RESPIRATORY ORGANS. 249 



erect with its apex directed craniad to allow the passage of air 

 to the lungs, or, as in the act of swallowing, it is turned caudad 

 over the aditus laryngis so as to allow food to pass over it and 

 into the oesophagus. 



The vocal cords are two fibrous elastic bands. Each is 

 attached at one end to the apex of the arytenoid cartilage, and 

 at the other end to the median ridge on the dorsal surface of 

 the thyroid. Each supports a projecting fold of mucous mem- 

 brane, the vibration of which causes the voice. 



MUSCLES OF THE LARYNX. i. Muscles moving the entire 

 larynx. 



A. Elevators. M. thyreohyoideus (Fig. 96, /, page 229). 

 A flat band on the lateral side of the larynx. 



Origin on the lateral part of the caudal border of the 

 thyroid cartilage. 



Insertion on the medial two-thirds of the caudal border of 

 the caudal cornu of the hyoid. 



Action. Raises the larynx. 



The stylohyoid (Fig. 65, d, page 109) and the median and 

 inferior constrictors (Fig. 96,7' and k] of the pharynx, already 

 described, have the same action. 



B. Depressors. The sternothyroid (Fig. 65, g'}, already 

 described (p. 141). 



2. Muscles which move the parts of the larynx one upon 

 another. 



A. Muscles on the Outer Surface of the Larynx. 



M. cricothyreoideus (Fig. 96, ). A broad flat band 

 which with its fellow covers the ventral surface of the cricoid 

 cartilage and the cricothyroid ligament. 



Origin. The lateral half of the ventral surface of the cricoid 

 cartilage. The muscles diverge so as to leave a part of the 

 cricothyroid ligament between them. 



Insertion. The ventral part of the caudal border of the 

 thyroid cartilage laterad of the median ventral notch. 



M. cricoarytenoideus posterior (Fig. 105, b}. The two 

 muscles cover the dorsal surface of the larynx. 



Origin. From the dorsal part of the caudal border of the 

 cricoid cartilage (3) and from its median dorsal crest. The 



