» The Lymphatic Structures of the Thorax

There are numerous lymph nodes within the thorax. Although most are collected in groups, these are sometimes less discrete than is often suggested. The following are the principal groups.

Small intercostal nodes lie in the dorsal part of some intercostal spaces. They receive lymph from the vertebrae and the adjacent muscles, the dorsal part of the diaphragm, and the local costal and mediastinal pleura. The efferent flow is to the thoracic duct.

The cranial mediastinal nodes are numerous and scattered about the esophagus, trachea, and vessels at the entrance to the thorax; usually some form a discontinuous chain that joins the caudal deep cervical nodes within the neck. The most caudal members reach the pericardium, where they overlap the nodes about the tracheal bifurcation that are assigned to the tracheobronchial and caudal mediastinal groups. Most efferent vessels pass to the thoracic duct; those from the most cranial nodes in the series may first perfuse deep cervical nodes.

The tracheobronchial group is scattered about the caudal part of the trachea and the chief bronchi (Fig. 20.5/3); left, middle, and right subdivisions are commonly distinguished. Small nodes within the peribronchial tissue of the lung may be regarded as members of this series. Most lymph passing through this group has origins within the lungs, but some comes from the pericardium, the heart, and the caudal mediastinal nodes. The efferent vessels are divided between those that go directly to the thoracic duct and those that first perfuse the cranial mediastinal nodes.

A number of small caudal mediastinal nodes lie directly in front of the diaphragm and between the esophagus and aorta. Lymph is received from the esophagus, the diaphragm, the liver, the mediastinal and diaphragmatic pleura, and, apparently, the lungs. The efferent lymph flow is divided between the thoracic duct and the tracheobronchial and cranial mediastinal lymph nodes.

The few ventral mediastinal lymph nodes are without significance.

The thoracic duct drains into one or another of the large veins at the entrance to the thorax, most commonly the cranial vena cava.

Comprehension Check

Delineate areas of cardiac and pulmonary auscultation and percussion on an equine skeleton or a cadaver, followed by practice on a live animal.

List the abdominal organs that may be within the rib cage.

Practice tracheotomy on the cadaver.

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* The hemorrhage from the pulmonary vasculature that is induced by severe exercise is a major concern of the horse-racing industry. Although the existence of the condition is rarely made evident by loss of blood externally or by abnormal distress during or immediately after a race, tracheobronchial endoscopy at the latter time reveals hemorrhage in the lungs of most (some would say all) Thoroughbreds subjected to the extreme demands of racing. There is some dispute concerning the origin of the blood leakage—whether it is from branches of the bronchial or the pulmonary arteries and whether it results from preexisting structural abnormality of the vessel wall. The condition impairs performance, worsens progressively, and is responsible for the premature retirement of many horses from racing. It often occurs incidental to other problems such as laryngeal hemiplegia in horses exposed to more moderate stress. Similar exercise-induced hemorrhage is recognized in racing Greyhounds, camels, and some elite human athletes.