1912.] Relation between Capillary Pressure and Secretion. 453 



tension be kept down so that the wound may heal, in the other case the 

 contents of the eyeball are diminished by the pressure and become set at a 

 lesser volume. It takes time for aqueous, blood, and tissue volume to return 

 to the old volume when the pressure is removed. The capacity of the capillary 

 vessels may be actually lessened by long applied pressure ; both tissue and 

 capillaries atrophy. 



Those who hold the mechanical theory of filtration of the aqueous assume 

 that there is a hydrostatic difference of pressure on either side of the capillary 

 wall. This assumption requires positive proof. It has never, in our opinion, 

 been demonstrated that the pressure in the capillaries is higher than that of 

 the surrounding tissue, or of the secretory fluid : neither in the case of the 

 glomerular secretion of the kidney, nor in the case of lymph formation, and 

 certainly not in the case of the cerebro-spinal or aqueous fluid. It has been 

 assumed that if the arterial and venous pressures are raised, that in the 

 capillaries must necessarily be raised also. This is not so. In the limbs the 

 capillaries are emptied by the compressive action of the muscles and by 

 change of posture. We have shown that in the dependent position of a limb 

 there may be a high pressure in the arteries and, owing to gravity, a very 

 considerable pressure in the large veins. But at the same time there may be 

 no positive pressure in the capillaries after they have been emptied by 

 muscular movement. Anyone can see that this is so, by holding his hand 

 down and clenching his fist several times. Each time the capillaries are 

 blanched, the valves in the veins permit this pumping action, and until the 

 capillary bed is again filled to distension no positive pressure results. 



The respiratory movements, the intestinal contractions and the move- 

 ments of the skeletal muscles are all designed to pump the blood 

 from the capillaries and keep a low pressure within them in any posture. 

 Distension of the capillaries leads to a sense of discomfort which results in 

 movement. In a case of hydrocephalus under Dr. Warner in the London 

 Hospital the fluid was drawn off; after the introduction of the needle, we 

 measured the pressure by the air index and counter-pressure method. It 

 equalled about 20 cm. of water. On lifting up the head of the child the 

 pressure of the fluid did not fall, as one might expect under the influence of 

 gravity, but rose owing to the compressive action of the hands which raised 

 the head, a bag of fluid. It is obvious in such a case that the cerebral- venous 

 pressure and the fluid must be the same, and that as the fluid pressure is 

 raised by compression of the head so must the cerebral-venous pressure rise 

 pari passu, since it is not conceivable that the wall of the pial veins could 

 support any part of the fluid pressure. On drawing off the fluid the pressure 

 sank to atmospheric pressure and the soft wall of the skull lost all its 



