264 



THE TRACHEA. 



Fig. 18a 



bifurcates into the two bronchi. It usually measures from four inches 

 to four inches and a half in length, and from three-quarters of an 

 inch to one inch in width ; but its length and width are liable to con- 

 tinual variation, according to the 

 position of the larjoix and the 

 direction of the neck ; moreover, 

 it usually widens a little at its 

 lower end, and its average diameter 

 is greater in the male than in the 

 female. In front and at the sides 

 the trachea is rendered cylindrical, 

 firm, and resistant, by a series of 

 cartilaginous rings ; these, how- 

 ever, are deficient behind, so that 

 the posterior portion is flattened 

 and entirely membranous (fig. 

 181). 



The windpipe is nearly every- 

 where invested by a loose areolar 

 tissue, abounding in elastic fibres, 

 and is very moveable on surround- 

 ing parts. Both in the neck and 

 thorax, it rests behind against the 

 gullet, which intervenes between it 

 and the vertebral column, but 

 towards its lower part projects 

 somewhat to the left side. The 

 recurrent laryngeal nerves ascend 

 to the larynx on each side in the 

 angle between these two tubes. 



In the neck the trachea is situa- 

 ted between the coimnon carotid 

 arteries ; at its upper end it is em- 

 braced by the lateral lobes of the 

 thyroid body, the middle part or 

 isthmus of which lies across it just 

 below the larynx. It is covered 

 in front by the sterno-thyroid and 

 sterno-hyoid muscles, between 

 which, however, there is left an 

 elongated lozenge-shaped interval 

 in the middle line : this interval 

 is covered in by a strong process 

 of the deep cervical fascia, while, 

 more superficially, another layer 

 not so strong crosses between the 

 sterno-mastoid muscles. The in- 

 ferior thyroid veins and the 

 arierkt ihyroidea ima, when that 

 vessel exists, also lie upon its 

 anterior surface ; whilst at the root of the neck, in the episternal notch, 

 the innominate artery and the left carotid pass obliquely over it as 

 they ascend to gain its sides. 



Fig. ISO. — Outline showin« the Gene- 

 ral Form of the Larynx, Trachea, 

 AND Bronchi, as seen prom before 

 (Allen Thomson). One-half the Natu- 

 ral Size. 



Ji, the great cornu of the liyoid bone ; c, 

 epiglottis ; t, superior, and t', inferior cornu 

 of _ the thyroid cartilage ; c, middle of the 

 cricoid cartilage ; t r, the trachea, showing 

 sixteen cartilaginous rings ; b, the right, 

 and U, the left bronchus. 



