THE TRACHEA 135 



above 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 epiglottis. It fits accurately over the opening of 

 the larynx, and during the act qf deglutition is closed to pre- 

 vent the entrance of food, saliva, etc. Except during deglu- 

 tition the epiglottis is raised and there is free passage of air 

 into and out of the laryngeal cavity. The vocal cords are 

 fixed anteriorly to a point between the alse 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. Dur- 

 ing 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 approxi- 

 mated. The expiratory act separates the cords and they af- 

 ford 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 re- 

 spiratory tract. It begins at the cricoid cartilage and 

 extends downward 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 num- 

 ber of cartilaginous rings, and an internal mucous mem- 

 brane. The rings are the most striking part of the tra- 

 chea. 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 interval between their ends is filled with fibrous 

 and non-striped muscular tissue. The mucous membrane is 



