106 
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, fig. 25), the central very 
Fig. 25. 
A side view of the larynx with peta gape attached. 
a, the ih o-hyoideus muscle; 6, the middle 
thyro-hyoid ligament; e, the pomum; d, the crico- 
thyroid ligament; m, the crico-thyroid muscle ; 
ON, the direction of the inferior fibres of the crico- 
thyroid lying nearly perpendicular to the axis of 
the crico-thyroid articulation; f, the trachea; nn, 
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, fig. 26), and conse- 
uently acting as if at the arm of a long lever. 
n this action the anterior superior margin of 
the cricoid is elevated towards the inferior 
edge of the thyroid from f to f” (fig. 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, (fig. 26), and as 
the space is greater trom A B' than A B, it is 
manifest that the space in the mesial plane 
NORMAL ANATOMY OF THE LARYNX. 
Fig. 26. 
A view of the left side of the larynx to ill 
functions of the thyro-arytenoid, the st 
and crico-thyroid muscles, 
The dotted line 1, 2, 3, 4, 5, shows the 
of the cricoid cartilage when the crico-thyroid 1 
cles have closed the crico-thyroid > Mm, 
crico-thyroid muscle; N, the crico- at 
lating axis; A B and BA, the directions o 
force of the thyro-arytenoideus muscle; R S, 
direction of the force of the sterno-thyr 
muscle meeting that of the thyro-arytenc 
R; RN, the resaltant of the combined m 
forces R P and RS; ON and P N are } 
cular lines drawn from the directions of the f 
of the thyro-arytenoideus and sterno-th roi 
muscles to the common axis of rotation; they 
also the cosines of the angles R N O, RN P, 
B N P, and show the amount of force on the; 
of the sterno-thyroideus and thyro-ary 
muscles respectively; R’ and A’ are the p 
which R and A mu&t pass through when the 
roid is rotated forwards on the cricoid; A, 
point opposite which the thyro-arytenoideus is 
serted into the posterior angle of the thy 
lage; B, the point on which the thyro 
acts in rotating B towards A; ff’, 
roid space; A, the trachea. 
must be enlarged to an amount equal to 
difference of the distance A Band A B’ (fig. 
The action of this muscle, therefore, i: 
stretch the thyro-arytenoid ligaments. — 
direction of the force of the inferior horiz 
fibres of the crico-thyroid which are }j 
parallel to the line O N (fig. 25 and 26) b 
nearly perpendicular to the axis of rotation, 
have, consequently, little or no effect, ur 
superior fibres have (by raising the cricoid) 
duced an angle with the axis N (fig. 25); 1 
assist only when the crico-thyroid spa 
diminished. It has been commonly sw 
that it is the thyroid which is drawn for 
on the cricoid, and Cruveilhier adopts this : 
position ; but it has been refuted by Magen 
and not only do we observe that the atta 
ments of the crico-thyroidei are mechanic 
directed to produce a rotatory motion ¢ 
cricoid, but the latter has no fixed point 
