterial tension ; after loosening 

 the ligature the capillary pressure remains for some minutes at a high 

 level, owing to the paralytic dilatation of the arterioles produced by the 

 venous blood. 1 



In the brain, the capillary venous pressure can be estimated by 

 finding the tension which is just sufficient to balance that which the 

 brain exerts against the skull (see p. 143). This pressure in the hori- 

 zontal position of the animal is usually about 10 mm. Hg. The capillary 

 pressure varies widely with changes in the general venous and arterial 

 pressures, and with the position of the animal. Thus, in the brain, 

 the pressure may fall below zero in the vertical feet-down position, 

 and rise to almost 50 mm. Hg during the height of strychnine convul- 

 sions. 



The capillary pressure stands in closer relationship to the venous 

 pressure than to the arterial pressure. Between an artery and its 

 capillaries lies the unknown and varying resistance of the arterioles ; 

 between the capillaries and vein there is no such resistance. It is most 

 fallacious to argue that a fall of arterial pressure necessarily produces a 

 fall of pressure in all the capillary areas of the body. In the want of a 

 good method of direct determination of the capillary pressure, we can 

 only judge of changes which take place under varying experimental con- 

 ditions in that pressure, by recording the tension in both the afferent 

 and the efferent vessel. Thus, for example, in dealing with the capillary 

 pressure in the liver, the tension both in the portal vein and vena cava 



1 Natanson, Arch. f. d. gcs. Physiol., Bonn, 1886, Bd. xxxix. S. 386. 



