308 THE CIRCULATION IN THE BLOOD-VESSELS [Oil. XXII 



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 vaso-constrictor and dilator fibres run separate 

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

 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 bv the use of various forms of manometer, which is the only 



FIG. 269. Plethysmograph. By means of this apparatus, the alteration in volume of the arm E, which 

 is enclosed in a glass tube A, filled with fluid, the opening through which it passes being firmly 

 closed by a thick gutta-percha band F, is communicated to the lever r>, and registered by a recording 

 apparatus. The fluid in A communicates with that in B, the upper limit of which is above that in 

 A. The alterations in volume are due to alterations in the blood contained in the arm. When 

 the volume is increased, fluid passes out of the glass cylinder, and the lever D also is raised, and 

 when a decrease takes place the fluid returns again from B to A. It will therefore be evident that 

 the apparatus is capable of recording alterations of the volume of blood in the arm. 



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 



