UPON THE VASCULAR SYSTEM. 129 



These results receive a feasible explanation by the supposition that the vagus, 

 like other mixed nerves, contain fibres which excite, and those which inhibit 

 contraction of the vessels. Because the vagus is a mixed nerve, the results 

 of its division must obviously be more trustworthy than the results of its 

 stimulation. We cannot suppose that while during digestion influences pass 

 from the stomach through the vagi to inhibit the gastric vessels, there are 

 also influences travelling from the same source which produce an opposite 

 effect ; and, therefore, we may expect that when we divide these nerves 

 during the dilatation of vessels which obtains during digestion, we shall simply 

 stop the transit of those vaso-inhibitory influences from the stomach, hence 

 the division of such nerves is a much simpler case than artificial stimulation, 

 seeing that during such stimulation we must throw into play fibres whose 

 functions are antagonistic. 



Seeing that the experiments just given show what are the evident changes 

 in the gastric vessels that follow stimulation of the vagi, it is convenient to give 

 here results of experiments which show the effect of this stimulation upon the 

 arterial blood-pressure. 



Effect upon the Arterial Blood-Pressure which follows Stimulation of 

 the Vagus after its section in the Cervical Region. 



(a.) Stimulation of the Lower End of the Nerve. 



If the lower end of the vagus be stimulated by a sufficiently powerful cur- 

 rent, the heart's action is retarded, the work done by that organ is diminished, 

 and in consequence the arterial blood- 

 pressure falls. The following tracing by 

 Ludwig's Kymograph from the carotid 

 artery of a rabbit affords a good illustra- 

 tion of the above fact. The tracing must 

 be read from right to left. The vertical 

 lines have been added to show when the 

 vagus was at rest and when it was stimu- A %^niatJtf^ Bef ° re ~ 



lated. The portion of the tracing between 



the two lines shows the influence of the cardio-inhibitory fibres of the vagus 

 upon the heart. 



The above trace shows the influence of the vagus upon the heart, but it con- 

 tains no indication as to whether or not the vagus contains vasomotor fibres. 

 This fact can only be ascertained by stimulating the nerve after its influence 

 over the heart has been got rid of. I accordingly paralysed the cardio-inhibi- 

 tory fibres by sulphate of atropia, and then stimulated the lower end of the 

 nerve as before. The results were various. If the animal were not paralysed 



