Definition: the trachea is a wide fibrocartilaginous  tube. It’s the continuation of the larynx.
Course: it commences in the neck below the cricoid cartilage at the level of the C6 vertebra 5cm above the jugular notch. It descends anterior to the esophagus. It enters the superior mediatinum  through the thoracic inlet and passes downwards and backwards posterior to the manubrium to bifurcate into the two main bronchi on a level just below the lower border of the manubrium.
Division: the trachea is divided into cervical part and the thoracic part
Measurement:
length-10cm, 15cm in deep inspiration
Diameter- 2cm
Relations:
A. Cervical part.
Anteriorly: the isthmus of the thyroid gland, inferior thyroid veins, sternohyoid and sternothyroid muscles
laterally: the lobes of the thyroid gland and the common carotid artery
posteriorly: the esophagus with the recurrent laryngeal nerve lying in the groove between esophagus and trachea.
B. Thoracic part.
Anteriorly: Brachiocephalic artery and left carotid artery and the brachiocephalic vein.
posteriorly: esophagus and left recurrent laryngeal nerve
left side: arc of the aorta, left common carotid and left subclavian arteries, left recurrent laryngeal nerve and pleura
Right side: vagus, azygos vein and pleura
Structure: it is kept patent as an airway by presence of 15-20 C-shaped hyalin cartilages (rings) joined together by fibroelastic membrane. Posteriorly the gaps in the rings are enclosed by a sheet of visceral muscle trachealis.
Blood supply: branches from inferior thyroid and bronchial arteries. Veins drain to the inferior thyroid plexus.
Lymph drainage: lymph drains to the posteroinferior group of deep cervical nodes and to the paratracheal nodes.
Nerve supply: fibres from vagi and recurrent laryngeal nerve. Sympathetic fibres from upper ganglia of the sympathetic trunks supply the smooth muscle and blood vessels.
Clinical correlates: the tracheal and tracheobronchial  lymph nodes may become enlarged when bronchogenic carcinoma develops. Widening or distortion of the Carina results from invasion and enlargement of these nodes by cancer.