308 THE VASCULAR SYSTEM 



the arch of the aorta, and it is formed by the union of a 

 cardiac branch from the left vagus with a cardiac branch from 

 the left sympathetic. A small ganglion is situated at the point 

 where the two nerves unite and it is believed to control the 

 rhythmical contractions of the heart, subject to the influence 

 of stimuli from the higher centres. 



It should be observed that the superficial cardiac plexus is 

 formed by branches from the vagus and the sympathetic, of 

 the left side only. This arrangement may possibly account for 

 the fact that the referred pains of angina pectoris are usually 

 limited to the left side of the body. 



The Deep Cardiac Plexus lies in front of the bifurcation of 

 the trachea and it is formed by branches from the sympathetic 

 and the vagi, of both sides of the body. 



The fibres derived from the vagus are both afferent and 

 efferent, the latter constituting the inhibitory nerves of the 

 heart. Irritation of the vagus causes a slowing of the heart- 

 rate, while paralysis leads to increased rapidity, since the 

 sympathetic accelerator fibres are then no longer opposed. 



The sympathetic fibres also are both afferent and efferent, 

 the latter constituting the accelerator nerves of the heart. In 

 fracture-dislocation of the vertebral column in the lower 

 cervical region, the sympathetic trunks are completely 

 paralysed (p. 189), while the vagi are not affected. In this 

 condition, therefore, there is usually a definite slowing of the 

 heart-rate. 



Cardiac Pain. When painful symptoms accompany 

 cardiac disturbances they are usually severe and often agonis- 

 ing in character. In the majority of cases of angina pectoris, 

 the pain is experienced at first in the praecordial region. It is 

 at present impossible to decide whether the intense pain which 

 is felt over the heart is actually experienced in the viscus, or 

 whether, as Mackenzie holds, it is experienced in the sensitive 

 tissues of the chest wall. In any case the pain is felt in the 

 areas supplied by the upper intercostal nerves, and, since the 

 sympathetic fibres which supply the heart are derived from 



