Recoil Curves as shown by the Hot- Wire Microphone. 295 



the variations which made their appearance in the normal subject under 

 different conditions. 



Curves were therefore taken before and after meals over a series of con- 

 secutive days at the same hour ; and before and after exercise. 



It was thought at first that the variations in the recoil might bear some 

 simple relation to the blood pressure. A number of healthy individuals 

 were, therefore, examined from this point of view, but yielded no definite 

 evidence that small differences of systolic, diastolic or pulse pressure readings 

 could be correlated with alterations in the size of the recoil. 



As Tigerstedt (13) had shown, with his Stromuhr, that nitro-glycerine 

 increased the minute-volume and lowered the blood pressure, while adrenalin 

 tended to the opposite effect, it was decided to try the effect of small doses 

 of vaso-constrictors and vaso-dilators, liquor adrenalin and nitro-glycerine 

 respectively, taken by the mouth. Accepting Tigerstedt's results as applic- 

 able to the normal human subject, it would seem that enlarged recoils are 

 the result of increases in volume output rather than raised blood pressure, 

 as is clearly shown by fig. 11. In this figure, the only curve showing a 

 definite increase of recoil is (d), i.e., the one taken five minutes after a small 

 dose of nitro-glycerine. Curve (c), taken after a small dose of adrenalin, is 

 not appreciably different from any of the control curves. 



It is true that the dose of liquor adrenalin is very small, and was given by 

 the mouth, but the effect of the nitro-glycerine is very striking. 



Curves were also obtained after taking by the mouth 0'02 grm. of the vaso- 

 constrictor tyramine, a drug which is definitely known to be absorbed, and 

 recoils of diminished amplitude were obtained. 



The results are not conclusive, but, taken in conjunction with the 

 variations during respiration, are suggestive that alterations in the volume 

 output are the chief factors affecting the magnitude of the recoil ; this agrees 

 with the theory of the action of the microphone dealt with above. 



The result of experiments carried out to show the effect of increased heart 

 work on the recoil indicates that all the features of the normal curve 

 become enlarged, especially the smaller peaks. An example of a curve taken 

 before, immediately after, and some time after exercise, is shown here to 

 illustrate this point (fig. 12, Plate 8). 



It would not, however, be justifiable to assume, at this stage, that a 

 curve such as (B) of fig. 10 is produced because the subject's heart is doing 

 more work. 



All the experiments that have been carried out have been directed solely 

 towards ascertaining the nature of a normal curve, and the variations to 

 which it is subject. As a single control to these experiments, the recoil 



