» The Lymphatic Structures Within the Thorax
The lymphatic drainage of the thorax is complicated and variable. Not every node is present in every animal, and some may be placed so that it is difficult to assign them to a particular group.
A series of small intercostal nodes is present directly below the pleura in certain spaces, and these are supplemented by a scattering of nodes along the aorta (Fig. 27.8). Both sets drain lymph from structures about the vertebral column and within the dorsal mediastinum. Most of their outflow is directed toward the cranial mediastinal nodes (Table 27.1).The thoracic duct, into which most of the lymph eventually flows, inclines ventrally over the left face of the trachea to end by opening into the cranial vena cava or one of its tributaries of the left side. The duct is often duplicated for all or part of its course.
FIG. 27.8 Lymph drainage of the bovine thoracic wall and mediastinum. 1, Thoracic duct; 2, cranial sternal lymph nodes; 3, caudal sternal lymph node; 4, cranial mediastinal lymph nodes; 5, middle mediastinal lymph nodes; 6, caudal mediastinal lymph nodes; 7, intercostal and thoracic aortic lymph nodes; 8, tracheobronchial node.
Comprehension Check
Demarcate the area of auscultation of the lungs, and practice auscultation on live animals. Compare the gross and subgross anatomy of the lungs of cattle with those of the horse, and discuss the functional implications of differences in structure.
» TABLE 27.1
Lymph Nodes of the Thorax
| Lymph Nodes | Location | Areas Drained | Efferent Flow |
| Caudal sternal | Below the Iransversethoracic muscle on the thoracic floor | Ventral parts of the thoracic and cranial abdominal floors and overlying muscles of the forelimb | To cranial mediastinal group |
| In front ofthecaudal sternal Iymphnodes | |||
| Cranial mediastinal | Near the entrance to the thorax | Drain adjacent mediastinal structures, plus from caudal and cranial sternal lymph nodes | To thoracic duct or one or two tracheal ducts |
| Middle mediastinal | To the right of the aortic arch | Mediastinal structures and from some of the tracheobronchial lymph nodes | To tracheal duct |
| Tracheobronchial | Directlyon the trachea and the bronchi | Lung | Various mediastinal lymph nodes |
| Caudal mediastinal: 1-2; could be 20 cm long | Dorsal to the hiatus over the esophagus | Adjacent structures | To thoracic duct; enlaιgement can compress the esophagus to cause bloating |