Respiratory System General, 109 



like a bulla, to receive a sacculus. The Cricoid is generally 

 smaller than the thyroid, but may be even three times greater 

 in size ; it is usually complete anteriorly, but may be almost 

 divided by a cleft : it is sometimes so extended posteriorly as 

 to cover the five anterior tracheal rings. The Epiglottis is 

 almost universally present, protecting the entrance into the 

 Larynx ; it may be entire, or notched at its apex ; its sides are 

 sometimes attached to the apices of the arytenoid cartilages ; 

 together with the arytenoids it may be enclosed by a sheath 

 of pharyngeal mucous membrane ; it may contain no true 

 cartilage ; and may be perforated by apertures leading into 

 sacculi. The Arytenoids sometimes extend half way across the 

 rima glottidis ; they maj'- be, as above said, enveloped with 

 the epiglottis in a membranous sheath ; and, together with 

 the cartilages of Santorini, are sometimes confluent at their 

 apices. The Vocal Chords are subject to great variations in 

 point of development ; both upper and lower ^chordae vocales ' 

 may be reduced to half their normal extent, owing to the 

 anterior extension of the arytenoids ; the superior ^ chordae ' 

 may be represented by a slight thickening of the lower border 

 of the lateral membranes continued from the arytenoids to the 

 base of the epiglottis ; again, the lower vocal chords may be 

 alone manifested, and the interchordal ventricle obsolete : or 

 there may be no vocal chords, and quasi- ventricles alone 

 present ; lastly, both the vocal chords and ventricles may be 

 wanting. Certain species vibrate their vocal chords by 

 currents of air in alternate opposite directions, but this is 

 exceptional. The interchordal ventricles when present vary 

 much in relative size ; they may be subdivided into several 

 pouches. Above the convergence of the upper chords is some- 

 times found the opening of a ' hyoid sac,' the basi-hyal bone 

 being hollowed out to receive this sac. Some species of Mam- 

 malia are always mute : others are mute except at the sexual 



