There are three single cartilages and one pair; the single cartilages
are the cricoid, thyroid, and epiglottis; the arytenoid cartilages are
paired.
[Illustration:
FIG. 349.—LARYNGEAL CARTILAGES OF HORSE, RIGHT VIEW.
]
The =cricoid cartilage= (Cartilago cricoidea) is shaped like a signet
ring. The dorsal part is a broad, thick, quadrilateral plate termed the
=lamina=; the outer (dorsal) surface of this is marked by a median ridge
separating two shallow cavities, which give attachment to the posterior
crico-arytenoid muscles. On either side of these depressions are two
articular facets. The =anterior facets= (Facies articulares arytænoideæ)
are placed at the anterior border, are oval and convex, and articulate
with the arytenoid cartilages. The =posterior facets= (Facies
articulares thyreoideæ) are situated on the front of a ridge, a short
distance from the posterior border; they are concave, and articulate
with the posterior cornua of the thyroid cartilage. The ventral and
lateral parts of the ring are formed by a curved band, called the arch
(Arcus), which is narrowest below. The lateral surfaces of the arch are
grooved for the crico-thyroid muscle. The anterior border of the lamina
is thick and slightly concave; the posterior border is thin and
irregularly notched. The anterior border of the arch is concave
ventrally and gives attachment to the crico-thyroid membrane; laterally
it is thicker and gives attachment to the crico-arytenoideus lateralis
muscle. The posterior border is attached to the first ring of the
trachea by an elastic membrane. The inner surface is smooth and is
covered with mucous membrane.
[Illustration:
FIG. 350.—CRICOID CARTILAGE OF HORSE, LEFT LATERAL VIEW.
]
The =thyroid cartilage= (Cartilago thyreoidea) consists of a median
thickened portion, termed the body, and two lateral laminæ or wings. The
=body= forms ventrally a slight prominence (Prominentia laryngea), which
can be felt, but is not visible in the living subject; it supports the
base of the epiglottis, which is attached to it by an elastic ligament.
The =wings= or =laminæ= spring from the body on either side and form a
large part of the lateral wall of the larynx. Each is a rhomboid plate,
presenting a slightly convex outer surface, which is divided into two
areas by an oblique line (Linea obliqua), on which the thyro-hyoid and
thyro-pharyngeus muscles meet. The superior (dorsal) border is nearly
straight; it gives attachment to the pharyngeal fascia and the
palato-pharyngeus muscle, and bears a cornu at each end. The =anterior
cornu= (Cornu orale) is attached to the thyroid cornu of the hyoid bone;
below it is a notch, which is converted into a foramen (Foramen
thyreoideum) by a fibrous band, and transmits the superior laryngeal
nerve to the interior of the larynx. The =posterior cornu= (Cornu
caudale) articulates with the cricoid cartilage. The inferior (ventral)
border is fused with the body in front; behind, it diverges from its
fellow to inclose a triangular space (Incisura thyreoidea caudalis),
which is occupied by the crico-thyroid membrane. The anterior border is
slightly convex, and is attached to the hyoid bone by the thyro-hyoid
membrane. The posterior border overlaps the arch of the cricoid
cartilage, and gives attachment to the crico-thyroid muscle. The inner
surface is concave, and is covered by mucous membrane to a small extent.
[Illustration:
FIG. 351.—THYROID CARTILAGE OF HORSE, VENTRAL VIEW.
]
The =epiglottis= (Cartilago epiglottica) is situated above the body of
the thyroid cartilage and curves toward the root of the tongue. It is
shaped somewhat like a pointed ovate leaf, and presents two surfaces,
two borders, a base, and an apex. The =oral= (or anterior) =surface= is
concave in its length, convex transversely; the =pharyngeal= (or
posterior) =surface= has the reverse configuration. The =borders= are
thin, denticulated, and somewhat everted. The =base= is thick, and is
attached to the dorsal surface of the body of the thyroid cartilage by
elastic tissue. From each side of it a cartilaginous bar projects
backward; these processes correspond to the cuneiform cartilages of man.
The =apex= is pointed and curved ventrally. Nearly all of the epiglottis
is covered with mucous membrane. It lies sometimes in front of,
sometimes behind, the soft palate.
[Illustration:
FIG. 352.—EPIGLOTTIS OF HORSE, RIGHT LATERAL VIEW.
]
The =arytenoid cartilages= (Cartilagines arytenoideæ) are situated on
either side, in front of the cricoid, and partly between the alæ of the
thyroid cartilage. They are somewhat pyramidal in form, and may be
described as having two surfaces, two borders, a base, and an apex. The
=internal surface= is concavo-convex, smooth, and covered by mucous
membrane. The =external surface= is divided by a ridge into two areas:
the lower flat area faces outward, and is covered by the thyro-arytenoid
and lateral crico-arytenoid muscles; the upper concave area faces
chiefly forward, and is covered by the transverse arytenoid muscle. The
=anterior border= is convex, and at the base of the cartilage forms a
projection termed the =vocal process= (Processus vocalis); it is so
named because it gives attachment to the vocal or thyro-arytenoid
ligament, which forms the basis of the true vocal cord. The =posterior
border= is straight, and extends from the vocal process to the facet on
the base. The =dorsal border= forms a deep notch with the apex. The
=base= is concave and faces chiefly backward; it presents an oval,
concave facet for articulation with the anterior border of the lamina of
the cricoid cartilage. The external angle of the base forms a strong
eminence termed the =muscular process= (Processus muscularis), which
gives attachment to the crico-arytenoid muscles. The internal angle is
attached to its fellow by the transverse arytenoid ligament. The =apex=
(Cartilago corniculata) curves upward and backward, forming with its
fellow the pitcher-shaped lip, from which the cartilages derive their
name.
[Illustration:
FIG. 353.—LEFT ARYTENOID CARTILAGE OF HORSE, INNER VIEW.
]
The cricoid and thyroid cartilages, and the greater part of the
arytenoid cartilages, consist of hyaline cartilage. The apices and vocal
processes of the arytenoid cartilages and the epiglottis (including the
cuneiform processes) consist of elastic cartilage; they show no tendency
toward ossification at any age. The thyroid and cricoid cartilages
regularly undergo considerable ossification; the process begins in the
body of the thyroid, and often involves the greater part of the
cartilage.