BATE OF MOVEMENT OF THE BLOOD 209 



measure, not the mean lineal velocity of the inflowing and outflowing 

 blood, but rather how many cubic centimeters of blood are traversing 

 the part per 100 grams of organ or tissue per unit of time. Such meas- 

 urements may be made in a variety of ways. If there are but one artery 

 and one vein to the part, the stromuhr may of course be employed, and 

 it may be inserted in either the arterial or the venous circuit. For 

 measuring the mass movement of blood through such large viscera as 

 the liver, this is indeed the only method that can be employed. The 

 stromuhr is inserted either in the course of the portal vein and he- 

 patic arteries, or, better still, in the vena cava just below the openings 

 of the hepatic vein, the vena cava being shut off for a moment between 

 the liver and the heart, and the blood, as it flows from the hepatic vein, 

 allowed to collect in the stromuhr. For other organs and tissues, how- 

 ever, methods which do not involve any interference with the blood 

 vessels may be employed. One of these is the so-called plethysmo graphic 

 method of Brodie. An organ, such as the kidney, is enclosed in a plethys- 

 mograph (see page 235), and while a record of its volume is being 

 inscribed on a quickly revolving drum, the vein is suddenly clamped, 

 with the result that the kidney volume expands in proportion to the 

 mass of blood flowing into it. When the expansion has reached a cer- 

 tain degree, the clamp is removed and the bloodflow allowed to pur- 

 sue its course. It is then an easy matter, by graduating the plethys- 

 mograph, to determine how many cubic centimeters of blood must have 

 flowed into the organ in a given time. To avoid serious local asphyxia 

 in the tissue, the clamp must be applied to the vein for only the briefest 

 period of time. This method may also be employed for measuring the 

 bloodflow through the extremities. Thus, if the arm is enclosed in the 

 plethysmograph (Fig. 62) and a band encircling the arm above the 

 plethysmograph is tightened so as to constrict the veins but not the ar- 

 teries, the rate at which the volume of the arm within the plethysmograph 

 expands will correspond to the rate at which blood is flowing into it 

 (Hewlett). 



For the purpose of measuring blood flow through the upper or lower 

 extremities, a much more serviceable clinical method is that of G. N. 

 Stewart. This depends on the principle that, provided the blood passing 

 from the thorax to the hands or feet is of constant temperature, the 

 rate at which heat is dissipated from the hands or feet will be directly 

 proportional to the rate of movement of the blood through these parts. 

 Fortunately for the method, the hands particularly, but also the feet, 

 are more or less perfect radiators at least they are to this extent, that 

 if the temperature in their environment is not much lower than the 

 temperature of the blood, then while this is traversing the part, it will 



