The trachea, 4 1/2 inches long, (Fig. XVI, 1.) commences immediately below the cricoid cartilage, on a level with the sixth cervical vertebra, and passing downwards through the superior mediastinum, bifurcates opposite the level of the lower part of the body of the fourth dorsal vertebra (Ludwig's plane).
The two bronchi diverge, (Fig. XVI, 2,3,4) the left being the longer and the narrower. The tendency of foreign bodies to pass more frequently into the right bronchus is explained by the fact that the septum between the two bronchi is placed to the left of the middle line of the trachea. The right bronchus, previous to the giving-off of the eparterial bronchus, is less obliquely inclined than the left bronchus, though subsequently it follows much the same course.
The greater obliquity of the left bronchus accounts also for the fact that the left pulmonary artery tends to lie at the higher level, whilst the right pulmonary artery lies below the level of the corresponding bronchus.
The oesophagus, 9 inches long, also commences at the level of the cricoid cartilage, (Fig. XIX, 1.) and passing downwards through the superior and posterior mediastina, pierces the diaphragm at the level of the tenth dorsal vertebra, entering the stomach at the level of the eleventh vertebra.
The entrance of the oesophagus into the stomach may be indicated by taking a point on the seventh left costal cartilage 1/2 inch away from the left side of the xiphisternal junction.
The thoracic duct, 15 to 18 inches long, commences as the receptaculum chyli, a spindle-shaped sac which lies opposite the bodies of the first and second lumbar vertebrae, (Fig. XX, 5.) and between the thoracic aorta on the left and the vena azygos major on the right. It may be represented on the surface by an oval enlargement placed just to the right of the middle line, occupying the upper two-thirds of the space between the transpyloric (first lumbar) and subcostal (third lumbar) planes.
The efferent duct pierces the diaphragm through the aortic orifice opposite the twelfth dorsal vertebra, and passes almost vertically upwards through the posterior mediastinum, (Fig. XX, 6.) just to the right of the middle line, as far as the lower part of the fourth dorsal vertebra (Ludwig's plane). The duct now crosses behind the oesophagus to the left of the middle line, and then again, passes vertically upwards through the superior mediastinum and into the neck as far as the level of the transverse process of the seventh cervical vertebra.
Finally, the duct curls outwards and downwards to open into the angle between the internal jugular and subclavian veins of the left side. (Fig. XX, 7.) The duct drains the whole of the lymphatic area of the body, except the right side of the head and neck, the right arm, the right side of the thorax and the convexity of the liver, the lymphatics from these regions draining into a smaller duct which opens into the angle between the right internal jugular and subclavian veins.