BRONCHI— THYROID GLAND 915 



THE BRONCHI 



Tlie right bronchus (fig. 536), from its commencement to the origin of its first 

 branch, i^ about one incli in length (25 nun.). It is shorter and wider than the 

 left, and in (Hrcction more horizontal in its passage to the root of the lung. 



Relations. — The vena azygos maj(n" arches above it from behind to end in the 

 superior vena cava, which latter is placed anteriorly. The right iiulnionary artery 

 is at first below, and then in front of it. 



The left bronchus is about two inches in length (50 mm.) outside of the 

 lung. It is more obH(|ue, longer, and narrower than the right. 



Relations. — It lies beneath the arch of the aorta, and rests upon the oesoph- 

 agus and the descending aorta. The left pulmonary artery lies in front of it. On 

 looking down the trachea the dividing ridge between the two bronchial orifices is 

 seen to be on the left of the middle line, more of the right orifice for this reason 

 being visible. This explains the fact that foreign bodies entering the trachea most 

 commoidy become lodged in the right bronchus. 



Above the point where it is crossed by the pulmonary artery the right bronchus 

 gives off an eparterial branch to the u})per lol)e of the rigid lung. Below thejioints 

 wdiere they are crossed by the pulmonary arteries, in the substance of the lungs, 

 both bronchi give off two series of hyparterial branches, lateral and dorsal, which 

 rise alternately. The dorsal branches pass towards the posterior borders of the 

 lungs, and the lateral branches curve outAvards and forwards towards the anterior 

 borders. The first lateral branch on the right side sui)])lies the middle lobe, and 

 the first lateral bi-anch on the left side the upper lobe, all the remaining hyi)arterial 

 branches are distril)uted to the lowxn* lobe. In addition to the two main sets of 

 hyparterial branches a number of small accessory 1:)ronchi arise from the antero- 

 lateral aspect of each stem bronchus; of these one Avhich rises on the right side a 

 short distance below the level of the first dorsal bronchus is called the cardiac 

 bronchus, ))ecause in some mammals it supplies a special lobe of the right lung 

 which lies l)elow and posterior to the heart. This small bronchus is the fourth 

 branch from the right stem bronchus. 



As the pulmonary artery passes dowaiwards in the lung it lies on the postero- 

 external aspect of the stem bronchus between the lateral and dorsal branches, and 

 the pulmonary vein is situated on the opposite side of the tul>e. 



THE THYROID BODY OR OLAXD 



The thyroid body is of a reddish e(jlour, and is classed amongst the ductless 

 glands. It consists of two lateral lobes, and a connecting isthmus which unites 

 them below. 



The gland is convex and rounded on its outer surface, but deeply it is moulded 

 to the parts which it overlies. It commonly weighs from one to two ounces, l)ut is 

 larger in the female, and is often increased in size during menstruation. 



The lateral lobes are pyriform in shape, with their broad ends below, and 

 measures about two inclies (50 nnii. ) in length, three-<iuarters of an inch (18 mm.) 

 in breadth, and al)out an inch (25 mm. ) in thickness near the middle. 



Relations of the lateral lobes. — Tiie ai)ex lies between the sterno-thyroid 

 and the inferior constrictor of the jiharynx, the latter separating it from the liinder 

 part of the ala of the thyroid cartilage and its inferior cornu. The base is rounded; 

 it lies under cover of the sterno-thyroid and sterno-hyoid muscles at the level of the 

 sixth ring of the trachea and about tliree-cpiarters of an inch (18 nnn.) aliove the 

 sternum. Some large radicles of the inferior thyroid vein issue from it. The exter- 

 nal surface is convex, it is covered l)y the sterno-hyoid. sterno-thyroid, and omo- 

 hyoid muscles, and it is overlapped by the anterior part of the sterno-mastoid. 

 The internal surface is concave; it is moulded on the trachea. Tlu^ j^osterior bor- 

 der is thick, it is in contact witli the carotid sluvith, and is grooved l>vthe common 

 carotid artery; on the left side, it lies in front of the opsophagus. and on both sides 

 in front of the recurrent laryngeal nerve and tlie inferior thyroid artery. The 

 anterior border is thin, above it is in relation with tlie internal terminal lirancli of 



