552 



PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



sc 



gastrics, it may be accelerated by stimulation of the cervical sympathetic 

 stimulation of the cervical spinal cord, or in a more marked degree by 

 stimulation of the communicating filaments between the spinal cord and 

 the inferior cervical and the first dorsal ganglia of the sympathetic. 

 These latter fibres are described as the accelerator nerves of the heart, 

 and when stimulated produce an increase in the rate of the heart's pulse, 

 but with a decreased force, the loss in power being, however, compensated 

 by the increase in rapidity. As a consequence, after stimulation of these 



nerves the blood pressure remains unchanged. 

 The course of these nerves is different in the 

 rabbit and the dog. 



The diagrams (Figs. 233 and 234) indicate 

 their most usual course. 



These accelerator fibres originate from 

 centres in the medulla oblongata and spinal 

 cord, though their exact location has not been 

 determined. It is by means of stimulation of 

 these fibres, therefore, that stimulation of either 



/ VSfl •' y the cervical spinal cord or cervical sympa- 



// a .\" ; thetic produces increase in the rate of the 



heart's pulsation. When the cervical spinal 

 cord is stimulated a great increase in blood 

 pressure follows from stimulation of vaso-motor 

 nerves, but that the increased rate of the 

 heart's contractions is not due to the high 

 blood pressure alone is proved by the occur- 

 rence of acceleration of the pulse when the 

 cervical cord is stimulated, even after section 

 of the splanchnic nerves, when increased blood 

 pressure is prevented. These nerves are not 

 in constant activity ; in other words, they do 

 not antagonize the pneumogastrics, and when 

 divided the heart does not beat slower. A 

 long latent period also characterizes these 

 nerves, and it takes considerable time, as much as ten seconds, from 

 the commencement of stimulation before the maximum acceleration of 

 the heart is reached ; while, again, it is only slowly and gradually that 

 the normal heart-rate is regained even at the cessation of stimulation. 



8. The Influence of the Nervous System on the Arteries. — 

 Anatomical examination of the walls of the minute arteries shows that 

 these vessels are not only supplied with circular muscular fibres, but are 

 also supplied with nerve-fibres which come both from the sympathetic and 

 cerebro-spinal nervous system, and numerous ganglia have also been 



Fig. 233. — Scheme op the 

 course of the cardiac 

 Accelerator Fibres. 

 (Landois.) 



P, pons ; MO, medulla oblongata ; 

 V, inhibitory centre for heart ; A, accel- 

 erator centre : Vag, vagus ; SL, supe- 

 rior, IL inferior laryngeal nerves ; SC, 

 superior cardiac fibres; H, heart: C, cere- 

 bral impulse ; S, cervical sympathetic ; 

 a, a, accelerator fibres. 



