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The mammal (rat) thymus is a bilobed structure located in the mediastinum anterior to the major vessels of the heart. It is largest at birth or shortly thereafter and then gradually decreases in size. The thymus is surrounded by a thin capsule of connective tissue. At intervals the capsule extends into the parenchyma of the thymus to form septa, which partially subdivide the lobes into lobules. Each lobule contains an inner or central medulla region and an outer cortex. Large numbers of closely packed cells can be observed in the cortex where they are more concentrated than in the medulla of the lobule. SEM X600.  3.5" X 4.5"  **On Page Credit Required**