406 SPLANCHNOLOGY, OR ANATOMY OF THE VISCERA 



THE TRACHEA 



The trachea is a membranocartilaginous tube, flattened 

 behind, continuous above with the larynx, and below dividing 

 into the two bronchi. 



The trachea consists of sixteen to twenty incomplete car- 

 tilaginous rings connected by a fibrous membrane. Their free 

 ends, which are directed posteriorly, are united similarly and 

 by plain muscular tissue. Its upper limit is at the sixth cervical, 

 its lower, opposite the body or upper border of the fifth thoracic 

 vertebra, and it measures about 4^ inches in length; trans- 

 versely, 2 to 1 inch. 



The relations are: In front, in the neck, the isthmus of the 

 thyroid, the sternohyoid and thyroid muscles and the cervical 

 fascia between them, the arteria thyroidea ima, the inferior 

 thyroid veins, and the communicating branches between the 

 anterior jugulars; in the thorax, the manubrium sterni, thymic 

 remains, the left innominate vein, arch of the aorta, innomi- 

 nate and left carotid vessels, and the deep cardiac plexus. 

 Behind is the esophagus. Laterally in fhe -neck, the common 

 carotids, the lateral lobes of the thyroid, the inferior thyroid 

 arteries, and the recurrent nerves (in the angles between esopha- 

 agus and trachea) ; in the chest, the pleura of each side and the 

 vagus. The trachea is supplied with blood by the inferior 

 thyroid arteries, branch of the thyroid axis. The veins empty 

 into the thyroid venous plexus. The nerves are derived from 

 the vagus, recurrent laryngeai, and the sympathetic. 



THE PLEURA AND MEDIASTINUM 



The pleurae are two separate serous sacs which invest each 

 lung to its root and are reflected on to the thoracic walls and 

 pericardium. That portion of the serous membrane invest- 

 ing the surface of the lung and extending into the fissures 

 between the lobes is called the visceral layer of the pleura 

 (pleura pulmonalis), while the portion lining the inner surface 

 of the thorax is called the parietal layer of the pleura (pleura 

 parietalis). The latter is subdivided into the cervical, the 

 costal, the diaphragmatic, and the mediastinal portions. The 



