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328 J. A. Mac WILL I AM 



(79) by the same method reported pressures of 17 to 25 mm. 

 Briscoe (21) using Hooker's capsule to cause paling estimated the 

 normal pressure at 23.5 cm. H2O. Hill and McQueen (66) taking the 

 returning flush as their criterion obtained values of about 10 mm. Hg. 



The interpretations put upon the results of blanching experiments 

 naturally depend on the different views held as to the causation of the 

 colour of the skin. There is thus much diversity of opinion as to what 

 is really being measured. While v. Kries, v. Basch, v. Recklinghausen 

 and Basler took the paling of the skin to be due to compression of the 

 capillaries, Lombard (85) and Danzer and Hooker (35) regard the empty- 

 ing of one or more of the venous plexuses in the dermis as the main 

 cause ; they find that paling is not necessarily accompanied by cessation 

 of flow in the capillaries. White (testing the influence of heat, etc.) 

 also concludes that the paling of the skin from external pressure is 

 not an indication of capillary pressure. He obtained values of 4 to 

 19.5 cm. H2O. Hill interprets the 10 mm. value (obtained as stated) 

 as indicating arteriolar, not capillary, pressure, plus the resistance of 

 the epidermis which has to be deducted. When a capillary area is 

 compressed the internal pressure is regarded as banking up to arteriolar 

 pressure. Such an arteriolar pressure value as 10 mm. Hg is about half 

 the amount reported for capillary pressures by many other observers. 

 The real capillary pressure Hill (62) estimates at something like 20 to 

 50 mm. H2O. Observing transparent parts of mammals and frogs by 

 Roy and Graham Brown's (115) method he found such a compressing 

 pressure to cause momentary checking of the blood flow and took this 

 as the true index — as distinguished from a compression stopping the 

 flow, which might require 350 mm. H2O. 



2. Pressure required to cause obliteration of capillaries under the 

 microscope. Lombard (85), who introduced the use of a drop of oil 

 on the skin to permit of direct examination of the capillaries, found 

 very different pressures necessary to obliterate the vessels of different 

 orders. Stated in millimeters H2O, the following values were obtained 

 — subcapillary venous plexus, 135 to 205; superficial venous branches 

 205 to 270, most compressible capillaries, 245 to 300, middle-size capil- 

 laries, 475 to 545, most resistant capillaries and arterioles, 815 to 

 950. Methods founded on Lombard's plan have been used by Krauss 

 (74), Basler (11) (capillary tonometer) and Kylin (78). The last 

 named recorded normal pressures of 8.5 to 14 mm. Hg and abnormal 

 ones up to 40 or 50 mm. in glomerulo-nephritis and scarlet fever. 

 Difficulty arises from the unequal compressibility of different capillaries 

 in the field of observation. 



