138 



330 J. A. MACWILLIAM 



average pressure in the capillary field as a whole, made up as it is of 

 a great and varying distribution of capillary pressure values in multi- 

 tudinous districts. It is obviously not permissible to speak of capillary 

 pressure in the same sense as arterial pressure, the latter being a defi- 

 nite measurement virtually the same in all the large arteries throughout 

 the body, while capillary pressures vary widely and in different senses 

 in numerous districts under physiological conditions. Still if there is 

 definite association of high readings of pressure in the minute skin 

 vessels in some category of high blood-pressure cases and not in others 

 this — even if not representative of the capillary system as a whole — 

 is obviously a matter of much interest calling for further investigation 

 as to its mechanism and significance. Boas and Mufson report close 

 correspondence in the capillary readings from the same individuals 

 taken many months apart — with some exceptions for which explana- 

 tions are offered. As to the relations of capillary and venous pressures 

 there is some conflicting evidence. While it has generally been accepted 

 that capillary pressures run much more nearly parallel with venous 

 than with arterial pressures. Boas and Doonieff (16) in a recent in- 

 vestigation (using a needle in a vein connected with a manometer) 

 find that a rise in venous pressure up to 39 cm. H2O may have no effect 

 on capillary pressure — evidence that the high capillary pressure which 

 may occur in hypertension is not accounted for by high venous pressure. 

 On the other hand, Danzer and Hooker found venous compression to 

 cause increased capillary pressure; Carrier and Rehberg observed a 

 parallelism between venous and capillary pressures. Von Basch and 

 Kraus had formerly emphasised the close relationship between these 

 pressures. 



The peripheral resistance. Recent work on the capillary system and 

 the very varied conditions that may obtain in it have re-opened the 

 question as to what the peripheral resistance in the circulation is con- 

 stituted by. Is the arterial resistance largely supplemented by resistance 

 in the capillary field and possibly also, as suggested by Hooker's (68) 

 work, in the venules, and capable of being altered in an important 

 degree by variations in these as well as in the small arteries? 



Favouring the commonly accepted view that the chief resistance is 

 in the small arteries is the greater internal friction depending on the 

 relatively rapid rate of flow in the arteries as compared with the slow 

 flow in the capillaries under ordinary conditions ; also such evidence as 

 is available to show that the loss of pressure in passing through the 

 capillaries is relatively small, the great fall from the arterial pressures 



