514 RESPIRATORY SYSTEM OF THE HORSE 



greater part oi the cavity (Regio respiratoria) is red in color, and is covered with a 

 stratified ciliated epithelium. It contains numerous tubulo-alveolar nasal glands 

 (Glandulse 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. In it are numerous olfactory glands (Glandulse olf actoriae) ; 

 these are long, tubular, and often branched. 



The vomero-nasal organ (Organon vomeronasal) ' 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 in a 

 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. 2.5-3 cm.) 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. ^ 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 oesophagus. 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, occipito-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 larjmx 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. 



> ■JiGommonly known as the organ of Jacobson. 

 _ , . ';In the ordinary position of the head and neck, and while the parts are at rest, about half 

 of 'the lar^ynx lies between the branches of the mandible; when the head and neck are extended, 

 ■pro^QBtJbnately more of the larynx lies behind a plane through the posterior borders of the rami. 



