1889.] The Innervation of the Pulmonary Vessels. 373 



yet none of them causes anything like a doubling of the pulmonary 

 pressure. 



In other words, when a great aortic rise has succeeded in producing 

 a pulmonary rise, the latter is not only small relatively to the aortic 

 rise but also relatively to the pulmonary pressure itself. We may 

 conclude that not only must a great rise of aortic pressure occur in 

 order to produce any appreciable rise of pulmonary pressure, but also 

 that this rise must be of some duration. 



The further discussion of the mode in which arise of aortic pressure 

 produces a rise of pulmonary pressure will be entered into at the close 

 of this communication. Having thus described shortly what may be 

 called the mechanical effects of rises of aortic pressure on the pul- 

 monary circulation, we will now consider the results of reflex 

 excitation of such nerves as the sciatic and vagus. 



Results of Excitation of the Central End of the divided Sciatic. 



It is well known that the rises of aortic pressure produced by the 

 excitation of this and other afferent nerves are frequently very con- 

 siderable. This is especially the case with the sciatic nerve. 



In one case the stimulation of the central end of this ner.ve gave an 

 aortic rise of 36 mm. Hg, and the accompanying pulmonary rise was 

 only 2 mm. Hg, i.e., one-eighteenth of the aortic rise, that is to say, 

 nearly the same ratio as that obtained in the previous experiments 

 described above in the passive reaction of the pulmonary vessels to 

 rises of general arterial tension. In another instance, with an aortic 

 rise of 30 mm. Hg, there was no simultaneous pulmonary rise. 



Eesults following Excitation of the Central End of divided Vagus. 



With this nerve somewhat different results are obtained. 



Thus, in one case, the aortic rise was 32 mm. Hg and the pulmonary 

 rise 4 mm. Hg, i.e., the relative ratio of the two effects being one-eighth. 

 This result was obtained in the same animal that previously gave with 

 the sciatic a ratio of one-eighteenth. In the case of the vagus the 

 pulmonary rise was double that observed with the sciatic, although the 

 aortic rises were almost the same in the two cases, i.e., 36 mm. and 

 32 mm. Hg. It is clear then that, although in this animal the vagus 

 and sciatic gave on stimulation practically equal effects in the systemic 

 vessels, yet the results on the pulmonary vessels were by no means the 

 same in the two cases. Hence the only conclusion is that excitation of 

 the central end of the divided vagus caused a reflex contraction of the 

 pulmonary vessels and thus caused a heightened pulmonary tension. 



In the cat frequently and in the dog occasionally the stimulation of 

 the central end of the vagus causes a fall of blood-pressure instead of 

 a rise, in many cases the fall of aortic pressure is considerable. Thus 



