330 DISSECTION OF THE THORAX. 



Dissection. The phrenic nerve is sufficiently denuded for its examina- 

 tion ; but the student should trace the vagus nerves through the thorax. 



The A'agus is to be followed, on both sides, behind the root of the lung, 

 and its large plexus in that position is to be dissected out: a few filaments 

 of the gangliated cord of the sympathetic coming forwards over the spinal 

 column to the plexus, must be looked for. In front of the root on the left 

 side, the nerve supplies a few pulmonary filaments. Beyond the root the 

 vagus is to be pursued along the oesophagus by raising the lung and re- 

 moving the pleura. 



The PHRENIC NERVE is a branch of the cervical plexus (p. 80). In its 

 course through the thorax it lies along the side of the pericardium, and at 

 a little distance in front of the root of the lung, with a small companion 

 artery. When near the diaphragm it is divided into branches ; these per- 

 forate the muscle, and are distributed on the under surface. The nerves 

 of opposite sides differ in length, and in their connections above the root 

 of the lung. 



The right nerve is deeper at first, and is also shorter and straighter than 

 the left. In entering the chest it crosses behind the subclavian vein, but 

 in front of the internal mammary artery ; and it lies afterwards along the 

 right side of the innominate vein and superior cava till it reaches the root 

 of the lung. 



The left nerve crosses the subclavian artery, but has the same position 

 as the right to the mammary vessels when entering the cavity. In the 

 thorax it is directed in front of the arch of the aorta to the root of the 

 lung, and makes a curve lower down around the projecting heart. Before 

 reaching the aorta the nerve is placed external to the left common carotid 

 artery ; and crosses the left vagus from without inwards, so as to be inter- 

 nal to that nerve on the arch. 



Branches. Some small filaments are said to be furnished from the 

 nerve to the pleura and pericardium. 



Internal mammary artery. A small part of this artery, which lies be- 

 neath the first rib, and winds round the phrenic nerve and the innominate 

 vein to reach the side of the sternum, is now to be learnt. It gives the 

 following offset : 



The superior phrenic branch (comes nervi phrenici) is a very slender 

 artery, which accompanies the phrenic nerve to the diaphragm, and is dis- 

 tributed to that muscle, anastomosing therein with other branches of the 

 aorta, and with the musculo-phrenic branch of the internal mammary (p. 239). 



The PXEUMOGASTRIC NERVE (p. 181) passes through the thorax to the 

 abdomen. In the lower part of the thorax the nerves of opposite sides 

 have a similar position, for they pass behind the root of the lung, each on 

 its own side, and along the oesophagus to the stomach. But above the 

 root of the lung, the right and left nerves differ much. Each supplies 

 branches to the viscera, viz., to the heart, the windpipe and lungs, and 

 the gullet. 



The right vagus is posterior to the left in position. It enters the tho- 

 rax between the subclavian artery and the innominate vein, and is directed 

 obliquely backwards, by the side of the trachea, and between this tube 

 and the oesophagus, to the posterior aspect of the root of the lung, where it 

 gives rise to the posterior pulmonary plexus. From the plexus two large 

 offsets are continued to the back of the gullet, and unite below into one 

 trunk, which reaches the posterior surface of the stomach. 



The left nerve appears in the thorax on the outer side of the left com- 



