Part 4: Nerves

Sympathetic Nerves

1. ALL specimens: On both sides, within the thorax, identify the sympathetic trunk, ganglia and ramus communicans.

• • On both left and right sides of the thorax, identify the sympathetic trunk and ganglia dorsal to the descending aorta.
    • Recall that the sympathetic trunk is found dorsally, inside the chest, running longitudinally along the ventral surface of the necks of the ribs.
    • Follow the sympathetic trunk using blunt dissection, with the tip of an iris scissors, and trace it cranially. Note the ganglia as you go; ganglia are the small enlargements in the trunk at each intercostal space. Attempt to identify a ramus communicans coming off the deep surface of a ganglion.

parasympathetic nerves

2. ALL specimens: On both sides, within the thorax, re-identify the vagus nerves (left and right). Trace each vagus nerve caudally to  locate where each divides into dorsal and ventral branches.

• • Each vagus nerve will split into dorsal and ventral branches, i.e., you will have a left dorsal and left ventral branch of the vagus AND a right dorsal and right ventral branch of the vagus.

3. ALL specimens: Continue tracing the left and right ventral branches of the vagus and identify where the (left and right) ventral branches unite just ventral to the esophagus; at this point of unification the ventral vagal trunk is formed.

• • Recall from Anatomy I, that the left and right ventral vagal branches combine to form the ventral vagal trunk on the ventral aspect of the esophagus. Identify the ventral vagal trunk and trace it caudally.
    • Dissection Note: Note that the ventral vagal trunk is usually formed just caudal to the heart and the root of the lungs.

4. ALL specimens: Continue tracing the left and right dorsal branches of the vagus and identify where the (left and right) dorsal branches unite dorsal to the esophagus; at this point of unification the dorsal vagal trunk is formed.

• • Recall from Anatomy I, that the left and right dorsal vagal branches combine to form the dorsal vagal trunk on the dorsal aspect of the esophagus. Identify the dorsal vagal trunk and trace it caudally.
    • Dissection Note: Note that the dorsal vagal trunk is usually formed farther caudally than the ventral vagal trunk, and is usually located closer to the diaphragm.

somatic nerves

5. ALL specimens: Identify the phrenic n. on both sides (left and right) and trace it to the diaphragm (Terminology note: phrenic = diaphragmatic).

• • Dissection Note: Note that cranial to the heart the vagus and phrenic nn. are closely related, but at the root of the lung the vagus n. passes dorsal to the root of the lung while the phrenic n. passes ventral to it (Figs. 4-2 and 4-3).

(Duplicate) Figure 4-2. Equine thorax, left lateral view. 1, rib 1; 2, rib 6; 3, diaphragm; 4, caudal mediastinum; 5, right auricle; 6, left auricle; 7, left ventricle; 8, pulmonary trunk; 9, ligamentum arteriosum; 10, cut surface of root of the lung; 11, esophagus; 12, trachea; 13, aorta; 14, vagus n.; 14’, dorsal and ventral vagal trunks; 15, phrenic n.; 16, thoracic duct; 12, tracheobronchial lymph node.

(Duplicate) Figure 4-3. Bovine thorax, left lateral view. The left azygous vein (16) passes around the cut surface of the root of the left lung. 1, external jugular vein; 2, sternocephalicus m.; 3, axillary a.; 4, axillary v.; 5, cervicothoracic ganglion; 6, esophagus; 7, left vagus n.; 8, (left) phrenic n.; 9, cardiac n.; 10, trachea; 11, internal thoracic a.; 12, mediastinal pleura; 13, reflected pericardium; 14, pulmonary trunk; 15, aorta; 16, left azygous v.; 17, sympathetic trunk; 18, left recurrent laryngeal n.; 19, ventral vagal trunk.; 20, dorsal vagal trunk.; 21, caudal mediastinal lymph node; 22, cranial extent of the diaphragm; 23, diaphragm; 24, internal intercostal m.; 25, external intercostal m.