AND BLOOD PRESSURE 137 



at the least or T V at the greatest estimate. So wide a difference makes 

 the calculation of little value except as an indication of the fact that 

 there is little resistance in the capillaries. In making such estimates 

 the velocity of flow in the capillaries is observed in their membranes 

 with the animal motionless and in the horizontal posture. We have, 

 moreover, no reason to think that the same laws hold good for glass 

 tubes and the living capillaries. The cross section of a capillary 10 /* 

 in diameter is some TirsTnr sq. mm., the outflow per second will be 

 TJiinr x i *f the velocity is 0*5 mm. per second, or 1 c.mm. in about 

 six hours, or 1 c.c. in 250 days (Stewart). 



The writer has measured the capillary-venous pressure in the 

 brain by finding the pressure which just overcomes the pressure 

 with which the brain bulges into a trephine hole. The " brain 

 pressure " is due to the blood pressure in the capillaries and veins 

 which is left over after the resistance in the arteries has been over- 

 come, and distends the brain substance. When the circulation 

 ceases the brain collapses away from the trephine hole and no 

 longer exerts any pressure. The intra-cranial pressure cannot be 

 greater than that in the pial veins, for otherwise these would be 

 obliterated, and the pressure within them must then rise to the 

 intra-cranial pressure. Observation can be made by screwing a 

 glass tube into the trephine hole, the tube being filled with water 

 and closed by a flaccid thin rubber membrane at the brain end, 

 and connected to pressure bottle and manometer at the other end. 

 An air bubble is introduced info the glass tube to mark the zero 

 pressure. When the tube is put in place, the air bubble is pushed 

 outwards, and on raising the pressure bottle and pushing it back 

 to zero the " brain pressure " is balanced and the manometer 

 indicates its amount. 



In the horizontal ansBsthetised animal the " brain pressure " 

 is about 100 mm. H 2 0, sinking even to in the vertical head up 

 position, and rising considerably in the vertical head down posture. 

 The capiUary pressure in the brain varies proportionately with the 

 rise and fall 01 vena cava pressure time being granted for the 

 capillaries to follow the alteration of venous pressure. The brain 

 pressure does not follow in exact proportion alterations of aortic 

 pressure, because between the capillaries and the aorta is placed 

 the varying resistance of the arterioles. The cerebral venous 

 pressure can be measured by inserting a tube into the torcular 

 Herophili, and the pressure of the cerebro-spinal fluid (the lymph 



