CHAP. XXXI.] 



ACTION OF MUSCLES. 



437 



Fig. 213. 



The extrinsic muscles are those which connect the larynx to the 

 sternum below and the hyoid bone above: the sterno-thyroid, the 

 thryo-hyoid, and, indirectly, the sterno-hyoid, omo-hyoid and 

 stylo-hyoid, and stylo-pharyngeal muscles. 



The intrinsic muscles are. the crico-thyroidei, the arytenoidei, the 

 crico-arytenoidei postici and laterales, and the thyro-arytenoidei. 

 We must refer to works on descriptive anatomy for the details of 

 the connexions of these muscles ; it must suffice here to state, that 

 their names denote very accurately their attachments to the seg- 

 ments of the larynx upon which they can exert their force. 



Action of Laryngeal Muscles. The arytenoid cartilages are at- 

 tached by ligaments to the posterior surface of the cricoid, which 

 permit a considerable extent of rotation upon an articulating 

 surface formed on the summit of the latter; by this arrange- 

 ment the chordae vocales may be stretched or relaxed, and the 

 aperture of the glottis increased or diminished according to the 

 direction of the force which acts upon the arytenoid cartilages. 

 The posterior crico-arytenoid muscles 

 render the vocal chords more tense, but 

 at the same time cause them to diverge 

 from each other, and so the width of 

 the aperture of the glottis is enlarged. 

 On the other hand, the thryo-arytenoid 

 muscles, by drawing the arytenoid car- 

 tilages towards the thyroid, relax the vo- 

 cal ligaments, while the crico-arytenoi- 

 dei laterales by causing the arytenoid 

 cartilages to rotate inwards upon their 

 axes approximate the vocal chords and 

 diminish the aperture of the glottis. 

 The arytenoid muscle draws the two 

 cartilages towards each other and so 

 tends to diminish the aperture of the 

 glottis, especially at its posterior part. 



These different movements are 

 further accelerated or increased in ex- 

 tent by the action of certain other 

 intrinsic and extrinsic muscles of the 

 larynx. 



Magnified view of internal parts of 

 Larynx of right side, after Willis, show- 

 ing actions of Laryngeal muscles. 



1, 2. Axis of articulation of Arytenoid 

 Cartilage. 3. Direction of force of Thy- 

 ro-Arytenoid Muscle. 4. Direction of 

 force of Crico-Arytenoideus Lateralis. 



Thus the arytenoid cartilages being * Of Crksp-Arytenoideaa Posticus. e. 



"Of Arytenoideus Transversus. 



fixed by the contraction of the ary- 

 tenoid and 



6. Crico-Arytenoid Ligaments, m. Cri- 

 co-Arytenoideus Lateralis. o. Crico-Ary- 

 pOStenor CriCO-arytenOld tenoideus Posticus. v. Vocal Ligament. 



