OF THE HEART. 85 



regulatory nervous system of the heart consists of (a) the 

 intra-cranial heart-centre (spinal accessory nuclei) ; (b) the 

 fibres of the spinal accessory and vagus, which are dis- 

 tributed to the heart, and (c) the accelerator nerves. 

 2. It was discovered by E. H. Weber in 1842 that through 

 the vagus the brain exercises an inhibitory influence on 

 the heart, i.e., that excitation of the cardiac fibres of the 

 vagus either arrests the heart in diastole, or, if less intense, 

 diminishes the frequency of its beats by prolonging each 

 diastolic interval, and thus diminishes the arterial pressure, 

 while it increases the amplitude of the arterial pulsation. 

 Between the excitation and the effect, a delay takes 

 place which (in the rabbit) amounts to second. 3. The 

 effect above described is produced reflexly by excita- 

 tion of various afferent nerves, e.g., in the mammal by 

 inhalation of irritant substances, in the frog by excitation 

 of the "rami mesenterici." 4. It is also produced by 

 direct excitation of the intra-cranial centre, by compres- 

 sion of the brain, by increase of arterial pressure in the 

 brain, or by the circulation in that organ of venous blood. 

 5. In most mammalia, particularly those in which, as in 

 the dog, the influence of the vagus centre on the heart is 

 constant, each inspiratory act is followed by increased 

 frequency of pulse. This may, with much probability, be 

 attributed to the inhibitory influence of the respiratory 

 centre on that of the heart. 



In all of these instances, the experimental proof that the vagus is the 

 channel by which the heart is acted on, is obtained by observing that the effect 

 is no longer produced after both vagi have been divided. 



6. In the frog, section of both vagi is almost without 

 effect on the rhythm of the heart, but in the dog, it is 

 followed by great increase of frequency and of arterial 

 pressure. Neither of these effects is obvious in the 

 rabbit. 



7. Accelerator Nerves. The increased frequency of the 

 heart-beats, which, in all animals, is produced along with 



