966 THE ORGANS OF VOICE AND RESPIRATION. 



covered from before backward by the first piece of the sternum, the remains of the 

 thymus gland, the left innominate vein, the arch of the aorta, the innominate and 

 left common carotid arteries, and the deep cardiac plexus. Posteriorly, it is in 

 relation with the oesophagus ; laterally, in the nc.ek, it is in relation with the com- 

 mon carotid arteries, the lateral lobes of the thyroid gland, the inferior thyroid 

 arteries, and recurrent laryngeal nerves ; and, in the thorax, it lies in the upper 

 part of the interpleural space (superior mediastinum), and is in relation on the 

 right to the pleura and right vagus, and near the root of the neck to the 

 innominate artery ; on its left side are the recurrent laryngeal nerve, the aortic 

 arch, the left common carotid and subclavian arteries. 



The Right Bronchus, wider, shorter, and more vertical in direction than the 

 left, is about an inch in length, and enters the right lung opposite the fifth dorsal 

 vertebra. The vena azygos major arches over it from behind; and the right 

 pulmonary artery lies below and then in front of it. About three-quarters of an 

 inch from its commencement it gives off a branch to the upper lobe of the right 

 lung. This is termed the eparterial branch, because it is given off above the 

 right pulmonary artery. The bronchus now passes below the artery, and is known 

 as the hyparterial branch. It divides into two branches for the middle and lower 

 lobes. 



The Left Bronchus is smaller and longer than the right, being nearly two inches 

 in length. It enters the root of the left lung, opposite the sixth dorsal vertebra, 

 about an inch lower than the right bronchus. It passes beneath the arch of the 

 aorta, crosses in front of the oesophagus, the thoracic duct, and the descending 

 aorta, and has the left pulmonary artery lying at first above, and then in front of 

 it. The left bronchus has no branch corresponding to the eparterial branch of the 

 right bronchus, and therefore it has been supposed by some that there is no upper 

 lobe to the left lung, but that the so-called upper lobe corresponds to the middle 

 lobe of the right lung. 



When the bronchi enter the lung they appear to divide into nearly equal 

 branches at the root of the lung, but a somewhat similar arrangement to what is 

 found in many animals may be made out where each bronchus passes downward 

 and backward toward the extremity of the lower lobe, giving off four branches at 

 intervals in two directions, dorsally and ventrally, and, in addition, accessory 

 branches, which arise from the front of the bronchus and pass mesially and dor- 

 sally into the inferior lobe. In the right bronchus the first ventral branch sup- 

 plies the middle lobe, the other three and all the dorsal going to the lower lobe ; in 

 the left bronchus, the first ventral supplies the superior lobe, and all the others, 

 both ventral and dorsal, go to the lower lobe. 



If a transverse section of the trachea is made a short distance above its 

 point of bifurcation, and a bird's-eye view taken of its interior (Fig. 534), the 

 septum placed at the bottom of the trachea and separating the two bronchi will be 

 seen to occupy the left of the median line, and the right bronchus appears to be a 

 more direct continuation than the left, so that any solid body dropping into the 

 trachea would naturally be directed toward the right bronchus. This tendency is 

 aided by the larger size of the right tube as compared with its fellow. This fact 

 serves to explain why a foreign body in the trachea more frequently falls into the 

 right bronchus. 1 



Structure. The trachea is composed of imperfect cartilaginous rings, fibrous 

 membrane, muscular fibres, mucous membrane, and glands. 



The cartilages vary from sixteen to twenty in number : each forms an imper- 

 fect ring, which surrounds about two-thirds of the cylinder of the trachea, 

 being imperfect behind, where the tube is completed by fibrous membrane. The 

 cartilages are placed horizontally above each other, separated by narrow mem- 

 branous intervals. They measure about two lines in depth, and half a line in 

 thickness. Their outer surfaces are flattened, but internally they are convex. 



1 Reigel asserts that the entry of a foreign body into the left bronchus is by no means so infre- 

 quent as is generally supposed. See also Med.-Chir. Trans., vol. Ixxi., p. 121. 



