Parasympathetic Vasodilators 127 



choline, chorda stimulation may diminish the venous outflow. 

 Fig. 7.2 shows one of the experiments of Graham and Stavraky. 

 Stimulation of the chorda increased the venous outflow before, 

 but decreased it after the intra-arterial injection of eserine. The 

 "reversal" was particularly marked when the blood pressure was 

 low. Nevertheless, the diminished venous outflow could not be 

 due to the factors discussed by Carlson et al. and by Dale, for 

 control experiments showed that the arterial inflow was similarly 

 diminished. The effect was therefore attributed to a vasoconstric- 

 tion, and numerous instances of other organs in which acetyl- 

 choline causes dilatation in low, and constriction in high concen- 

 tration, were quoted from the literature. 



It may thus be said that it is possible to elicit a vasoconstriction 

 by stimulating the chorda ; but there is little evidence to show that 

 the chorda contains nerve fibres, the normal function of which is 

 to cause a vasoconstriction. The fibres causing the constriction in 

 the experiments of Graham and Stavraky may have been the sec- 

 retory fibres, or fibres which ordinarily cause vasodilatation, or 

 both. 



PARASYMPATHETIC VASODILATORS 



When Claude Bernard (1858) instilled some vinegar into the 

 mouth of a dog he observed that the dark blood emerging from 

 the submaxillary gland quickly became brighter ; this he assumed 

 to be due to a gustatory reflex, the efferent pathway being the 

 chorda tympani, since the response was not obtained when that 

 nerve had been cut. Stimulation of the distal end of the cut nerve 

 gave the same change in colour, and when the vein of the gland 

 was cut open it could be seen that the blood flowed at a much 

 higher speed and could even show pulsations. The effect of the 

 chorda stimulation was obviously to dilate the vessels of the gland 

 to such an extent that the blood passed into the veins "sans perdre 

 l'impulsion cardiaque". 



When the rate of blood flow was measured, chorda stimulation 

 was found to cause a fourfold increase. Some later investigators 

 have described a much bigger augmentation. The outcome prob- 

 ably depends, among other things, on whether the sympathetic 

 constrictor tone has been abolished or not. Even when the sym- 

 pathetic has been cut, chorda stimulation will cause a considerable 

 increase in the blood flow. The chorda, on the other hand, seems 



