2S6 CIRCULATION OF THE BLOOD. 



production, and therefore that the relation between cause and 

 consequence is in this case not dependent on the lengthening 

 of the systole by resistance, as supposed by Marey. The 

 question, however, remains, whether the mechanical explana- 

 tion may not be accepted as regards the remainder of effect 

 which is observed after the vagi are divided. There are two 

 reasons why this is not possible. One is, that here, as in other 

 cases when the pulse rate is retarded, the retardation does not 

 signify that the S3 r stole is lengthened, but that the diastolic 

 intervals arc more protracted. The other reason is, that even 

 after section of the vagi, the retardation of pulse produced by 

 increased arterial pressure is postponed, whereas if it were 

 merely mechanical it would certainly be immediate. We must 

 therefore turn to the nervous system for its explanation 

 either to some influence exercised on the heart by means of 

 accelerator nerves, which after section of the vagi are the only 

 channel by which the heart is in communication with the cere- 

 bro-spinal centres, or to excitation of the inhibitory nerves in 

 the heart itself. Considering that in the frog the same effects 

 are produced by exciting the ganglion of the vagus in the cut- 

 out heart as by exciting the vagus itself, and that we have 

 no reason to believe that increased pressure produces any 

 paralyzing influence on the accelerators, we need have little 

 hesitation in concluding that the effect of increased blood- 

 pressure in retarding the heart's rhythm is exercised entirety 

 through the inhibitory heart-nerves ; and that it is due princi- 

 pal^' to the increased supply of blood to the intra-cramal vagus 

 centre i. e., to the medulla oblongata, but partly also to the 

 influence of the increased endocardial pressure on the vagus 

 ends in the heart itself. 



80. Demonstration of the Functions of the Accele- 

 rator Nerves. It has been already seen that when, after 

 severance of the spinal cord just below the medulla oblongata, 

 the organ is excited electrically below the section, two effects 

 are produced the arterial pressure, reduced by the section, is 

 enormously increased, and the heart beats much more fre- 

 quently. Bezold thought that both of these effects were due 

 to the direct action of the spinal cord on the heart. Ludwig 

 and Thiry showed that, as regards arterial pressure, this was a 

 mistake. They also showed that the acceleration of the pulse 

 was in part a secondary effect of the increased resistance to the 

 flow of blood ; for they found that even after the complete 

 severance of all nervous communication between the heart and 

 the spinal cord, the pulse became markedly more frequent an 

 excitation of the cord. Hence Ludwig was led to doubt 

 whether, after all, the central nervous system exercised any 

 direct accelerative influence on the heart. We now know that 

 while v. Bezold was wrong in believing that the spinal nerves 



