THE TRACHEA 135 



and parallel with the true cords. A small triangular leaflet of 

 fibre-cartilage is attached by its base to the base of the tongue 

 and to the upper anterior part of the larynx. This is the epi- 

 glottis. It fits accurately over the opening of the larynx, and 

 during the act of deglutition is closed to prevent the entrance of 

 food, saliva, etc. Except during deglutition the epiglottis is 

 raised and there is free passage of air into and out of the laryn- 

 geal cavity. The vocal cords are fixed anteriorly to a point 

 between the alae of the thyroid and posteriorly to the movable 

 arytenoids. Intrinsic muscles have the power of so moving 

 the arytenoids as to separate and approximate the posterior 

 attachments of the cords and thus increase or decrease the size 

 of the rima glottidis. During inspiration these muscles act to 

 separate the cords and allow free entrance of air into the trachea. 

 When this act has ceased they relax and the cords are passively 

 approximated. The expiratory act separates the cords and they 

 afford no obstruction to the exit of air. The inspiratory act, on 

 the other hand, tends to draw the cords together and the active 

 intervention of the muscles is necessary to keep the glottis open. 

 The Trachea. The trachea succeeds the larynx in the respi- 

 ratory tract. It begins at the cricoid cartilage and extends down- 

 ward for about four and a half inches where it bifurcates to form 

 the right and left bronchi, one of which goes to each lung. The 

 trachea consists of an external fibrous membrane, between the 

 layers of which are a number of cartilaginous rings, and an inter- 

 nal mucous membrane. The rings are the most striking part of 

 the trachea. They serve to keep the canal open at all times. 

 The inspiratory effort would otherwise collapse the walls and 

 prevent the entrance of air. These rings are sixteen to twenty 

 in number, and are lacking in the posterior third or fourth of the 

 circumference. They are, therefore, not true rings. The inter- 

 val between their ends is filled with fibrous and non-striped mus- 

 cular tissue. The mucous membrane is lined by ciliated epithe- 

 lium, and has mucous glands in its substance (Figs. 47, 48). 



