224 



same. Tissue fluid pressure and capillary pressure constantly balance 

 each other. 



The capillary pressure stands in far 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 veins there is no such resistance. 



Although often put forward, the view is erroneous, that fluid 

 niters through the capillary wall under the influence of the capillary 

 blood -pressure. No measurable difference in pressure normally exists 

 between the capillary pressure and that of the tissue fluids, such as 

 the aqueous or cerebro-spinal fluid. The wet films of the protoplasm 

 which form the walls of capillaries cannot act as rigid sieve-like struc- 

 tures. The tissue cells are by their colloidal structure endowed with 

 the power of linking up or setting free the crystalloids brought to them 

 in solution. They are the seat of play of complex physical forces, 

 such as imbibition and osmosis, as well as of chemical reaction, selective 

 in character, and dependent on the enzymic contents of the cells. 

 The cells control the passage of fluid in one of the other directions 

 in just the same way as do the unicellular organisms, in which, as 

 regards the secretory processes, there can be no question of filtration. 

 This view is illustrated in Figs 119A, 119B, 119c. 



Apart from other experiments which tell against the filtration 

 hypothesis, and quoted in their respective sections, there remains 

 the outstanding fact that in the brain and eye, where measure- 

 ments have been made, the capillary-venous pressure and the tissue- 

 fluid pressure are not measurably different. Moreover, the membranse 

 propria? are arranged to allow the tissue cells to produce osmotic and 

 secretory pressures, not to support the capillaries as rigid filtering 

 membranes. Leakage occurs when the skin, or capsule fan organ, is 

 wounded, because capillary pressure is then no longer balanced by 

 tissue fluid pressure. 



The organs rhythmically pulse full with systole and shrink with 

 diastole, and the pulse furthers the flow of tissue lymph as well as 

 the circulation. Every muscular movement, by compressive action 

 and the action of valves in veins and lymphatics, aids the circulation. 

 The membranae propriae, by limiting expansion, allow secretory cells 

 to raise the pressure of the tissue fluid and produce, for example, the 

 mtra-ocular pressure. The capillary-venous pressure within the eye- 

 ball adjusts itself to this secretory pressure, for the pressure must 

 just exceed the intra-ocular pressure for the circulation to continue. 



