956 



THE ORGANS OF VOICE AND RESPIRATION. 



muscles, and the portion of cartilage included between it and the posterior border 

 to part of the Inferior constrictor muscle. 



The inner surface of each ala is smooth, slightly concave, and covered by 

 mucous membrane above and behind ; but in front, in the receding angle formed by 

 their junction, are attached the epiglottis, the true and false vocal cords, the Thyro- 

 arytenoid and Thyro-epiglottidean muscles, and the thyro-epiglottidean ligament. 



The upper border of the thyroid cartilage is sinuously curved, being concave 

 at its posterior part, just in front of the superior cornu, then rising into a convex 

 outline, which dips in front to form the sides of a notch, the thyroid notch, in 



the middle line, immediately above the pomum 

 Adami. This border gives attachment through- 

 out its whole extent to the thyro-hyoid mem- 

 brane. 



The lower border is nearly straight in front, 

 but behind, close to the cornu, is concave. It 

 ^ S connecte d to t' ie cricoid cartilage, in and near 

 the median line, by the middle portion of the 

 crico-thyroid membrane ; and, on each side, by 

 the Crico-thyroid muscle. 



The posterior borders, thick and rounded, 

 terminate, above, in the superior cornua, and 

 below, in the inferior cornua. The two superior 

 cornua are long and narrow, directed upward, 

 backward, arid inward, and terminate in conical 

 extremities, which give attachment to the lateral 

 thyro-hyoid ligament. The two inferior cornua 

 are short and thick ; they pass downward; with 

 a slight inclination forward and inward, and 

 present, on their inner surfaces, a small oval 

 articular facet for articulation with the side of 

 the cricoid cartilage. The posterior border 

 receives the insertion of the Stylo-pharyngeus 

 and Palato-pharyngeus muscles on each side. 



During infancy the alre of the thyroid carti- 

 lage are joined to each other by a narrow, lozenge-shaped strip, named the infra- 

 thyroid cartilage. This strip extends from the upper to the lower border of the 

 cartilage in the middle line, and is distinguished from the alas by being more 

 transparent and more flexible. 



The cricoid cartilage is so called from its resemblance to a signet ring 

 (x t olxo~, a ring). It is smaller, but thicker and stronger than the thyroid carti- 

 lage, and forms the lower and back part of the cavity of the larynx. It consists 

 of two parts : a quadrate portion, situated behind, and a narrow ring or arch, one- 

 fourth or one-fifth the depth of the posterior part, situated in front. The posterior 

 square portion rapidly narrows at the sides of the cartilage, at the expense of the 

 upper border, into the anterior portion. 



Its posterior portion is very deep and broad, and measures from above down- 

 ward about an inch (23 cm.) ; it presents, on its posterior surface, in the 

 middle line, a vertical ridge for the attachment of the longitudinal fibres of the 

 oesophagus ; and on either side a broad depression for the Crico-arytenoideus 

 posticus muscle. 



Its anterior portion is narrow and convex, and measures vertically about one- 

 fourth or one-fifth of an inch (57 cm.) ; it affords attachment externally in 

 front and at the sides to the Crico-thyroid muscles, and behind, to part of the 

 Inferior constrictor. 



At the point of junction of the posterior quadrate portion with the rest of the 

 cartilage is a small round elevation, for articulation with the inferior cornu of the 

 thyroid cartilage. 



FIG. 527. Side view of the thyroid and 

 cricoid cartilages. 



