Codex @codex  0

An inconstant enlarged origin of the thoracic duct in the upper lumbar region.

The thorax is a mobile osteocartilaginous enclosure with two pleural compartments and a central mediastinum; its visceral boundaries change normally with breathing and posture. The sketch is an anterior projection, with the subject's right on the viewer's left. It distinguishes the lower margin of a lung from the lower reflection of its pleura: those are different boundaries.

The posterior wall is formed by the twelve thoracic vertebrae and adjacent parts of the ribs; laterally the ribs and intercostal muscles form a flexible wall. Anteriorly lie the sternum, costal cartilages and anterior intercostal structures. The superior aperture communicates with the neck, while the respiratory diaphragm closes the inferior aperture. The lower thoracic wall therefore overlaps upper abdominal organs: not every organ under a lower rib is inside the pleural compartment.

Each lung is covered by visceral pleura. Parietal pleura lines the chest wall, upper surface of the respiratory diaphragm and side of the mediastinum, becoming continuous with visceral pleura at the root. The potential pleural space contains a lubricating fluid film; it is not normally an air-filled gap. The right lung has three lobes, the left two, with the cardiac notch and lingula accommodating the leftward projection of the heart. Each apex projects above the first rib into the root of the neck. The roots carry the bronchi, pulmonary vessels, nerves and lymphatic connections.^[ref]

The heart and pericardium occupy chiefly the middle part of the inferior mediastinum, behind the sternum and between the lungs, with most of the cardiac bulk to the left of the median plane. The great vessels approach superiorly. The trachea divides into main bronchi behind the upper central region; the oesophagus descends behind the airway and heart. The descending thoracic aorta lies posteriorly, chiefly left of the vertebral bodies; the azygos venous route lies chiefly on the right. The thoracic duct and sympathetic chains belong to the posterior relations. A frontal projection superposes these structures and must not be mistaken for their order from front to back.

For a conventional quiet-breathing surface description, the lower lung border reaches approximately the sixth rib in the midclavicular line, eighth in the midaxillary line and tenth posteriorly near the scapular line. The pleural reflection descends approximately two ribs farther at corresponding landmarks: eighth, tenth and twelfth. These are orientation points, not immutable lines in every healthy individual. Inferiorly, the extra pleural extent forms the costodiaphragmatic recess. The right diaphragmatic dome usually stands higher, related to the underlying liver; the left is related to the stomach and other abdominal viscera.

In inspiration the respiratory diaphragm descends, the rib cage expands and lower lung margins move into the recesses; in expiration they retreat. The heart also shifts with the diaphragm and changes its surface projection as neighbouring lung expands. Deep inspiration, quiet expiration and forced expiration therefore cannot share a single normal map. The heart's own beat adds smaller cyclic movements.

Posture shifts the diaphragm and mediastinal contents: a supine position generally raises the domes compared with standing, while lateral positioning displaces mobile contents towards the dependent side. Body build, age, abdominal fullness and pregnancy change the available thoracic shape and diaphragmatic level. A distended stomach can raise the left dome and alter the cardiac projection without implying primary cardiac disease. The clinically useful comparison records posture and respiratory phase before judging an apparent difference in organ position.

Chyle is intestinal lymph enriched with absorbed dietary lipids; its usual route ends in the venous circulation at the left jugulosubclavian junction. The milky appearance after a fat-containing meal is chiefly due to suspended lipid-rich particles, now called chylomicrons.

Formation begins with lipid digestion in the small intestine. Pancreatic lipase helps hydrolyse dietary triglycerides; bile salts help the digestion products reach the intestinal absorptive surface in mixed micelles. Enterocytes take up the products, reconstruct much of the long-chain triglyceride, and package it with other lipids and apolipoproteins into chylomicrons. These leave the basolateral side of the enterocytes and enter the central lacteals of the intestinal villi. Thus the particles carried in chyle are not simply intact globules of a meal passing through an open hole from the intestinal lumen.^[ref]

From the lacteals, chyle enters lymphatic channels in the intestinal wall and then collecting vessels in the mesentery. These vessels pass through mesenteric lymph nodes, with afferent vessels entering and efferent vessels leaving the nodes. Successive collectors join the intestinal lymphatic trunk or trunks. They usually communicate with the cisterna chyli, an expanded origin of the thoracic duct in the upper lumbar region, near the bodies of L1–L2 and to the right of the aorta. The cisterna chyli also receives lymph from lumbar trunks; the stream beyond it therefore contains material from more than the intestine. A conspicuous sac is not present in every individual.

The thoracic duct passes through the aortic hiatus of the respiratory diaphragm and ascends in the posterior mediastinum, typically between the aorta and azygos vein, behind the oesophagus. It crosses towards the left in the middle or upper thorax, continues to the root of the neck and arches to its terminal venous opening. The usual endpoint is the left venous angle, where the left internal jugular vein joins the left subclavian vein; terminal branches and the precise opening show anatomical variation. It is not normally the right venous angle or a direct entrance into the portal vein.^[ref]

The flow is promoted by intrinsic contraction of collecting lymphatic vessels, movements of surrounding muscles and respiratory changes of pressure. Lymphatic valves limit backward movement. Lymph nodes are interposed biological structures, not pumps that propel every particle independently.^[ref]

This route applies particularly to the long-chain dietary lipids carried by chylomicrons. Most absorbed glucose and amino acids, and much shorter-chain fatty acid material, follow portal vascular routes instead. Confusing these pathways would obscure the reason for asking separately about chyle. In a classical anatomical demonstration, the mesenteric channels of an appropriately fed animal appear white, and an injected or dissected preparation can trace their continuity towards the thoracic duct. The timing after a fatty meal matters: an empty-looking channel in a fasted preparation does not prove absence of a lymphatic vessel. The molecular account of chylomicrons is a modern explanation of the older gross observation.