UPON THE VASCULAR SYSTEM. 131 



lation always raises the blood-pressure, but if the cerebrum be removed, 

 or if it be paralysed by morphia, vagus stimulation always lowers the 

 pressure. 



This is simply untrue. Stimulation of the nerve may increase or lower the 

 pressure whether morphia narcotism be induced or not. Since I performed 

 my experiments on this subject, Kowalewsky and Adamuk,* Aujbert and 

 RoEVERt have published the results of their researches regarding this question, 

 and I am glad to say that these exactly agree with what I had previously found. 

 Seeing that these authors have already published results similar to mine, I 

 need not do more than briefly say, that when the upper end of the vagus is 

 stimulated, the respiration is very apt, more especially in rabbits, to come to a 

 stand-still. As a result of this, carbonic acid accumulates and oxygen diminishes 

 in the blood, thereby bringing about a condition of that fluid which acts as an 

 irritant to the vasomotor centre in the medulla, and increases the tonicity of 

 the blood-vessels so that the blood-pressure is raised. This source of fallacy 

 must be guarded against by using artificial respiration. Struggling, too, is apt 

 to result from stimulation of the upper end of the vagus, to guard against which 

 we may narcotise the animal by means of opium, or may produce paralysis by 

 curara. When we give opium or curara and then stimulate the nerve, a rise in 

 the blood-pressure is not so frequently observed as when the nerve is stimulated 

 before these poisons are administered; obviously because extraneous convulsive 

 movements have been got rid of. However, whether we give these toxic agents 

 or not, stimulation of the upper end of the vagus in rabbits and cats (where the 

 depressor nerve is a separate branch) may be followed by increase or by 

 diminution of the blood-pressure, most frequently the latter. I have often ob- 

 served that in the same animal a rise or fall of the blood-pressure may be 

 obtained by using for the production of the latter a more powerful stimulus than 

 that which may have been found sufficient to produce the former. The explana- 

 tion of this seems to be, that in the case of the vessels, as in that of the heart, 

 a weaker stimulus suffices to throw the excito-motor nerves into action than is 

 necessary to cause the inhibitory nerves to produce their effect. The following 

 tracings illustrate the results of stimulating the upper end of the vagus. They 

 must be read from left to right. 



The fibres in the vagus, then, which influence blood-vessels, all convey in- 

 fluences towards the medulla oblongata, and these fibres appear to be both 

 vaso-inhibitory and excito-vasomotor, the former causing dilatation of blood- 

 vessels and consequent lowering of the blood-pressure (fig. 5), the latter caus- 

 ing contraction of blood-vessels and consequent increase of the blood-pressure 

 (fig. 6). Doubtless the influences which travel through these two kinds of 



* Centralblatt, 1868, p. 546. + Ibid. p. 578. 



VOL. XXVI. PART I. 2 M 



