1048 PHYSIOLOGY 



as the products of their metabolism. There is evidence that in some 

 situations the cells forming the capillary wall may be contractile. 

 According to Strieker and others, the cell substance is arranged in 

 strands running from the nuclei around the capillary. By the con- 

 traction of these strands the vessel may be narrowed to obliteration. 

 These phenomena have been observed in the nictitating membrane 

 of the frog, but it is doubtful how far they may be extended to the 

 other capillary systems of the body. If the contractile power is at all 

 universally present, it must play an important part in determining the 

 amount of blood-flow through the capillaries of an organ, and will 

 doubtless be largely affected by chemical substances 

 produced as the result of the metabolism of the sur- 

 rounding tissues. 



The average length of a capillary is between 0'4 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 velocity 

 determined in this way varies from about 0'5 to 0'8 mm. 

 per second. Since the red corpuscles travel in the 

 FIG. 420. Ap- axial part of the stream, the mean velocity of the total 

 Kries for mea- blood will be rather smaller, and may be taken at about 

 suring capillary Q'5 mm. per second. 



blood pressure. . . . 



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. 



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 calculation 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 = 1000 c.nim. H 2 



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



1000 

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



considerable, 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 also 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 



