136 



THE VASCULAR SYSTEM 



under ordinary conditions. Moreover the pressure so applied is 

 spent largely in deforming the horny convex plate of the epidermis, 

 and in the case of v. Kries' method the size of the horny layer 

 which is depressed is much larger than the square glass plate. 

 This can easily be seen by taking a set of broom bristles and fasten- 

 ing them to matches in lengths of about an inch, and then finding 

 the bristle which will just blanch the skin when pressed on it till 

 it bends. It can be seen that the bristle depresses a horny plate 

 selected, say, on the back of the hand, much larger than its own 

 sectional area. If some of the horny layer be carefully removed 

 with a razor (without producing hypereemia) a weaker bristle will 

 effect the blanching. Observations of this kind show that these 

 methods are too inaccurate to use as a measure of capillary pres- 

 sure ; nor can they be used to give exact comparative differences of 

 pressure under varying conditions, for the error cannot be taken 

 the same all through. V. Kries found that the blanching pressure 

 at the root of the nail did not give the full gravity effect on change 

 of posture. Here the time of fixation in any posture is an im- 

 portant and undetermined factor. 



Lewy has calculated the pressure required jx> overcome the re- 

 sistance in the capillaries from the known facts concerning pressure, 

 velocity, and the viscosity of the blood. The average length of the 

 capillaries is 0'4 to 0'7 mm., their radius 0*0045 mm., and velocity 

 0'5 to 0*9 mm. per second. There flows per second through a capillary 

 a quantity varying from IT x 0*5 x 0'0045 2 up to tr x 0*9 x 0'0045 2 c.c. 

 From the Poisenille formula determined for flow in capillary glass 



tubes Q = K x -j-, where Q = the quantity per second, h the pressure 



difference between beginning and end of tube, d the diameter and I the 

 length of the tube, and K a constant representing the coefficient of 

 viscosity, from this formula Lewy calculates h to be equal to 10 to 

 150 mm. of blood. The mean arterial pressure being taken as 115 mm. Hg 

 or 1500 mm. of blood the full of pressure in the capillaries is T }^ of this 



