This space, little more than a potential one, lies between the pericardium and sternum. It is overlapped by the anterior edges of both lungs. It contains the thymus (or its remnants), sternopericardial ligaments, a few lymph nodes and branches of the internal thoracic vessels.
Thymus
The thymus may appear to be a single organ, but in fact it consists of right and left lobes closely applied to each other for much of their extent (Fig. 4.15). It is usually most prominent in children, where it may extend from the level of the fourth costal cartilages to the lower poles of the thyroid gland. In front of it lie the sternohyoid and sternothyroid muscles, the manubrium and upper part of the body of the sternum and their adjacent costal cartilages. Behind it are the pericardium, the arch of the aorta with its three large branches, the left brachiocephalic vein and the trachea.
Blood supply
Small branches enter the thymus from the inferior thyroid and internal thoracic arteries, and there are corresponding veins. Frequently a relatively large short thymic vein enters the left brachiocephalic vein (Fig. 4.10) and needs to be secured before the thymus is retracted after median sternotomy.
Lymph drainage
Efferent channels drain into parasternal, tracheobronchial and brachiocephalic nodes. The thymus does not receive any afferent lymphatics.
Development
The epithelium of the thymus develops mainly from the endoderm of the third branchial pouch. Some of the epithelial cells become the thymic (Hassall's) corpuscles; others form a network of epithelial reticular cells believed to be the source of thymic hormones concerned with the differentiation of T lymphocytes. Connective tissue elements are derived from surrounding mesoderm, but the original colonizing lymphocytes have migrated from the bone marrow. The developing thymus descends from the neck into the mediastinum in front of all the major contents. It doubles its weight rapidly after birth and then maintains that level although the lymphoid content decreases with age, being replaced by fat and fibrous tissue. However, the secretion of thymic hormones and its influence on lymphocytes that migrate to it continue throughout life.
Surgical approach
Median sternotomy provides surgical access to the thymus.