RESPIRATION. 455 



From these connections, its apex projects upwards and for- 

 wards in its ordinary condition, but on the passage of a bolus 

 of food is passed backwards and downwards to close the open- 

 ing of the glottis. On looking into the larynx from above, 

 we see that, for some distance below the orifice, it is toler- 

 ably capacious, though slightly flattened from side to side; 

 below this is a constricted triangular portion, the base of 

 which is turned posteriorly; and, still lower, a widened and 

 circular part, leading into the trachea. The median con- 

 stricted part, or glottis, is bounded laterally on each side by two 

 folds of mucous membrane, the superior and inferior vocal 

 cords, separated from each other by a cavity called the ven- 

 tricle of the larynx. The superior or false vocal cords are 

 simply folds of mucous membrane, with a few elastic fibres as 

 a basis, while the lower or true vocal cords contain each a 

 considerable elastic bundle. The latter extend from the inner 

 aspect of the arytenoid cartilages, forwards to the thyroid. 

 They are chiefly put upon the stretch by two pairs of muscles, 

 the crico-arytenoideus posticus, and the thyro-arytenoideus 

 transversus, whose function is to pull the arytenoid cartilages 

 backwards and outwards. Two other muscles assist in the 

 dilatation of the glottis, while five pairs and a single muscle 

 serve to relax the vocal cords, and constrict the glottal orifice. 

 Any alteration in the size of this orifice modifies the pitch of 

 the voice. In ruminants the vocal cords are much less promi- 

 neDt than in the horse. The whole internal surface of the 

 larynx is covered by mucous membrane, provided with ciliated 

 epithelium, and attached to the subjacent parts by a conside- 

 rable amount of connective tissue. It contains numerous 

 racemose mucous glands, especially towards the base of the 

 epiglottis, and near the entrance of the larynx. This mem- 

 brane, which is supplied by the superior laryngeal nerve, is 

 extremely sensitive, a provision which leads to the ready 



