58 



THE MECHANISM OF THE CIRCULATION. 



On the whole, there is much to be said in favour of the supposition that 

 the terminations of the afferent nerves on the inner wall of the heart 

 may be stimulated by intracardiac pressure, and thus the controlling 

 centres of the circulation — vagus, vasomotor, and respiratory — excited 

 or depressed. If this be the case, the heart possesses a reflex mechanism 

 for regulating, not only its own rate, but the whole circulation. 



By moderate expansion of the lungs the heart is accelerated. 1 The 

 afferent nerves of this reflex arc are the vagi. The frequency of the 

 pulse is increased by singing, coughing, sipping water, reading aloud, and 

 forced breathing. By starvation the excitability of the vagus centre is 

 said to be greatly increased. The normal difference between the pulse 

 frequency in inspiration and expiration becomes then very marked. 2 

 The heart is normally greatly accelerated by severe muscular exertion, 

 but slightly so after division of the vagi, and not at all after also 

 dividing the accelerators. These observations have been made on the 

 rabbit. 3 



According to Francois- Franck, 4 stimulation of the mucous membrane 

 of the lungs and the upper part of the larynx produces slowing of the 

 heart. Similarly, excitation of the nasal branch of the fifth nerve always 



Fig. 38. — Reflex inhibition produced by inhalation of ammonia. The upper line is 

 the trace of arterial pressure, the lower line that of the pulse. — Francois-Franck. 



diminishes the frequency of the pulse (Fig. 38). 5 Thus, cardiac inhibi- 

 tion is of usual occurrence during the primary stage of inhalation of 

 chloroform. 



In the frog, inhibition of the heart is produced with the greatest 

 ease by tapping the intestines. 6 The reflex arc for this effect is formed 

 by the abdominal sympathetic, the spinal cord and bulb, and the vagi. 7 



Almost any form of stimulation of the abdominal viscera produces 

 slowing of the heart. Sudden distension of the stomach may be especi- 

 ally mentioned as a means by which this reflex may be evoked. 8 Ex- 

 citation of the central end of the sciatic or splanchnic nerves arouses to 

 activity both the vagal and the accelerator centres. The accelerator 

 centre is usually the more strongly excited of the two, so that increased 

 frequency is usually the resultant effect. The excitation of the vagal 

 centre either lasts the longer or else becomes relatively increased by 



1 Hering, Sitzungsb. d. k. Akad. d. IFisscnsch. , math.-naturv. CI., Wien, 1871, Bd. lxiv. 

 S. 332 ; Sommerbrodt, Ztschr. f. klin. Med., Berlin, 1881, Bd. ii. S. 601. 



2 Aducco, Arch. ital. de biol., Turin, 1894, vol. xxi. p. 419. 



3 H. E. Hering, Central 'bl. f. Physiol., Leipzig u. Wien, 1894, Bd. viii. S. 75. 



4 "Trav. du labor, de Marey," 1880, tome iv. p. 378. 



5 Francois-Franck, " Ibid.," 1876, tome ii. p. 226. 



6 Goltz, Virchow's Archiv, 1863, Bd. xxvi. S. 10. 



7 Bernstein, Arch. f. Physiol., Leipzig, 1864, S. 622. 



8 Mayer and Pribram, Sitzungsb. d. k. Akad. d. Wissensch., inatli.-natiirv. CI., Wien, 

 1872, Bd. lxvi. Abth. 3, S. 102. 



