930 PHYSIOLOGY 



by chemical substances produced as the result of the metabolism of the 



surrounding tissues. 



The average length of a capillary is between 04 and 0-7 mm. The 



velocity of blood-flow can be directly determined by observing under the 



microscope the time taken by any given corpuscle to travel a measured 

 distance on the microscope stage. The mean velo- 

 city determined in this way varies from about 0-5 

 to 0-8 mm. per second. 



The blood-pressure in the capillaries may be 

 measured approximately by applying pressure to 

 the outer surface of the skin or mucous membrane 

 and noticing the point at which blanching of the 

 surface is produced. 



FIG. 418. Apparatus of 

 von Kries for measuring 

 capillary blood-pressure. 



In von Kries' method a small glass plate, from 2 to 

 5 sq. mm. in area, is placed on the last joint of the finger. 

 Attached to this glass plate is a small scale pan on which 

 weights are placed until the pressure is just sufficient to 

 blanch the underlying skin. In using this method the cal- 

 culation of the capillary pressure is made as follows : 

 Supposing that the size of the glass plate is 4 sq. mm. and 1 grm. in the scale pan is 

 just sufficient to cause a change of colour in the skin, then 



a weight of 1 grm. = 1 c.c. H 2 O = 1000 c.mm. H 2 O 



is present on an area of 4 sq. mm. The height of the column of water supported by 

 1 sq. mm. is therefore - = 250 mm. H 2 0. The errors of this method are consider- 

 able, since the pressure thus determined is not the total capillary pressure, but this 

 minus the pressure in the tissue spaces on the outer side of the capillary wall. The 

 result will therefore vary not only with capillary pressure but ako with the tension of 

 the skin and the amount of fluid in the tissue spaces. 



The pressure in the capillaries as found by this method necessarily varies with the 

 position of the part under investigation, i.e. with the hydrostatic pressure of the column 

 of blood between it and the heart. The following figures were found by von Kries : 



Finger : Mm. H 2 O 



Distance of finger 

 below head 



328 . . mm. 



329 .. 205mm. 

 513 . . 490 mm. 

 738 . . 840 mm. 



Ear : 20 mm. Hg. 

 Gums of Babbits : 33 mm. Hg. 

 Frog's Web (Roy) : 100-150 mm. H 2 O. 

 Capillary venous pressure of brain (Hill) : 



(1) Animal in horizontal position : 10 mm. Hg. 



(2) ,, ,, feet-down position : zero or less. 



(3) During strychnine convulsions : 50 mm. Hg. 



Owing to the fact that a varying and unknown resistance that of the 

 arterioles lies between the capillaries and the arteries, the pressure in 

 the capillaries must stand in much closer relationship to that in the veins 

 than to that in the arteries. One cannot therefore argue that a fall of 



