440 RESPIRATORY SYSTEM OF THE HORSE 



turbinal and the floor of the cavity. It is much larger than the other two, and is 

 the cUrect passage between the nostrils and the pharynx. 



The common meatus (Meatus nasi communis) is situated between the septum 

 and the turbinals, and is continuous externally with the other meatuses. It is 

 very narrow dorsally, but widens ventrally. 



The lateral masses of the ethmoid bone project forward into the posterior 

 part (fundus) of the nasal cavity (Fig. 347). Between the ethmoturbinals of 

 which each mass is composed there are three principal and numerous small passages, 

 the ethmoidal meatuses (Meatus ethmoidales). 



The posterior nares (C'hoanse) are two elliptical orifices by which the nasal 

 cavity and pharynx communicate. They are in the same plane as the floor of the 

 nasal cavity, and are separated from each other by the vomer. They are, taken 

 together, about two inches (ca. 5 cm.) wide and three to four inches (ca. 8 to 

 10 cm.) long. 



The nasal mucous membrane (Membrana mucosa nasi) is thick, highly vas- 

 cular, and is, in general, firmly attached to the underlying periosteum and peri- 

 chondrium. It is continuous in front with the skin which lines the nostrils, 

 and behind with the mucous membrane of the pharynx. It is also continuous 

 at the naso-maxillary fissure with the very thin and only slightly vascular mucous 

 membrane which lines the sinuses. In the anterior part of the cavity it forms 

 prominent thick folds on the lateral wall, which extend from the turbinal 

 bones to the nostril. There are usually two superior turbinal folds which unite 

 anteriorly. The upper one incloses a thin plate of cartilage which is continuous 

 with the superior turbinal bone. The inferior turbinal fold is curved, and incloses 

 an '^^^ shaped cartilaginous plate which prolongs the inferior turbinal bone; this 

 fold is continuous with the alar fold of the nostril, and forms with it the upper 

 margin of the entrance from the true nostril to the nasal cavity. Below this there 

 is a rounded ridge produced by the nasal process of the premaxilla. The mucous 

 membrane of the greater part of the cavity (Regio respiratoria) is red in color, 

 and is covered with a stratified ciliated epithelium. It contains numerous acinous 

 nasal glands (Glandulse nasales). The submucosa contains rich venous plexuses 

 which form in certain situations a sort of cavernous tissue (Plexus cavernosus), 

 composed of several strata of freely anastomosing veins, between which there are 

 unstriped muscle-fibers. This arrangement is most marked in the turbinal folds, 

 on the lower part of the inferior turbinal bone, and the lower part of the septum. 

 On the posterior part of the lateral masses of the ethmoid and the adjacent part 

 of the superior turbinal and the septum (Regio olfactoria) the mucous membrane 

 is brownish-yellow in color and thicker; it contains the olfactory nerve-endings 

 in a special non-ciliated epithelium. In it are numerous olfactory glands (Glandulse 

 olfactorise) ; these are long, tubular, and often branched. 



The vomero-nasal organ of Jacobson (Organon vomeronasale) is situated on 

 the floor of the nasal cavity, on either side of the ventral l)order of the septum (Fig. 

 345.) It consists of a tubular cartilage lined with mucous membrane, to which 

 fibers of the olfactory nerve may be traced. Its anterior part communicates 

 with the nasal cavity by a slit-like orifice. 



The paranasal sinuses are described in the Osteology. 



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 internal pterygoid muscles, partly in the neck 



