THE LARYNX 901 



The middle mediastinum contains the heart, the ascending portion of 

 the aorta and tlic ])uhnonary artery, the superior vena cava (lower half), all of 

 which are placed within the pericardium; and in addition the phrenic nerves and 

 their companion arteries, the termination of the azygos vein, the roots of the lungs, 

 with some bronchial lym])liatic ghmds. 



The anterior mediastinum is narrow above, lying Ijehind the sternum op])osite 

 the second, thiid, and luuith cartilages. Below, it o])ens out into a quadrangular 

 space, having nearly the whole of the lower lialf of the sternum, the lower edge of 

 the fourth, the fifth, part of the sixth, and the termination of the seventh cartilages 

 of the left side, Avith the triangularis sterni muscle in front (fig. 521). The pericar- 

 dium forms its posterior wall. The space merely contains some small lymphatic 

 glands and areolar tissue, and a few irregular bands, the inferior sterno-pericardiac 

 ligaments, with the thymus gland or its vestiges. It corresponds to the area of 

 cardiac dullness. 



The posterior mediastinum is bounded in front by the roots of the lungs and 

 posterior surface of the pericardium, and behind by the thoracic vertebral column 

 below the fourth thoracic vertebra. On each side it is bounded by the pleural sacs. 

 It contains the descending aorta, oesophagus, and pneumogastric nerves, azygos 

 veins, thoracic duct, and lym])hatic glands. 



THE ORGANS OF VOICE 



THE LARYNX 



The larynx is the organ of the voice, and the protector of the air passages from 

 the intrusion of foreign bodies. By its closure, moreover, it retains the air within 

 the chest cavity, contril)uting to the fixity of the thorax, a condition which is 

 essential to the due performance of a variety of muscular actions. 



It consists of a framework of cartilages held together by ligaments and acted 

 U])()n by muscles (extrinsic and intrinsic). It is lined by a very sensitive mucous 

 membrane, and sup])lied with blood-vessels, nerves, and lymphatics. 



The cartilages of the larynx are nine in numl)er. ' Some of these are single, 

 some are arranged in pairs. 



Single cartilages : — Paired cartilages : — 



Thyroid. Arytenoid. 



Cricoid. Cornicula larvngis (Santorini). 



Epiglottis. Cuneiform ( Wrisberg). 



From their general structure they may be arrranged as follows: — 



Hyaline : — Yellow elastic : — 



Thyroid. E})igl()ttis. 



Cricoid. Cornicula larvngis. 



Arytenoid (the tip of the arytenoid Cuneiform, 

 is yellow elastic). 



In the anteater there extends backwards from each side of the epiglottis to the summit of 

 the arytenoids a continuous rim of yellow elastic cartilage. It is the broken remnant of this 

 which in man forms tlie cornicula and the cuneiform cartilages. (Blaud Sutton.) 



The THYROID CARTILAGE forms the front and sides of the upper ])art of the 

 l.'irvnx, being placed al)Ove the cricoid. It consists of two nearly square symmetrical 

 plates, or alte, united in front, where they form the ponunn Adami, but widely 



