1218 THE RESPIRATORY SYSTEM 



The cartilago triticea is the remains of a connection between the thyreoid and hyoid present 

 in the embryo. It persists in adult hfe in some lower animals. 



The hyo-epiglottic ligament [lig. hyoepiglotticum] (figs. 978, 981) connects the 

 anterior surface of the epiglottic cartilage with the superior margin of the body and 

 the greater cornua of the hyoid. It is a broad sheet, lying above a mass of fat 

 which stands between the median hyo-thyreoid membrane and the epiglottis 

 and spreading laterally to join the pharyngeal aponeurosis in the region of the 

 piriform recess. 



The name glosso-epiglottic ligament is given to the elastic fibres extending 

 between the root of the tongue and the epiglottis within the median glosso-epi- 

 glottic fold. 



The corniculo -pharyngeal ligament (fig. 977) extends from the corniculate cartilage down- 

 ward and toward the median hne, attaching to the mucosa of the pharjoix and joining its 

 fellow behind the arytaenoid muscle. From this point a single band, the cr ice -pharyngeal liga- 

 ment [lig. cricopharyngeum], which may enclose a nodule of cartilage (the interarytaenoid 

 or procricoid cartilage), descends in the middle hne, to be fixed to the cricoid lamina and into 

 the pharyngeal mucosa. 



The larynx and trachea are united by fibrous membrane, the crico-tracheal 

 liganaent [lig. cricotracheale] (figs. 974, 978), between the inferior margin of the 

 cricoid cartilage and the upper margin of the first tracheal ring. Posteriorly 

 the ligament is continued into the membranous wall of the trachea. 



MUSCLES OF THE LARYNX 



Of the many muscles connected with the larynx, two groups may be recog- 

 nised, the members of one coming from neighbouring parts, fixing themselves to 

 the larynx and acting upon the organ as a whole; the members of the other 

 group confining themselves exclusively to the larynx and acting so as to affect its 

 parts. The muscles composing the first group are described elsewhere. (See 

 Section IV.) The muscles of the second group are composed of striated fibres 

 and are supplied by the vagus nerve through its laryngeal branches. These 

 muscles are all more or less under cover of the thyreoid cartilage, with one ex- 

 ception, the crico-thyreoid. 



The crico-thyreoid muscles [m. cricothyreoideus] (fig. 980) are placed one on 

 either side of the outer surface of the larynx in its lower part. Each muscle is 

 partially separated into an anterior straight [pars recta] and a posterior oblique 

 portion [pars obliqua], which together arise from the arch of the cricoid. The 

 fibres of the straight part ascend steeply and are inserted into the inferior margin 

 of the thyreoid cartilage. The obhque portion is inserted into the inferior cornu 

 and into the lower margin and inner surface of the thyreoid cartilage. 



The straight part elevates the arch of the cricoid, causing the lamina, and with it the 

 a,rytaenoid cartilages, to sink, while the obhque part draws forward the thyreoid; thus the vocal 

 hgaments are made tense. The muscle is supplied by the external branch of the superior laryn- 

 geal nerve. A connexion between the posterior part of this muscle and the inferior constrictor 

 of the pharynx and their common nerve-supply indicate their genetic relationship. 



The posterior crico-arytaenoid muscle [m. cricoarytaenoideus posterior] (figs. 

 980, 981, 982), paired, is situated at the back of the larynx, covered by the submu- 

 cous coat of the pharynx. It is a thick, triangular mass which takes origin from 

 the posterior surface of the cricoid lamina, the two muscles being well separated 

 by the median crest of the cartilage. The lower fibres ascend and the upper 

 ones pass horizontally lateralward and are inserted into the muscular process 

 of the arytaenoid cartilage on its posterior surface and tip. 



When these muscles contract, the muscular processes of the arytajnoids are pulled back- 

 ward and downward, while the vocal processes travel lateralward and a little upward, so that 

 the rima glottidis is widened and the vocal ligaments made tense (fig. 982). The innervation 

 is by the posterior branch of the inferior laryngeal nerve. 



In ether narcosis the dilator muscle is later paralyzed and afterward earher restored than 

 the con.strictor3 of the larynx. 



At the lower margin of this muscle a small shp, the cerato-cricoid muscle [m. ceratocri- 

 coidnusl, is sornctimos found, extending between the lamina of the cricoid and the inferior cornu 

 of the thyreoid cartilage. 



The constrictor laryngis. — Whereas the crico-aryta^noideus posterior is a dila- 

 tor^of the larynx, the several muscles now to be considered are in the main con- 



