380 NERVOUS SYSTEM. 



tributed to the pharyngeal muscles and cranial part of the 

 oesophagus. 



c. N. laryngeus superior (//). — This arises from the gan- 

 glion nodosum, passes ventrad, crossing the medial surface of 

 the carotid artery, and reaches the larynx at about the level 

 of the caudal end of M. thyreohyoideus. It passes into the 

 larynx and supplies its mucosa. 



Thoracic Portion of the Vagus (Fig. 157). — As the vagus 

 (^b) and sympathetic (<:) separate to enter the thorax, the vagus 

 (/^) lies more ventrad. The right vagus now lies along the 

 lateral surface of the trachea, the left one (Fig. 157) along that 

 of the oesophagus. At about the level of the first rib or further 

 craniad there arise, apparently from the vagus (but really from 

 the sympathetic), two slender nerves which pass caudad parallel 

 and close to the vagus, lying slightly dorsad of it. These are 

 the cardiac nerves (/); they arise from the sympathetic farther 

 craniad in the neck region, accompany the vagus, and seem 

 to branch from it. In cases where vagus and sympathetic are 

 separate throughout their courses, these nerves arise clearly 

 from the sympathetic. The cardiac nerves pass to the heart 

 and aid in forming a network of nerves known as the cardiac 

 plexus {k). 



The vagus usually receives also, at about the level of the 

 first rib, one or two communicating branches from the middle 

 cervical ganglion {d) of the sympathetic. It then passes to 

 the roots of the lungs, crossing the lateral surface of the aortic 

 arch on the left side, and the medial surface of the azygos vein 

 on the right side. As it crosses the aortic arch the left vagus 

 gives off N. laryngeus inferior (7). This curves around 

 the caudal side of the aortic arch to the lateral surface of the 

 trachea [o), then extends on the lateral and ventral surface of 

 the trachea craniad into the neck region. In the neck it passes 

 to the dorsolateral side of the trachea and reaches the larynx. 

 It passes into the larynx between the cricoid and thyroid carti- 

 lages, and supplies the muscles p{ the larynx. It anastomoses 

 with the superior laryngeal. 



The right inferior laryngeal nerve is given off from the 

 right vagus much further craniad than the left; it curves around 



