214 



CIRCULATION. 



t mm 



(b) Vein choose one of the large veins on the back of the 

 hand, and find the positions of two valves in its course. 

 Place the hand flat on a chair close to that upon which the 

 subject is seated. Hold the sphygmometer upon the distal valve, 

 and empty the vein above by pressing a finger along it heart- 

 ward, and determine the pressure which must be 

 exerted to prevent the vein from filling from 

 the periphery. This will just balance the blood 

 pressure. 



Pressure in the capillaries. Lay the subject's 

 hand palm down on the table, rest the square 

 end of the little glass staff upon the skin at 

 the root of the nail of the middle finger, and 

 press upon the upper end of the staff with the 

 ampulla of the sphygmometer until the skin just 

 blanches, as seen through the end of the staff, 

 and read the pressure. The end of the staff has 

 an area of half a centimetre square and itself 

 weighs 1 gramme. 1 Add this to the pressure indicated by 

 the sphygmometer ; the sum multiplied by four gives the 

 capillary pressure per square centimetre. 



The pressure of a vertical column of mercury one millimetre long 

 and one quarter of a square centimetre in section, presses with a force 

 of 1-359S. 



Rate of blood flow in the capillaries. Arrange the 

 web of a frog that is little pigmented under the microscope 

 under the high power, and adjust a chosen length of capillary 

 across a space of the eye-piece micrometer, and determine its 

 length. 



FIG. 49. 

 Glass Staff 

 for taking* 

 the capillary 

 pressure with 

 the sphygmo- 

 meter. 



i A column of mercury 760 mm long exerts a pressure of r033 kilo upon the square 

 centimetre; e.g., one atmosphere. 



