CII. XXI.] PLETHYSMOGRAPHY 307 



It is, however, probable that all the vessels of the body receive 

 both constrictor and dilator nerves. But the presence of the latter 

 is difficult to determine unless they are present in excess ; if they 

 are not, stimulation affects the constrictors most. The effect of 

 section is also inconclusive ; for if a mixed nerve is cut, the only effect 

 observed is a dilatation due to removal of the tonic constrictor influence. 



To solve this difficult problem, three methods are in use : 



1. The method of degeneration. If the sciatic nerve is cut, the 

 vessels of the limb dilate. This passes off in a day or two. If the 

 peripheral end of the nerve is then stimulated, the vessels are dilated, 

 as the constrictor fibres degenerate earliest, and so one gets a result 

 due to the stimulation of the still intact dilator fibres. 



2. The method of slowly interrupted shocks. If a mixed nerve is 

 stimulated with the usual rapidly interrupted faradic current, the 

 effect is constriction ; but if the induction shocks are sent in at long 

 intervals (e.g., at intervals of a second), vaso-dilator effects are 

 obtained. This can be readily demonstrated on the kidney vessels 

 by stimulation of the anterior root of the eleventh thoracic nerve in 

 the two ways just indicated. 



By studying the rate of flow of the blood through the submaxillary 

 gland, in which the vase-constrictor and dilator fibres run separate 

 courses, it has been shown that if both sets of fibres are simultane- 

 ously excited, constriction is produced during the stimulation, while 

 marked dilatation follows after the stimulation has ceased. Excitation 

 of the constrictors alone is not followed by dilatation. These results 

 explain the mode of action of slowly interrupted shocks, for with each 

 there will only be a very slight constriction, while the dilator effects 

 which run a much slower course will be summed up to produce a 

 marked effect. 



3. The influence of temperature. Exposure to a low temperature 

 depresses the constrictors more than the dilators. If the leg is 

 placed in ice-cold water, stimulation of the sciatic, even if it has only 

 been recently divided, produces a flushing of the skin with blood. 



Plethysmography. 



The action of vaso-motor nerves can be studied in another way 

 than by the use of various forms of manometer, which is the only 

 method we have considered so far. The second method, which is 

 often used together with the manometer, consists in the use of an 

 instrument which records variations in the volume of any limb, or 

 organ of an animal. Such an instrument is called a plethysmograph. 

 One of these instruments applied to the human arm is shown in the 

 accompanying figure (fig. 301). 



Every time the arm expands with every heart's systole, a little 



