THE THORAX. 345 



The Thoracic Sympathetic Nervous System. Fig. 74. 



This consists of a nerve cord which extends through the 

 chest alongside of the dorsal vertebrae (continuous above with 

 the cervical and below with the lumbar cord), and a series 

 of enlargements or ganglia, twelve in number, which rest 

 upon the heads of the ribs (excepting the last two, which 

 lie upon the side of the body of the corresponding ver- 

 tebrae). 



The sympathetic branches are (external) to the intercostal 

 nerves and (internal) to the thoracic viscera. 



From the lower ganglia are given off the important 

 splanchnic nerves, which, formed in the thorax, pass to the 

 abdomen. 



(i) The great splanchnic nerve is formed by filaments 

 from the fifth to the ninth thoracic ganglia inclusive, passes 

 through the crus of the diaphragm, and terminates in the 

 semilunar ganglion of that side. (2) The lesser splanchnic 

 nerve is composed of filaments from the eleventh and 

 twelfth ganglia, takes the same course through the diaph- 

 ragm as the great, and ends in the solar or renal plexus. 

 (3) The smallest splanchnic nerve is a small branch from the 

 last thoracic ganglia to the renal plexus. 



The Branches of the Thoracic Aorta. (For the artery see 

 page 338.) Figs. 72, 74. 



(i) Pericardial, a few small arteries to the posterior sur- 

 face of the pericardium. (2) Bronchial, three in number. 

 The right usually rises from the first right (aortic) intercos- 

 tal artery or from the front of the thoracic aorta by a trunk 

 common to it and the left upper bronchial artery. It runs 

 outward to the right lung on the back of the bronchus. The 

 left bronchial are two in number, arising from the thoracic 

 aorta one below the other, and both pass to the left lung 

 along the back of the bronchus. The left upper may arise 



