514 RESPIRATORY SYSTEM OF THE HORSE 
greater part ot the cavity (Regio respiratoria) is red in color, and is covered with a 
stratified ciliated epithelium. It contains numerous tubulo-alveolar nasal glands 
(Glandule nasales). The submucosa contains rich venous plexuses which form 
in certain situations a sort of cavernous tissue (Plexus cavernosus nasalis), composed 
of several strata of freely anastomosing veins, between which there are unstriped 
muscle-fibers. This arrangement is most marked in the turbinate folds, on the 
lower part of the ventral turbinate bone, and the lower part of the septum. The 
mucous membrane of the vestibule presents numerous small depressions (openings 
of gland ducts), and is covered with stratified squamous epithelium. On the 
posterior part of the lateral masses of the ethmoid and the adjacent part of the 
dorsal turbinate and the septum (Regio olfactoria) the mucous membrane is brown- 
ish-yellow in color and thicker; it contains the olfactory nerve-endings in a special 
non-ciliated epithelium. Init are numerous olfactory glands (Glandulz olfactoriz) ; 
these are long, tubular, and often branched. 
The vomero-nasal organ (Organon vomeronasale)* is situated on the floor of 
the nasal cavity, on either side of the ventral border of the septum (Fig. 452). 
It consists of a tubular cartilage (Cartilago vomeronasalis) lined with mucous 
membrane (Ductus vomeronasalis), to which fibers of the olfactory nerve may be 
traced. Its anterior part communicates with the nasal cavity by a slit-like orifice 
in common with the incisive or naso-palatine duct. The posterior blind end is ina 
transverse plane through the second or third cheek tooth. The average length of 
the organ is about five inches (ca. 12 cm.). 
The ductus incisivus (or naso-palatine duct) is a small mucous tube, an inch 
or more (ca. 2.5-8 em.) in length, which extends obliquely through the palatine 
fissure. Its nasal end communicates with the nasal cavity through a slit-like 
opening in common with the vomero-nasal organ. The palatine extremity ends 
blindly in the submucous tissue of the hard palate. 
The paranasal sinuses are described in the Osteology. 
Vessels and Nerves.—The arteries are branches of the ethmoidal, spheno- 
palatine, palatine, superior labial, and lateral nasal arteries: The blood is carried 
away by corresponding veins. The lymph vessels go to the mandibular and 
pharyngeal lymph glands. The nerves come from the olfactory and trigeminal 
nerves. 
THE LARYNX 
The larynx is a short tube which connects the pharynx and trachea. It is a 
complex valvular apparatus, which regulates the volume of air in respiration, 
prevents aspiration of foreign bodies, and is the chief organ of voice. } 
It is situated partly between the medial pterygoid muscles, partly in the neck | 
between the parotid glands.2 Its long axis is practically horizontal in the ordinary 
position of the head and neck. It is related dorsally to the pharynx and the origin | 
of the esophagus. Ventrally it is covered by the skin, fascia, and sterno-hyoid 
and omo-hyoid muscles. Laterally it is related to the parotid and mandibular | 
glands and to the medial pterygoid, oceipito-mandibularis, digastricus, stylo-hyoid, | 
and pharyngeal constrictor muscles. It is attached to the body and thyroid cornua 
of the hyoid bone, and thus indirectly to the base of the cranium. Its cavity 
communicates dorsally with the pharynx, posteriorly with the trachea. The | 
skeleton of the larynx consists of a framework of cartilages, which are connected by | 
joints and ligaments or membranes, and moved by extrinsic and intrinsic muscles. 
It is lined with mucous membrane. 
1Commonly known as the organ of Jacobson. | 
2Tn the ordinary position of the head and neck, and while the parts are at rest, about half 
of the larynx lies between the branches of the mandible; when the head and neck are extended, | 
proportionately more of the larynx lies behind a plane through the posterior borders of the rami. 
