MECHANISM OF LAlllW \ 405 



of the ciliated variety. Except also on the free edges of the vocal 

 folds the mucous nienibrane is studded with nnicous glands. These 

 are especially plentiful upon the epiglottis (q.v.) where they are 

 lodged in little pits. 



The cavity of the larynx is divided into three parts by two pairs 

 of folds of the mucous membrane which project from the sides of 

 the cavity into its interior. The upper or ventricular folds are the 

 so-called false vocal cords. Each encloses a narrow band of 

 fibrous tissue (the ventricular ligament), which is fixed in front 

 to the thyreoid cartilage and behind to the arytsenoid cartilage. 

 The fissure between the false cords is termed the rima vestibuli. 

 The lower or vocal folds are the true vocal cords. Each encloses a 

 band of yellow elastic tissue (the vocal ligament), which is fixed to 

 the same cartilages as the corresponding ventricular ligament, but 

 a little lower down. The vocalis muscle lies lateral to and parallel 

 with the vocal ligament. The fissure between the true cords is 

 called the rima glottidis. Between the false and the true cords is a 

 recess known as the ventricle of the larynx (or of Morgagni), 



False Cords. The ventricular folds play only a protective part 

 in phonation, keeping the true cords moistened by the secretion of 

 the numerous mucous glands with which they and their appendices 

 are provided. They are of use in " holding the breath." Animals 

 which have to do this often in fight or flight have w^ell-developed 

 ventricular folds. 



True Cords. The alterations in the width and shape of the 

 rima glottidis, brought about by the movements of the vocal folds 

 and arytaenoid cartilages during phonation, can be studied readily 

 by the use of the laryngoscope. This instrument consists of two 

 mirrors, a small one which is held by means of a long handle at the 

 base of the uvula with the mirror directed at an angle towards the 

 larynx, and a larger mirror with a central hole through which the 

 observer examines the image in the small mirror. The practical 

 details of this method of examination are given in any practical 

 text-book of physiology (" Practical Physiology," by Anrep and 

 Harris) and are out of place here. 



In order that the vocal cords may set the air current into vibra- 

 tion they must be put into a state of tension. In the dead 

 larynx it is possible to produce sounds by forcing air from bellows 

 through the larynx, meanwhile applying tension to the cords by 

 pulling the arytaenoid cartilages backw^ards. 



From experiments on the cadaver and on tracheotomised 

 patients it has been found that with a constant tension on the 

 cords sounds vary in loudness and in pitch with the air pressure 

 developed. In the following table (LXI.) is given the limits of 



