374 Dr. J. R. Bradford and Mr. H. P. Dean. [Feb. 21, 



in one experiment the central end of left vagus was excited for 

 28 seconds and the aortic pressure fell from 112 mm. to 66 mm. Hg, 

 i.e., a fall of 56 mm. Hg. The pulmonary pressure fell from 17 mm. 

 to 14 mm. Hg, i.e., a fall of 3 mm. Hg. 



This pulmonary fall is rather greater in amount than that previously 

 described as occurring after section of the cord in the dorsal region, 

 but it is not too large to be explained on the grounds of a passive effect 

 owing to the large aortic fall. 



It is, however, with stimulation of the posterior surface of the 

 spinal cord that the greatest relative effects are seen. When this 

 mode of excitation is used the rise of pulmonary pressure is frequently 

 as much as one-tenth of the simultaneous aortic rise, i.e., the ratio is 

 higher than with any of the previous methods of experimentation. 



No doubt part of this effect may be due to the direct excitation of 

 the pulmonary vaso-motor fibres, as will be shown below. Probably, 

 however, the result is mostly due to reflex effects dependent on the 

 cord stimulation, and this is confirmed by the fact that excitation of 

 the central end of a divided posterior root of the upper nerves will 

 cause a great relative rise of pulmonary pressure. 



On the other hand, the excitation of the central end of a divided 

 intercostal nerve causes but slight effects both on the pulmonary and 

 on the aortic blood-pressures. Occasionally the central end of an 

 intercostal produces depressive effects similar to those just described 

 for the vagus. 



Having thus determined the relation existing between a given rise 

 of aortic pressure and the coincident passive pulmonary rise, and also 

 the effects resulting from reflex excitation of the cord, vagus and 

 sciatic, we will now pass on to the question of the existence and 

 paths of the vaso-motor fibres. 



If the upper part of the medulla oblongata be excited it will, of 

 course, be found that a large rise of aortic and pulmonary pressure 

 will be observed. If, now, the spinal cord be divided at about the 

 level of the 7th dorsal nerve and its lower end excited, then just as 

 great or perhaps greater rise of aortic pressure will be observed, but 

 the pulmonary rise will be either very small indeed or else entirely 

 absent. 



If the upper part of the medulla be now again excited, the rise of 

 aortic pressure is small owing to the section of the cord, but the pul- 

 monary rise is as great as before. With stronger excitation this rise 

 of pulmonary pressure becomes greater whilst the accompanying aortic 

 rise is still comparatively small. Thus, in one case, the excitation of 

 the lower end of the divided cord caused an aortic rise of 150 mm. Hg. 

 The accompanying pulmonary rise was less than 2 mm. Hg. On now 

 exciting the medulla in the same animal the aortic pressure rose 

 55 mm. only, owing to section of the cord, but the pulmonary pressure 



