282 THE CIRCULATION OF THE BLOOD 



by Stewart that T may be taken as the same as that of the mouth, or 0.5° 

 C. below that of the rectum, and 2" as the average temperature of the 

 water in the calorimeter during the observation. To allow for the specific 



heat of blood, the result is multiplied by-^-, the reciprocal of the specific 



heat of blood. 



Theoretically, then, the method is very simple, and there are no un- 

 usual technical difficulties in applying it. The only special precaution 

 is that the air surrounding the calorimeter should be kept fairly con- 

 stant in temperature, so that we may be enabled to allow in our calcula- 

 tions for the loss of heat from the calorimeter itself, this value being 

 obtained by observing the change of temperature in the calorimeter for 

 a certain period of time after the hand has been removed from it. 



The Normal Flow 



The results are calculated on the basis of grams of blood flowing 

 through 100 c.c. of tissue in one minute. The volume of the hand or foot 

 is ascertained by placing it in water contained in a small-sized irrigation 

 can, the tube of which is connected with a burette. The height to which 

 the water rises in the burette is noted, and after withdrawing the hand, 

 water is added from a graduate to the irrigation can until the same 

 height is reached on the burette. The number of cubic centimeters re- 

 quired gives the volume of the hand. In a normal, healthy individual 

 the average flow in the hand is from 12 to 13 gm. for the right hand, 

 and about half a gram less for the left. This difference between the two 

 hands corresponds, of course, with their relative degree of development. 

 The average foot flow is much less, and varies according to whether the 

 patient is sitting up or lying down while the measurement is being made. 

 In a normal individual, while lying down, it was 5.11 gm. in the right 

 foot and 5.23 gm. in the left, per 100 c.c. of foot; but only 2.96 gm. for 

 the right and 4.1 gm. for the left foot, while sitting up. The results have 

 been found to undergo only a slight variation from month to month in a 

 given healthy individual, provided the air temperature during the dif- 

 ferent observations is the same and the person has been some time in the 

 room before the observations are begun.' This precaution is especially 

 important if he is a dispensary patient and has been in the open air with 

 bare hands. The flow varies in different individuals both with regard 

 to absolute amount and the ratio between the two hands or feet. When 

 the total flow in the hands is compared with that in the feet, a ratio of 

 about 3 to 1 is usually obtained. 



'ilu Physiological Causes for Variations in Blood flow. — As above indicat- 

 ed, the most marked of these is probably the temperature of the room. The 



