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We investigated the morphology of the visceral pleura of 36 sheep, using macroscopic, histologic, and ultrastructural approaches to quantify regional pleural thickness, blood supply, and lymphatic drainage, including the pulmonary ligament and hilar lymphatic distributions. Pleural thickness increased caudally and dorsally, such that the costal pleura of the caudal lobes had a mean minimum pleural thickness of 83 microns. The blood supply to the entire visceral pleura came exclusively from the bronchial arteries. Lymph vessels formed an extensive plexus throughout the serous membrane of all lobes. Trunk lymphatics (greater than 100 microns diameter) had a density of about 2/cm of pleural length on all lobar surfaces except for the cranial and middle lobes, where their density on the costal surfaces was less than 1/cm. Pleural trunk lymphatics coursed to the pulmonary ligaments and to the hilum on their way to regional lymph nodes. At the hilum they anastomose with intrapulmonary lymphatic trunks. The principal lymph nodes to receive pulmonary lymph were the caudal mediastinal node and tracheobronchial nodes. The visceral pleura of sheep is thick, showing considerable regional diversity in morphology.

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