CHAP. XXXI.] HUMAN LARYNX. 433 



trical) essential to this mechanism, and there are four smaller, 

 accessory cartilages. The first are the cricoid and the thyroid 

 cartilages, the epiglottis, and the arytenoid cartilages, which are 

 symmetrical. To these may be added the cuneiform cartilages 

 and the cornicula, which are merely sesamoid bodies destined to 

 keep the folds of mucous membrane in proper position. 



The cricoid cartilage is a ring, whose lower margin is parallel to, 

 and united by fibrous membrane with, the first ring of the trachea. 

 Its upper margin slopes from behind forwards, and from above 

 downwards, so that the posterior surface of the cartilage is con- 

 siderably deeper than the anterior, and affords two large concave 

 surfaces for the attachment of the posterior crico-arytenoid muscles. 

 Its upper border is connected in front, with the lower margin of 

 the alse of the thyroid cartilage by an expansion of yellow fibrous 

 tissue, which is particularly thick in front, called the crico-thyroid 

 ligament, and which fills up the space called by surgical anatomists 

 the crico-thyroid space. This space and ligament, bounded on 

 each side by the crico-thyroid muscle, are penetrated by the trocar 

 in the operation of laryngotomy. The posterior half of the upper 

 border of the cricoid cartilage exhibits on each side an oval convex 

 articular surface, on which plays one of the arytenoid cartilages. 



The thyroid, cartilage consists of two square plates of cartilage 

 (alee) united at an acute angle in front ; each of these is prolonged 

 at the upper and lower corners behind, into a long superior process 

 (superior cornu), and a very short inferior process (inferior cornu). By 

 the superior process, and along the whole superior border of its 

 alse, the thyroid cartilage is united to the os hyoides by the thyro- 

 hyoid ligament. Each inferior process rests upon the outer surface 

 of the cricoid cartilage, and plays upon a small circular plane 

 articular facet situated thereon. 



By the angular union of the two alse of the thyroid cartilage 

 in front, a projection is formed beneath the integuments which is 

 most prominent in the male, and is commonly known as the pomum 

 Adami. Into the hollow angle behind, the vocal chords are in- 

 serted, and also, the stalk and ligament of the epiglottis. The 

 broad outer surfaces of the alse, give attachment to muscles along 

 an oblique line, which is sufficiently conspicuous between the 

 middle and lower third of each. The thyro-hyoid and sterno- 

 thyroid muscles are thus attached. To the inner surface of each 

 ala are inserted the thyro-arytenoid and the crico-thyroid muscles. 



By the gliding of the inferior horns of the thyroid upon the 

 articular facet on the outer surface of the cricoid cartilage, a 



