(4.39)

Now that we’ve looked at the arteries and veins of the thorax, we’ll move on to look at the nerves. The nerves that we’ll see are the phrenic nerve, the vagus nerve, the sympathetic trunk, and the intercostal nerves.

We’ll look at the phrenic and vagus nerves first. The phrenic is the motor and sensory nerve of the diaphragm. The vagus provides the parasympathetic supply for all the organs of the thorax and abdomen.

The courses of these two nerves are similar: they both start in the neck, run downward in the mediastinum, and pass through the diaphragm. We’ll look at them first in the neck.

In this dissection the clavicles and the sternocleidomastoid muscles have been removed. Here’s the phrenic nerve. It runs down on the front of the anterior scalene muscle, and passes in front of the subclavian artery, and behind the subclavian vein.

To see the vagus nerve we’ll retract the internal jugular vein. Here’s the vagus nerve. It lies behind and between the internal jugular vein and the common carotid artery. It passes in front of the subclavian artery.

On the right side only, the vagus gives off this branch, the recurrent laryngeal, which curls around the artery and passes upwards to the larynx. To follow these two nerves, we’ll remove the anterior chest wall.

Here’s the phrenic nerve, here’s the vagus nerve. The phrenic nerve passes behind the subclavian vein, which has been divided here, and runs downward in the mediastinum in front of the root of the lung, close to the superior vena cava and right atrium. The phrenic nerve passes through the diaphragm.

In the intact mediastinum, the phrenic nerve runs here, just beneath the pleura. On the left side, the course of the phrenic nerve is similar: in its course in the mediastinum it passes over the aorta, the pulmonary trunk, and the left ventricle.

To see the course of the vagus nerves, we’ll remove the brachiocephalic veins, and the superior vena cava. On the right side, the vagus nerve passes downward and backward, close to the trachea, to reach the esophagus. It breaks up into several branches as it runs down along the esophagus.

On the left side, the vagus nerve crosses the arch of the aorta, and passes backward to run down alongside the esophagus and through the diaphragm. On the left side, the recurrrect laryngeal branch is given off here, and curls around the arch of the aorta to return to the neck.

Now we’ll look at the intercostal nerves and at the sympathetic trunk.

Here are the vertebral bodies, here’s the proximal end of one of the ribs. Here are three of the intercostal nerves. They’re the direct continuation of the anterior rami of the thoracic spinal nerves. They give motor innervation to the intercostal muscles, and sensory innervation to the chest wall. They run closely with the intercostal blood vessels, which have been removed in this dissection. This slender, irregular cord is the sympathetic trunk.

We won’t go into a description here, of the many functions of the sympathetic system, or of its somewhat complex arrangement. The details that follow are shown on the premise that you’ve either just learned about these things, or that you’re just about to do so.

The sympathetic trunk runs alongside the vertebral column, all the way from T1, down to the sacrum. This thickening is one of the ganglia of the sympathetic trunk. These fine connections between the sympathetic trunk and the anterior rami of the spinal nerves, are the rami communicantes.

The nerves passing medially from the sympathetic trunk are the splanchnic nerves, on their way to the celiac and mesenteric gangla.