NORMAL ANATOMY OF THE LARYNX. 
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 (fig. 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 fill the space TN X V 
(fig. 29), and to close tightly the sides of the 
passage below the vocal ligaments; thus clos- 
the anterior and central portions of the 
_ The question as to how A is made a fixed 
point, in the above demonstration, remains 
tobe solved. Mr. Willis remarks that while 
ull writers agree that the crico-thyroidei 
serve to approximate the cricoid cartilage to 
he thyroid, either by raising the cricoid or 
by depressing the thyroid, none of them have 
shown how the cartilages ate to be separated 
. 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 
2 as (fig. 26), by which f’ ascends to f; the 
object in question (fig. 26). It is clear that 
the crico-thyroid muscle cannot be employed 
in this instance, as it has heen already shewn 
that its action is to force B to B’ and f tof’; 
whereas we have now to reverse the direction, 
_and to bring back B’ to B, so that f’ may de- 
_ seend to f. The sterno-thyroidei are the only 
muscles, which by their origin, insertion, and 
direction of force are calculated to effect this 
P| se; the insertion of one of these muscles 
itis about the point R at an angle with the 
_ axis R N (fig. 26), its force in the line ROS 
(fg. 26) cutting the right line O N at O; the 
_ efiect of which will be to draw forwards and 
_ downwards the thyroid cartilage from A to A’, 
and the point R I’; 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 RS is acting simultaneously 
with that of the thyro-arytenoideus, in the di- 
rection ARP B perpendicular to RS, the 
- composition of these forces RS and ARP B 
will produce a resultant in the diagonal R N, 
feats will cut the axis N; and as by hypo- 
thesis the forces RO and RP 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 BN, 
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 f” to 
J; and thus the question is solved. In the 
preceding demonstration it must be remem- 
109 
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 ridge, but any of its fibres 
below R will have the same effect as if at 
R, provided they are in the line OS. It 
must also be borne in mind that the thyro- 
hyoid prolongs the action of the ste: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 
BBPA must 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 Vorce. 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-arytenvidei 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 altozether a different tendency. 
They are a pair of muscles arising at the pos- 
terior surface of the cricoid (e e, figs. 27 & 28); 
the superior and middle fibres ascend obliquely, 
the inferior nearly vertical'y to be inserted into 
the lateral prominences at the bases of the ary- 
tenoid cartilazes, 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 
NW, 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 
ligameuts, 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 
