THE ESOPHAGUS, TRACHEA, AND THYMUS
Although the esophagus still lies partly to the left on entering the chest, it quickly regains a position dorsal to the trachea; thereafter, it pursues a median course, apart from slight deflections as it passes the aortic arch and again just before the esophageal hiatus.
The striated muscle of the cranial part of the esophagus is gradually replaced by smooth muscle as the heart is approached; the color change makes the transformation obvious. The muscle is somewhat thicker immediately before the diaphragm, and this part of the tube is commonly contracted in the dead specimen. There is no evidence that the diaphragm embraces the esophagus tightly at the hiatus, as sometimes alleged. Indeed the free movement of the diaphragm over the esophagus is facilitated by the peritoneum pouching through the hiatus on the right and ventral side of the esophagus.The trachea becomes median soon after entering the thorax. It then lies against the longus colli muscles but soon diverges to run lower within the mediastinum. After passing over the left atrium, it bifurcates at about the level of the fifth rib (or space) (Figure 20-7Z3). The bifurcation is not symmetrical; the right bronchus is larger.
The thymus is prominent in early life but soon regresses. Its formation from right and left parts is not obvious because they are closely applied together. In the young foal it completely fills the ventral part of the mediastinum cranial to the heart and may even extend over the left side of the pericardium; part may also pass into the neck beside the trachea, very occasionally reaching the thyroid gland. At this stage the thymus is clearly lobulated and bright pink. It is largest about 2 months after birth and thereafter regresses, although the rate is variable. Usually little remains after 3 years, when the vestige consists largely of fatty fibrous tissue. At its apogee the thymus makes contact with most structures within the cranial mediastinum.