20 



the two stopcocks, C, C", are properly arranged. For accuracy of control the 

 plunger of the syringe is moved by a screw adjustment. 



After the pipette has been introduced into the capillary the stopcocks are so 

 turned that the pressure is applied to the dye in the pipette by the column of 

 water, W, the height of which can be changed rapidly by the movement of the 

 syringe plunger. To correct for capillarity in the upright tube the instrument 

 has been calibrated by balancing the water column against known pressures. 



For measuring the higher pressures found in the arteries a mercury column is more 

 convenient. This is connected through a U-tube with the water manometer. If the 

 opening at the tip of the tube, W, is open the whole system functions as a column 

 of water, but if it is closed by a ringlet of rubber tubing, T, the pressure ap- 

 plied is measured by the mercury manometer providing correction be made for the 

 column of water between the surface of the mercury and the tip of the pipette. This 

 arrangement allows a more rapid succession of pressure determinations in arterioles 

 and arteries than would otherwise be possible. 



The material used by Landis consisted of mesenteric capillaries of the frog. A loop 

 of the intestine was brough through an incision in the abdominal wall and the mesen- 

 tery laid loosely over a transparent glass stage. The preparation was kept moist 

 and clean by a slow constant drip of- saline solution, 



B. FLOREY METHOD 



A method by means of which fine blood vessels are made accessible to manipulation 

 is also described by Florey. This method seems equally well adapted for study and 

 manipulation in vivo of any group of tissue cells thin enough to allow the penetra- 

 tion of sufficient light for critical observation. 



Florey' s method is as follows i "A trough made from sheet tin was fashioned to ac- 

 comodate the body of the rat. In one side of the trough, at the appropriate place, 

 a rectangular piece of metal was cut away so that the gut, g, with its mesentery, m, 



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