NORMAL ANATOMY OF THE LARYNX. 



109 



passing through A and B, this muscle must 

 consequently relax them. 



The closing of the anterior and central por- 

 tion of the glottis by these muscles, or that part 

 lying between T and V (Jig- 29), is effected, 

 according to Mr. Willis, partly by the approach 

 of the point V of the arytenoids towards T 

 arising from the obliquity of the axis of rota- 

 tion, and partly by the swelling of the muscle 

 whilst contracting to approximate the arytenoid 

 cartilages tending to nil the space T N X V 

 (Jig. 29), and to close tightly the sides of the 

 passage below the vocal ligaments; thus clos- 

 ing the anterior and central portions of the 

 glottis. 



The question as to how A is made a fixed 

 point, in the above demonstration, remains 

 to be solved. Mr. Willis remarks that while 

 all writers agree that the crico-thyroidei 

 serve to approximate the cricoid cartilage to 

 the thyroid, either by raising the cricoid or 

 by depressing; the thyroid, none of them have 

 shown how the cartilages are to be separated 

 again. Let us investigate this proposition. 

 In order that the motions necessary to dilate 

 the crico-thyroid space be effected by mus- 

 cular motion, it is obvious that A must be 

 made a fixed point, so that B' may be drawn 

 to B (fig. 26), by which _/' ascends to j\ the 

 object in question (Jig. 26). It is clear that 

 the crico-thyroid muscle cannot be employed 

 in this instance, as it has been already shewn 

 that its action is to force B to B' andy'toy"; 

 whereas we have now to reverse the direction, 

 and to bring back B' to B, so thaty'' may de- 

 scend to _/. The sterno-thyroidei are the only 

 muscles, which by their origin, insertion, and 

 direction of force are calculated to effect this 

 purpose ; the insertion of one of these muscles 

 being about the point 11 at an angle with the 

 axis It N (Jig. 26), its force in the line R O S 

 (fig. 26) cutting the right line O N at O ; the 

 effect of which will be to draw forwards and 

 downwards the thyroid cartilage from A to A', 

 and the point R R'; these muscles have the 

 advantage of acting on the extremities of a 

 lever equal to the line O N. When any force 

 equal to that in R S is acting simultaneously 

 with that of the thyro-arytenoideus, in the di- 

 rection A R P B perpendicular to R S, the 

 composition of these forces R S and A 11 P B 

 will produce a resultant in the diagonal R N, 

 which will cut the axis N ; and as by hypo- 

 thesis the forces R O and R P are equals, R 

 and consequently A will be fixed points ; but 

 the attachment of the thyro-arytenoid at B 

 makes an angle with the axis in the line B N, 

 and the perpendicular cutting the direction of 

 the force of this muscle produced to the axis is 

 PN; thus whilst the sterno-thyroid has, by 

 its action on the lever O N, fixed the points A 

 and R, the thyro-arytenoid may act with an 

 equal force at the point B on the lever P N ; 

 but as the force P N is produced on the cricoid 

 (which is free to move by the relaxing of the 

 crico-thyroid), the result will be to rotate the 

 point B towards A, and depress the point/' to 

 f, and thus the question is solved. In the 

 preceding demonstration it must be remem- 



bered that the point R is assumed to be that 

 in which the whole of the sterno-thyroid is in- 

 serted, whereas it is expanded upon the surface 

 around the oblique Ttde,e, but any of its fibres 

 below R will have the same effect as if at 

 R, provided they are in the line O S. It 

 must also be borne in mind that the thyro- 

 hyoid prolongs the action of the stei no-thyroid 

 to the os hyoides ; but in this instance the os 

 hyoides itself descends simultaneously with 

 the expansion of the crico-thyroid space, and 

 we know that the sterno-thyroid is always in 

 action during the descent of the larynx. There 

 is, however, very little muscular force required 

 for rotating the cricoid in the direction in ques- 

 tion. It is therefore evident that the sterno- 

 thyroid is the antagonist to the thyro-arytenoid, 

 and that, in this instance, during the rotation 

 of the cricoid on the thyroid in the direction 

 B B P A must be the antagonist to the crico- 

 thyroid. 



From the preceding demonstrations we con- 

 clude that when the crico-thyroidei, the thyro- 

 arytenoidei, the crico-arytenoidei laterales, and 

 the arytenoid muscles are acting simultane- 

 ously, the chink of the glottis is entirely closed. 

 Another function of the thyro-arytenoidei re- 

 lates to their effects during the production of 

 vocal sounds, which will be considered in the 

 article VOICE. In order that the glottis may 

 be closed in the manner just described, it is 

 necessary that the crico-thyroid assisted by the 

 sterno-thyroid should have fixed the fulcrum 

 for the play of these muscular motions. 



The crico-arytenoidei postici. The intrinsic 

 muscles of the larynx already described tend, 

 more or less, to affect the antero-posterior di- 

 ameter of the laryx, the tension of the thyro- 

 arytenoid ligaments, or the contraction of the 

 chink of the glottis. The crico-arytenoidei 

 postici have altogether a different tendency. 

 They are a pair of muscles arising at the pos- 

 terior surface of the cricoid (e e,fgs. 27&28); 

 the superior and middle fibres ascend obliquely, 

 the inferior nearly vertically to be inserted into 

 the lateral prominences at the bases of the ary- 

 tenoid cartilages, anterior to the crico-aryte- 

 noidei laterales. 



These muscles lie under the mucous mem- 

 brane of the pharynx, and upon the posterior 

 surface of the cricoid. 



The contraction of these muscles is gene- 

 rally said to draw the arytenoids backwards, 

 outwards, and downwards, and to open the 

 glottis posteriorly. This view is in a great 

 degree, but not strictly, accurate. The crico- 

 arytenoideus posticus being inserted into the 

 arytenoid cartilage at N has the effect of acting 

 as on the arm of a short lever at N, and of 

 rotating it upon the axis O P, in the direction of 

 N W, which is directly opposed to the direc- 

 tion of the force of the crico-arytenoideus late- 

 ralis, which is represented by W N, therefore 

 the effect of this muscle is to separate the vocal 

 ligaments, and consequently to open the chink 

 of the glottis. Mr. Willis remarks that the 

 thyro-arytenoidei postici do not draw the ary- 

 tenoids backwards, as described by anatomists, 

 which implies that the posterior fasciculi of the 



