THE CIRCULATION OF THE BLOOD. 133 



epigastrium may be referred to. The stoppage of the heart's action in 

 this case, is due to the conveyance of the stimulus by fibres of the sym- 

 pathetic (afferent) to the medulla oblongata, and its subsequent reflection 

 through the vagi (afferent) to the inhibitory ganglia of the heart. It is 

 also believed that the power of the medullary inhibitory centre may in a 

 similar manner be reflexly lessened so as to produce accelerated action of 

 the heart. 



Acceleration of the Heart's Action. The heart receives an ac- 

 celerating influence from the medulla oblongata through certain fibres of 

 the sympathetic. These accelerating nerve-fibres, issuing from the spinal 

 cord in the lower cervical and upper dorsal regions, reach the inferior 

 cervical ganglion of the sympathetic, and pass thence to the cardiac 

 plexus, and so to the heart. Their function is shown in the quickened 

 pulsation which follows stimulation of the spinal cord, when the latter 

 has been cut off from all connection with the heart, excepting by these 

 accelerating filaments. Unlike the inhibitory fibres of the pneumogas- 

 tric, the accelerating fibres are not continuously in action. 



The accelerator nerves must not, however, be considered as direct an- 

 tagonists of the vagus ; for if at the moment of their maximum stimula- 

 tion, the vagus be stimulated with minimum currents, inhibition is 

 produced with the same readiness as if these were not acting. Nor is 

 there any evidence that these fibres are constantly in action like those of 

 the vagus. 



The connection of the heart with other organs by means of the ner- 

 vous system, and the influence to which it is subject through them, are 

 shown in a striking manner by the phenomena of disease. The influence 

 of mental shock in arresting or modifying the action of the heart, the 

 slow pulsation which accompanies compression of the brain, the irregu- 

 larities and palpitations caused by dyspepsia or hysteria, are good evi- 

 dence of the connection of the heart with other organs through the ner- 

 vous system. 



Other Influences affecting the Action of the Heart. 



The healthy action of the heart no doubt very materially depends (1) 

 upon a due supply of healthy Uood to its muscular tissue. It is not un- 

 likely that the apparently contradictory effect of poisons may be ex- 

 plained by supposing that the influence of some of them is either 

 partially or entirely directed to the muscular tissue itself, and not to the 

 nervous apparatus alone. 



As will be explained presently, the heart exercises a considerable 

 influence upon the condition of the pressure of blood within the arteries 

 but in its turn (2) the blood pressure within the arteries reacts upon the 

 heart, and has a distinct effect upon its contractions, increasing by its 



