456 THE EESPIEATOEY APPARATUS IN MAMMALIA. 



inferior border and inner face of the vocal cords. The presence of nervous 

 filaments, analogous to those of the trachea, in the sub-glottal part of the 

 larynx may explain the difference, well known to physiologists, that exists 

 between the sensibility of the entrance to the glottis and that of the inferior 

 border of the vocal cords. 



EXTERNAL SURFACE OF THE LARYNX. It is divided into four planes : a 

 superior, inferior, and two lateral. The superior plane, formed by the ary- 

 tenoid and posterior crico-arytenoid muscles, is covered by the pharynx 

 and oesophagus ; in its anterior moiety, it is directly covered by the pharyn- 

 geal mucous membrane. 



The inferior plane presents, from before to behtnd, the thyro-hyoid 

 membrane, the body of the thyroid cartilage, the crico-thyroid membrane, the 

 inferior part of the cricoidxsartilage, and the crico-trachealis ligament. There 

 is remarked, laterally, the inferior border of the thyro-hyoid muscle. This 

 plane corresponds to the scapulo-hyoideal muscles, which entirely cover it. 



The lateral planes exb'bit the external faces of the thyro-hyoid and 

 crico-thyroid muscles, that of the cricoid cartilage, and the alee of the 

 thyroid. They also show the opening through which passes the superior 

 laryngeal nerve ; they are related to the crico- and thyro-pharyngeal muscles, 

 as well as the maxillary gland. 



INTERNAL SURFACE OF THE LARYNX. This surface is divided into three 

 perfectly distinct regions : a middle one, named the glottis ; a superior, called 

 the supraglottic portion ; and an inferior, designated the subglottic portion. 



The glottis (rima glottidis) is a narrow space which affects the figure of a 

 very elongated isoscelated triangle, its base being uppermost. This irregular 

 fissure is comprised between the elastic structures known as the vocal cords. 

 It is the narrowest part of the larynx. 



The supraglottic portion, wider than the glottis, but always greatly 

 depressed on each side, particularly in the region comprised between the 

 arytenoids, presents: 1, The two ventricles of the larynx, lateral excavations, 

 dilated at the bottom, and which penetrate between the anterior border of 

 the vocal cords and the prolongations of the base of the epiglottis, insinuat- 

 ing themselves even between the fasciculi of the thyro-arytenoid muscle 

 (in the Ass and Mule the ventricles are proportionately larger than in the 

 Horse, and open close to the base of the epiglottis) ; 2, The subepiglottic 

 sinus, a deep depression at the base of the epiglottis, which is provided, in 

 the Ass and Mule, with a thin membrane, capable of vibrating ; 3, The 

 entrance of the larynx, or pharyngeal opening of the cavity, a vast, gaping 

 aperture of an oval form, circumscribed by the anterior border of the 

 arytenoids and the lateral border of the epiglottis, and making a remark- 

 able projection at the bottom of the pharyngeal space. 



The subglottic portion of the larynx is the widest of the three; it is 

 directly continuous with the internal canal of the trachea. In front is seen 

 the prominence formed by the posterior border of the vocal cords : above, 

 a diffused and shallow excavation, placed at the point of junction of the 

 ary tenoid and cricoid cartilages, and which is named 'the subarytenoid sinus. 



FUNCTIONS. As a tube intended for the passage of a column of air 

 during the act of respiration, the larynx does not give rise to any very 

 interesting physiological considerations. It is, nevertheless, worthy of remark 

 that this organ, in imitation of the nostrils, dilates or contracts, according 

 to the volume of the column of air introduced into, or expelled from, the 

 lungs, and that its paralysis, during rapid movements, causes an embarrass- 

 ment in the respiration which betrays itself in " roaring." But a physio- 



