174 CONDITIONS AFFECTING CAPILLARY PRESSURE. 



parent membrane covering the upper end of a small brass cylinder, whose lower 

 end contains a piece of glass fitted air-tight into it. The interior of the brass 

 cylinder communicates by means of a tube with an arrangement for obtaining any 

 desired pressure, and the amount of the pressure is indicated by a manometer. 

 Air pressure is used, and this is obtained by compressing a caoutchouc bag between 

 two brass plates. The membrane to be investigated lies between two transparent 



Fig. 79. Fig. 80. 



Apparatus used by v. Kries for estimating the capillary pressure a, the small 

 square of glass. In Fig. 79 the scale-pan for the weights is below, and in 

 Fig. 80, above. 



media, an upper one of glass and a lower one of transparent membrane, on which 

 the pressure acts. Any change in the vessels is observable by means of the micro- 

 scope. These observers conclude from their experiments that the capillaries are 

 contractile, and that their contractility is, to all appearance, in constant action. 

 The regulation of the peripheral blood-stream is due not only to the cerebro-spinal 

 vaso-motor centres, but also to independent peripheral vaso-motor mechanisms, 

 which may be nervous in their nature, or are due to some direct action on the walls 

 of the vessels (p. 126).] 



Conditions influencing Capillary Pressure. The intra-capillary blood- 

 pressure in a given area increases (1.) When the afferent small 

 arteries dilate. When they are dilated the blood-pressure within the 

 large arteries is propagated more easily into them. (2.) By increasing 

 the pressure in the small afferent arteries. (3.) By narrowing the 

 diameter of the veins leading from the capillary area. Closure of the 

 veins may quadruple the pressure (v. Kries). (4.) By increasing the 

 pressure in the veins (e.g., by altering the position of a limb). A 

 diminution of the capillary pressure is caused by the opposite 

 conditions. 



Changes in the diameter of the capillaries influence the internal pressure. We 

 have to consider the movements of the capillary wall itself (protoplasma-movements, 

 Strieker p. 125), as well as the pressure, swelling, and consistence of the surround- 



