974 PHYSIOLOGY 



capillaries before they are finally swept off by the stream into one or the 

 other. 



The capillary wall is composed of a single layer of elongated flattened 

 cells which present little resistance to the passage through them by diffusion 

 of dissolved substances, such as sugar, salts, oxygen, or carbon dioxide. 

 In this way the tissue cells obtain oxygen from the red blood corpuscles 

 and nutriment from the plasma, and give off to the circulating blood carbon 

 dioxide and other effete substances 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 sub- 

 stance is arranged in strands running from the nuclei around the capillary. 

 By the contraction of these strands the vessel may be narrowed to oblitera- 

 tion. 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 

 .surrounding tissues. 



The average length of a capillary is between 04 and O7 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. 



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. 

 Fio. 428. Apparatus of Attached to this glass plate is a small scale pan on which 

 von Kries for measuring weights are placed until the pressure is just sufficient to 

 capillary blood pressure. 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 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 



1000 



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 also with the tension of 

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



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



