THE TRACHEA 339 



affected, but, owing to the narrowness of the rima glottidis, 

 inspiration becomes laboured and difficult, and a slight degree 

 of oedema glottidis may cause complete obstruction. 



Bilateral paralysis of the crico-thyreoid results in relaxation 

 of the vocal folds owing to the increased tonus of the 

 unopposed vocalis and thyreo-arytaenoid muscles. On laryngo- 

 scopic examination, the rima glottidis is observed to become 

 slightly oval in outline when the vocal folds are adducted. 

 The condition never occurs alone, and it is usually accom- 

 panied by other signs of vagus paralysis. 



THE TRACHEA AND BRONCHI 



The Trachea begins at the lower border of the cricoid 

 cartilage and extends downwards through the neck into the 

 thorax. It terminates at the upper border of the fifth thoracic 

 vertebra, which corresponds, in level, to the sternal angle 

 (p. 294) on the anterior surface of the body and to the tip of 

 the third thoracic spine on the posterior surface. 



Except at its termination, which is often displaced slightly 

 to the right, the trachea lies in the median plane, and it is 

 separated from the vertebral column only by the oesophagus. 

 This posterior bony relation is of importance, for it renders the 

 trachea liable to become narrowed when compressed in an 

 antero-posterior plane. 



The isthmus of the thyreoid gland lies in front of the 

 second, third and fourth rings of the trachea, and, when it 

 becomes enlarged, it can exercise considerable backward 

 pressure. Since the lobes of the gland are also involved, 

 the trachea is gripped by the tumour and compressed against 

 the vertebral column. This produces a mechanical obstruction 

 to respiration, which is indicated by the characteristic "brassy" 

 sound of the cough, and the condition is often aggravated by 

 abductor paralysis of the vocal folds, due to pressure on the 

 recurrent nerves (Fig. 50). 



Within the thorax, the trachea is crossed by the arch of the 



