THE TRACHEA. 457 



logical study of the larynx acquires a real interest v/hen it is examined with 

 regard to the articulation of sounds, or as an organ of phonation. This 

 study, however, does not come within our province ; though what has been 

 said concerning the vocal cords will give a summary, but satisfactory, idea 

 of the mechanism which presides over this function, and the part the larynx 

 plays. 



It may also be added, that nearly all the muscles of the larynx are con- 

 cerned in phonation ; by modifying the tension and the separation of the 

 vocal cords they determine differences in the sounds. One only is concerned 

 in respiration ; this is the posterior crico-arytenoideus, which is a dilator of 

 the glottis. 



2. The Trachea. (Figs. 230, 234.) 

 Preparation. Follow the same procedure as for the dissection of the oesophagus. 



The trachea is a flexible and elastic tube, formed of a series of incom- 

 plete cartilaginous rings, which succeed the larynx, and terminate above the 

 base of the heart by two divisions, which constitute the bronchi. 



Form. This tube is cylindrical, and (slightly) flattened on both sides. 

 Its inferior face and two borders are regularly rounded, and offer transverse 

 grooves, which correspond to the intervals between the constituent pieces of 

 the trachea. The superior face, nearly plane, shows the thin and widened 

 extremities of these cartilaginous arcs. 



Course. Leaving the posterior extremity of the larynx, the trachea 

 descends backwards to the entrance of the chest, by following the inferior 

 border of the neck below the longus colli muscle. It afterwards becomes 

 inflected, superiorly, to pass between the two first ribs, enters the chest in 

 traversing the anterior mediastinum, proceeds directly backwards, and finally 

 arrives above the left auricle of the heart, to the right of the posterior aorta, 

 where the tube presents its terminal bifurcation. 



Relations. In its cervical portion, the trachea, surrounded by a loose 

 and abundant cellular tissue, lies in a kind of muscular envelope which the 

 majority of the muscles of this region form around it, and which are : the 

 sterno-hyoid and sterno-thyroid, placed in front ; the sterno-maxillaries, 

 situated at first in front, and afterwards on the sides towards their termina- 

 tion ; the subscapulo-hyoideii, above and in the middle of the lateral parts ; 

 the scalenii, altogether below and at the sides ; the longus colli, behind ; 

 and outside all these muscles, the superficial expansion of the subcutaneous 

 muscle of the neck. This envelope is thinnest in front of the middle portion 

 of the neck ; and this is the place where the operation of tracheotomy 

 should be performed. 



The trachea is also in relation, in its cervical portion: 1, With the 

 oesophagus, which descends, as we know, at first in the middle of the 

 posterior face, then to the left side of the air-tube ; 2, With the carotid 

 arteries, which pass along both sides of the tube, accompanied by their 

 satellite nerves the pneumogastric, great sympathetic, and recurrent 

 nerves. 



After clearing the two first ribs, where it reaches its thoracic portion, 

 the trachea responds, superiorly, to the longus colli and the cssophagus ; 

 below to the brachial trunks, to the anterior aorta which furnishes them, to 

 the anterior vena cava, the cardiac and recurrent nerves, and to the base 

 of the heart ; laterally, to the inferior cervical ganglia of the great sympa- 

 thetic, the vertebral vessels cervical and dorso-muscular and to the two 

 layers of the anterior mediastinum ; to the right, the vena azygos ; to the 



