The larynx is the enlarged upper end of the air-passage which leads from
the pharynx to the lungs. It is a box composed of pieces of cartilage
connected by ligaments and moved by muscles, and it is lined by mucous
membrane. At the root of the tongue about one centimeter caudad of the
body of the hyoid bone is the triangular leaf-like =epiglottis= (Fig.
95, _e_, page 227). When food is taken this closes the opening into the
larynx, and the food passes over it into the œsophagus. It is so curved
that its apex is directed craniad. A fold of mucous membrane, the =plica
glossoepiglottica= or =frenulum= of the epiglottis, extends from the
middle of its cranial surface to the root of the tongue, and on each
side of this fold is a depression.
From each side of the base of the epiglottis a fold, =plica
aryepiglottica= (Fig. 95, _f_), extends caudad to the base of the
arytenoid cartilage (_g_). Dorsad of this fold and separated from it by
a depression is a ridge which marks the position of the caudal hyoid
cornu. The plicæ aryepiglotticæ (_f_) and the epiglottis (_e_) form the
boundaries of the =aditus laryngis=, or opening into the larynx.
The cavity of the larynx is divided into three portions. The upper one
of these is the =vestibule= of the larynx. It is bounded caudad by two
folds of mucosa (_i_) that stretch from the caudal surface of the
epiglottis near its base to the tips of the arytenoid cartilages. These
folds are the =false vocal cords= (_i_). Their vibration is said to
produce purring. Caudad of the false vocal cords two folds of the mucosa
stretch from the apices of the arytenoid cartilages to the thyroid
cartilages, near the base of the epiglottis. These folds are nearer the
median plane than the false vocal cords. They are the =true vocal cords=
(_j_). The =middle portion= of the laryngeal cavity is that between the
true and the false vocal cords. It is produced laterally into a very
small pouch or pocket on each side, the =ventriculus=. The narrow slit
between the true vocal cords is the =glottis= (_h_). It can be narrowed
and widened by the action of muscles. The vocal cords, which bound it,
are set vibrating by currents of air transmitted from the lungs, and the
voice-sounds are thus produced. The =caudal portion= (inferior portion)
of the laryngeal cavity is that between the glottis and the first
tracheal cartilage. It is narrowed near the glottis.
CARTILAGES OF THE LARYNX (Fig. 104).--There are three unpaired
cartilages, the =thyroid= (1), =cricoid= (3), and =epiglottic= (2), and
two paired cartilages, the =arytenoids= (4).
[Illustration: FIG. 104.--CARTILAGES OF LARYNX, WITH SIDE VIEW OF
HYOID BONE.
_b_, ceratohyal; _c_, epihyal; _d_, stylohyal; _e_, tympanohyal; _f_,
thyrohyal. 1, thyroid cartilage; 2, epiglottis; 3, cricoid cartilage;
4, arytenoid cartilage; 5, cricothyroid ligament; 6, thyrohyoid
ligament; 7, trachea.]
The =thyroid= cartilage (=cartilago thyreoidea=) (1) has nearly the form
of a visor of a cap, but is relatively broader at its ends than a cap
visor. It forms about two-thirds the circumference of a circle, and is
so situated that it embraces the other cartilages ventrally and
laterally. Its caudal and cranial borders are oblique to its
caudocranial axis and are directed dorsocaudad. To the middle of the
cranial border is attached the epiglottic cartilage (2), and the whole
cranial margin is connected by membrane (6) to the body and caudal
cornua (_f_) of the hyoid bone. The dorsal border projects craniad into
a considerable cornu which is attached to the free end of the caudal
hyoid cornu (_f_). The border also projects caudad into a process which
articulates with a facet on the lateral surface of the cricoid cartilage
(3). In the middle of the dorsal surface is a longitudinal ridge for
attachment of the vocal cords and origin of the thyroarytenoid muscles
(Fig. 105, _d_), and at the caudal end of this ridge the caudal border
presents a considerable rounded notch.
The =cricoid cartilage= (=cartilago cricoidea=) (3) has the form of a
seal ring with its broader part dorsad. The broad dorsal part of the
ring is partly embraced by the wings of the thyroid cartilage (1). Its
caudal border is undulating and nearly at right angles to its
caudocranial axis. It is connected by membrane with the first tracheal
ring. The cranial border is oblique to the long axis and lies in a plane
which passes from the ventral side craniodorsad. Near the median line on
each side it articulates by an oblique facet with one of the two
arytenoid cartilages (4). Near the midventral line the cricothyroid
ligament (5) is attached and it stretches thence to the midcaudal notch
of the thyroid cartilage (1).
The outer surface presents at the middle of each side a facet for
articulation with the thyroid, and is marked in the median dorsal line
by a ridge for the posterior cricoarytenoid muscle (Fig. 105, _b_).
The =arytenoid cartilages= (=cartilago arytenoidea=) (Fig. 104, 4) are
triangular pyramids with base and sides nearly equilateral triangles.
One side articulates with an oblique facet on the cranial border of the
cricoid near the mid-dorsal line. Near the opposite apex is attached the
vocal cord. At the lateral angle of the base is attached on its dorsal
side the posterior cricoarytenoid muscle (Fig. 105, _b_), and on its
ventral side the thyroarytenoid (Fig. 105, _d_) and lateral
cricoarytenoid (Fig. 105, _c_) muscles. The movements produced by these
muscles carry the arytenoid ends of the vocal cords toward or from the
median plane and thus open or close the glottis.
The =epiglottic cartilage= (Fig. 104, 2) is flexible (fibro-cartilage)
and of a cordate form. It is so curved that its caudal surface is convex
dorsoventrally and concave from side to side. It supports the
epiglottis. Its cranial surface presents a slight median ridge for
attachment of muscles. By its base it is attached to the midventral part
of the cranial border of the thyroid cartilage (1). Its position varies
so that it either stands erect with its apex directed craniad to allow
the passage of air to the lungs, or, as in the act of swallowing, it is
turned caudad over the aditus laryngis so as to allow food to pass over
it and into the œsophagus.
The =vocal cords= are two fibrous elastic bands. Each is attached at one
end to the apex of the arytenoid cartilage, and at the other end to the
median ridge on the dorsal surface of the thyroid. Each supports a
projecting fold of mucous membrane, the vibration of which causes the
voice.
MUSCLES OF THE LARYNX.--1. Muscles moving the entire larynx.
A. =Elevators. M. thyreohyoideus= (Fig. 96, _p_, page 229).--A flat band
on the lateral side of the larynx.
_Origin_ on the lateral part of the caudal border of the thyroid
cartilage.
_Insertion_ on the medial two-thirds of the caudal border of the caudal
cornu of the hyoid.
_Action._--Raises the larynx.
The stylohyoid (Fig. 65, _d_, page 109) and the median and inferior
constrictors (Fig. 96, _j_ and _k_) of the pharynx, already described,
have the same action.
B. =Depressors.=--The sternothyroid (Fig. 65, _g′_), already described
(p. 141).