CIRCULATION. 177 



spinal cord near its junction with the bulb, and that the sudden decrease in 

 the number of afferent impulses caused by section of the splanchnic nerve 

 quickens the pulse-rate. 



Irradiation. — The slowing of the rate of beat observed chiefly during the 

 expiratory portion of respiration disappears after the section of both vagus 

 nerves. The slowing may perhaps be due to the stimulation of the cardiac 

 inhibitory centre by irradiation from the respiratory centre. 1 



Origin of Cardiac Inhibitory Fibres. — Since the researches of Waller and 

 others, it has been generally believed that the cardiac inhibitory fibres enter 

 the vagus from the spinal accessory nerve, for the reason that cardiac inhibi- 

 tion was not secured in animals in which the fibres in the vagus derived from 

 the spinal accessory nerve were made to degenerate by tearing out the latter 

 before its junction with the vagus. These results have lately been called in 

 question by Grossmann. 2 The method employed by his predecessors, according 

 to him, probably involved the destruction of vagus roots as well as those of 

 the spinal accessory. Grossmann finds that the stimulation of the spinal 

 accessory nerve before its junction with the vagus does not inhibit the heart. 

 Nor does inhibition follow the stimulation of the bulbar roots supposed to be 

 contributed to the mixed nerve by the spinal accessory. 



Augmentor Centre. — The situation of the centre for the augmentor 

 nerves of the heart is not definitely known, although from analogy it seems 

 probable that it will be found in the bulb. That this centre is constantly in 

 action is indicated by the lowering of the pulse-rate after section of the vagi 

 followed by the bilateral extirpation of the inferior cervical and first thoracic 

 ganglia. 3 The division of the spinal cord in the upper cervical region after the 

 section of the vagi has the same effect. Vagus inhibition, moreover, is said 

 to be more readily produced after section of the augmentor nerves. 



McWilliam ' has remarked that the latent period and the character of the 

 acceleration often accompanying the excitation of afferent nerves may differ 

 entirely from the characteristic effects of the excitation (if augmentor nerves. 

 The stimulation of the latter is followed by a long latent period, after which 

 the rate of beat gradually increases to its maximum and, after excitation is 

 over, as gradually declines. The excitation of an afferent nerve, on the con- 

 trary, causes often, with almost no latent period, a remarkably sudden accel- 

 eration, that reaches at once a high value and often suddenly gives way to a 

 slow heart-beat. These facts seem to show that reflex acceleration of the heart- 

 beat is due to chaDges in the cardiac inhibitory centre, and not to augmentor 

 excitation. This view is strengthened by the fad that if the augmentor nerve- 

 are cut, the vagi remaining intact, the stimulation of afferent fibres, for exam- 

 ple in the brachial nerves, can still cause a marked quickening of the pulse- 

 rate. In short, the action of afferent nerves upon the rateofbeal is essentially 



L Laulani£: Comptes rendus Soci&i de Biologie, 1893, p. ~'2'.\. Compare Wood: American 



Journal of I'/iiisi<>h,,/,i, Is'.i'.i, ii. p. :>.VJ. 



' < rrossmann : Archiv fur die gesammte Physiologic, 1895, liv. p. 6 

 3 Hunt: Amrrirun Jniirwtl <>f /'//i/.si'o/of/;/, IS",)',), ii. p. 397. 

 * McWilliam : Proceedings Royal Society, L893, liii. p. I7"_\ 



Vol. 1.— 12 



