AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



cartilage. To the elevators of the larynx belong the genio-hyoid, the mylo- 

 hyoid, the digastric, the stylo-hyoid, and the hyo-glossus. The muscles of the 

 palate and the constrictors of the pharynx enter into coordinated action with the 

 above. When food is passing through the pharyx in the act of swallowing, 

 the hyoid bone is drawn upward and forward, raising the larynx with it ; the 

 tongue is thrown backward so that the epiglottis covers the entrance into the 

 larynx, and the constrictors of the larynx contract, completely closing the 

 entrance into that organ. 



The intrinsic muscles of the larynx are the crico-thyroids, the lateral crico- 

 arytenMs, the posterior crico-arytenoids, the arytenoid, the aryteno-epiglot- 

 tideans y and the thyro-arytenoids ; all being in pairs except the arytenoid, 

 which crosses the middle line. The crico-thyroid muscle arises from the front 

 and side of the cricoid cartilage and, passing upward and backward, is inserted 

 into the lower edge of the thyroid cartilage (Fig. 298). The action of the crico- 

 thyroid muscle is to diminish the distance between the thyroid and cricoid car- 

 tilages in front, either by depressing the front of the thyroid or by elevating 

 that of the cricoid cartilage, or both. In the first case the distance between 

 the anterior attachment of the vocal cords and the vocal processes of the 



aryteuoid cartilages is increased by movement of 

 the thyroid, and in the second case the same effect 

 is produced by backward rotation of the edge of 

 the cricoid upon which the arytenoid cartilages are 

 seated (Fig. 297). The muscle, therefore, is a 

 tensor of the vocal cords. It is, probably, the 

 mechanism we ordinarily use in raising the pitch 

 of the voice when the vocal machinery has been 

 " set " by the other muscles (see below). If the 

 fingers be placed on the cricoid ring and on the 

 pomum Adami while the ascending scale is sung 

 in the middle chest register, both descent of the 

 front of the thyroid and ascent of the cricoid can 

 be made out. The lateral crico-arytenoid muscle 

 arises from the upper, lateral border of the cricoid 



FIG. 298. Lateral view of the ., , , , , , -, , 



cartilages of larynx with the crico- cartilage, and passes upward and backward to be 



thyroid muscle (Quoin'* Anatomy, i nserte( } i nto the outer edge of the arytenoid car- 

 after Willis) : 1, crico-thyroid mus- 



tilage, on and in front of the lateral prominence 

 (Fig. 299). Its main action is to wheel the 

 vocal process of the arytenoid toward the middle 

 line and thus approximate the vocal cords. The posterior crico-arytenoid is a 

 large muscle, which rises from the median posterior surface of the cricoid car- 

 tilage and passes upward and outward to be inserted into the outer surface of the 

 arytenoid cartilage, behind and above the insertion of the lateral crico-arytenoid 

 (Fig. 300). Its action is to turn the vocal processes outward and thus abduct the 

 vocal cords. The posterior crico-arytenoid occupies an important position in the 

 group of respiratory muscles; during vigorous inspiration it is brought into action 



cle ; 2, crico-thyroid membrane ; 3, 

 cricoid cartilage; 4, thyroid carti- 

 lage ; 5, upper rings of the trachea. 



