106 



NORMAL ANATOMY OF THE LARYNX. 



thick, almost quadrangular-shaped muscles, 

 situated on each side of the anterior part of 

 the larynx : they arise from the anterior and 

 inferior surface of the cricoid cartilage, on each 

 side of the median line. The fibres are fleshy : 

 the most internal directed obliquely upwards 

 and outwards (m, Jig. 25), the central very 



Fig. 25. 



Fig. 26. 



A side view of the larynx with the os hyoidei attached, 

 a, the thyro-hyoideus muscle ; b, the middle 

 thyro-hyoid ligament ; e, the pomum ; d, the crico- 

 thyroid ligament; m, the crico-thyroid muscle; 

 O N, the direction of the inferior fibres of the crico- 

 thyroid lying nearly perpendicular to the axis of 

 the crico-thyroid articulation ; f, the trachea ; n n, 

 the insertion of the thyro-hyoid muscle and mem- 

 brane to the inner margin of the os hyoides. 



obliquely, and the inferior almost horizontally 

 to the inferior margin of the thyroid and to the 

 inferior horn : others are inserted into the pos- 

 terior surface of the thyroid. A portion of this 

 muscle is prolonged to the inferior constrictor 

 of the pharynx. 



Each crico-thyroid muscle is covered by the 

 sterno-thyroideus, and lies external to the crico- 

 arytenoid lateralis and the thyro-arytenoideus. 

 The triangular space between the crico-thyroidei 

 is occupied by the crico-thyroid membrane. 



The action of the crico-thyroidei is to rotate 

 the cricoid on the thyroid. The superior and 

 middle fibres are at the greatest distance from 

 the axis of rotation (N, Jig. 26), and conse- 

 quently acting as if at the arm of a long lever. 

 In this action the anterior superior margin of 

 the cricoid is elevated towards the inferior 

 edge of the thyroid from /to/' (Jig. 26), by 

 which the posterior upper margin of the cricoid 

 is carried backwards from B to B' indicated by 

 the dotted line 1, 2, 3, 4, 5, (fg. 26), and as 

 the space is greater from A B'' than A B, it is 

 manifest that the space in the mesial plane 



A view of the left side of the larynx to illustrate the 

 functions of the thyro-arytenoid, the sterno-thyroid, 

 and crico-thyroid muscles. 



The dotted line 1, 2, 3, 4, 5, shows the position 

 of the cricoid cartilage when the crico-thyroid mus- 

 cles have closed the crico-thyroid space ; m, the 

 crico-thyroid muscle ; N, the crico-thyroid articu- 

 lating axis ; A B and B A, the directions of the 

 force of the thyro-arytenoideus muscle ; R S, the 

 direction of the force of the sterno-thyroideus 

 muscle meeting that of the thyro-arytenoideus in 

 R ; R N, the resultant of the combined muscular 

 forces R P and R S ; O N and P N are perpendi- 

 cular lines drawn from the directions of the forces 

 of the thyro-arytenoideus and sterno-thyroideus 

 muscles to the common axis of rotation ; they are 

 also the "cosines of the angles R N O, R N P, and 

 B N P, and show the amount of force on the axis 

 of the sterno-thyroideus and thyro-arytenoideus 

 muscles respectively; R' and A' are the points 

 which R and A must pass through when the thy- 

 roid is rotated forwards on the cricoid ; A, the 

 point opposite which the thyro-arytenoideus is in- 

 serted into the posterior angle of the thyroid carti- 

 lage ; B, the point on which the thyro-arytenoid 

 acts in rotating B towards A ; ff, the crico-thy- 

 roid space ; h, the trachea. 



must be enlarged to an amount equal to the 

 difference of the distance A B and A B' (fig. 26). 

 The action of this muscle, therefore, is to 

 stretch the thyro-arytenoid ligaments. The 

 direction of the force of the inferior horizontal 

 fibres of the crico-thyroid which are lying 

 parallel to the line O N (Jig. 25 and 26) being 

 nearly perpendicular to the axis of rotation, can 

 have, consequently, little or no effect, until the 

 superior fibres have (by raising the cricoid) pro- 

 duced an angle with the axis N (Jig. 25) ; they 

 assist only when the crico-thyroid space is 

 diminished. It has been commonly supposed 

 that it is the thyroid which is drawn forwards 

 on the cricoid, and Cruveilhier adopts this sup- 

 position ; but it has been refuted by Magendie, 

 and not only do we observe that the attach- 

 ments of the crico-thyroidei are mechanically 

 directed to produce a rotatory motion of the 

 cricoid, but the latter has no fixed point of 



