9 8 



THE NERVOUS SYSTEM 



Cardiac branches arise from both vagi in the neck and from 

 the right vagus and left recurrent (laryngeal) nerves in the 

 thorax. These branches constitute the inhibitory nerves of the 

 heart, and they are, therefore, antagonistic to the cardiac 



FIG. 50. Transverse Section through the Neck at the level of the 

 First Thoracic Vertebra. 



1. Isthmus of thyreoid gland. 



2. Sterno-hyoid and sterno-thyreoid 



muscles. 



3. Right lobe of thyreoid gland. 



4. Sterno-masto'd muscle. 



5. Right recurrent nerve. 



6. Internal jugular vein. 



7. Common carotid artery. 



8. Oesophagus. 



9. First thoracic vertebra. 



branches from the sympathetic, which act as accelerators of 

 the heart-rate. UNILATERAL LESIONS of the vagus cause only 

 a transitory disturbance of the heart's action, but BILATERAL 

 LESIONS produce a profound effect. Irritative lesions, such as 

 occur in the early stages of post-diphtheritic neuritis, stimulate 



