238 THE CIRCULATION OF THE BLOOD 



If the writing points of the recording levers in this experiment are made 

 of very delicate strips of note paper, so as to offer little resistance to the sur- 

 face of the drum, the detail of the pulse wave at the two points will be 

 accurately transcribed and may be compared. 



17. The Capillary Circulation. The capillary circulation is best 

 demonstrated in the laboratory by direct observation on the web of the frog's 

 foot by the use of the compound microscope. Give a 40-gram frog a hypo- 

 dermic injection of 0.3 c.c. of ether under the skin of the back. Wet a piece 

 of cheese cloth the size of a handkerchief with tap water and wrap the etherized 

 frog so as to cover the entire body with the exception of the foot. When the 

 anesthesia has progressed so as to destroy voluntary movements, bind the 

 foot on an ordinary frog board and spread the web over the window in the 

 board. Choose an area of the skin which shows small arteries, capillaries, 

 and veins, and in which the blood is flowing freely and rapidly. Examine 

 with a low-power compound microscope. In a favorable field small arteries, 

 capillaries, and veins with blood flowing rapidly through them will be easily 

 found. Choose one such field, cover with, a piece of thin cover glass, moisten- 

 ing with a drop of water if necessary, and examine with a high pow r er. Note 

 in the small artery the pulsating current; the border of clear fluid along the 

 side of the main stream of blood; the slight pulsations; and the white cor- 

 puscles that will be found flowing along the borders of the current. In the 

 small veins there are usually no pulsations and the speed of the current is 

 somewhat less. In the capillaries a careful examination will reveal a deli- 

 cate wall, the individual corpuscles, and the fact that the red .corpuscles are 

 actually larger than the diameter of the capillary at some points and must 

 be bent to pass through. Note that the capillaries form an intricate and 

 anastomosing network; ttiat the current may occasionally reverse itself in 

 some of the anastomoses. 



The anesthetizing effect of the dose of ether recommended will usually con- 

 tinue about 15 to 20 minutes. If the observation is more prolonged a second 

 dose of ether should be given. The capillaries in the tails of small fish are 

 of ten very readily observed and these may be substituted for the frog's web. 



18. Capillary Blood Pressure. Measure the capillary blood pres- 

 sure in your own finger by von Krie's method. This apparatus consists 

 of a small piece of glass an inch square, or less, which is placed across the 

 knuckle of the finger just back of the nail. A small weight pan is suspended 

 by a loop of thread over this glass plate so that weights put in the pan will 

 bring varying pressure on the plate above. Add weights to the pan until 

 an area of the skin, about 5 mm. in diameter, is blanched by the pressure. 

 Mark the outline of this bloodless area on the glass, take off the apparatus 

 and measure the exact area of glass so marked, weigh the entire apparatus 

 and compute the pressure per square centimeter for the area. This pres- 

 sure in terms of mercury represents the capillary blood pressure in the vessels 



