CONTRIBUTIONS TO ZOOLOGICAL PATHOLOGY. 
351 
prominent and tense. Associated with this movement is that of 
slight elevation and protrusion of the os hyoides, in consequence of 
its intimate articulation with the anterior cornua of the thyroid car- 
tilage; and with this movement the base of the tongue is carried 
forward, the epiglottic fibro-cartilage elevated, and the aryteno- 
epiglottidean folds of mucous membrane that form the anterior or 
superior aperture of the larynx also become elongated and ren-r 
dered more tense, especially if the arytenoid cartilages remain 
fixed in their situation. 
This complex series of direct and indirect movements are all 
produced by the contraction of the crico-thyroidei muscles, aided 
by that of the sterno-hyo-thyroidei ; and which, by directly ap- 
proximating the thyroid to the cricoid cartilages inferiorly, indi- 
rectly lengthen, superiorly, the rima glottidis, and thereby render 
tense the true and false vocal chords. The thyro-hyoidei muscles, 
taking their point of fixion at the os hyoides, carry forwards the 
thyroid cartilage under cover of the os hyoides; and thus diminish- 
ing the length of the skeleton of the larynx from below, upwards, 
causes the epiglottis to be passively depressed, and placed securely 
as a coverlit, over the anterior aperture of the laryngeal cavity. 
The crico-arytenoidei superiores, or postici muscles, revolve the 
arytenoid cartilages on their axes, upwards, backwards, and out- 
wards, producing oblique eversion of their apices, and, thus 
separating them from each other at their bases, open the rima 
glottidis to its fullest extent at its base or superior angle. These 
muscles, acting in association with the crico-thyroidei and the 
sterno-hyo-thyroidei, form, with the thyro-arytenoidei transversi, 
the antagonizing points for the former ; and thus the vocal chords 
will be rendered tense to their utmost by the perfect action of 
either set. of muscles. 
The crico-arytenoidei inferior es , or laterales muscles, carry the 
bases of the arytenoid cartilages outwards, forwards, and down- 
wards, across their general axes of motion, and thus, diminishing 
the distance between these cartilages and the thyroid, indirectly 
relax the vocal chords ; and also, causing to approximate the edges 
of the rima glottidis, they tend to diminish in the centre the space 
between them. They are not muscles, however, that specially 
act in relaxing the vocal chords and closing the rima glottidis, but 
they rather act in steadying and guiding the bases of the arytenoid 
cartilages in their movements over the oblique articular surfaces 
of the cricoid ; and thus they enable those other muscles, which, by 
the axes of their movements, tend to produce the oblique inversion 
and eversion of the arytenoid cartilages, on which the state of 
relaxation or tension of the vocal chords and the size of the rima 
glottidis depend. 
