448 RESPIRATORY SYSTEM OF THE HORSE 



It is bounded on either 8i(l(^ by the true vocal cord and the inner surface of the ary- 

 tenoid cartilage. The true vocal cords (Labia vocales) are situated behind the 

 false cords and the lateral ventricles. They extend from the angle between the 

 body and laminae of the thyroid cartilage to the vocal processes of the arytenoid 

 cartilages. They are prismatic in cross-section, and their free edges look forward 

 and somewhat upward. The mucous membrane of the cord (Plica vocalis) is very 

 thin and smooth, and is intimately attached to the underlying ligament. In 

 ordinary breathing the rima is somewhat lanceolate in form; when dilated, it is 

 diamond-shaped, the widest part being l;)etween the vocal processes. The narrow 

 part of the glottis between the vocal cords is termed the glottis vocalis (Pars 

 intermembranacea), while the wider part between the arytenoid cartilages is the 

 glottis respiratoria (Pars intercartilaginea). 



The posterior compartment of the laryngeal cavity is directly continuous with 

 the trachea. It is inclosed by the cricoid cartilage and the crico-thyroid membrane. 

 It is oval in form, the transverse diameter being an inch and a half to two inches 

 (ca. 4 to 5 cm.), and the dorso-ventral diameter two to two and a half inches (ca. 

 5 to 6 cm.). 



The mucous membrane which lines the larynx (Tunica mucosa laryngis) is 

 reflected around the margin of the aditus to become continuous with that of the 

 pharynx, and behind it is continuous with that which lines the trachea. It is 

 closely attached to the pharyngeal surface of the epiglottis, but elsewhere in the 

 aditus and vestibule it is loosely attached by submucous tissue which contains 

 many elastic fibers.^ It is thin and very closely adherent over the vocal cords 

 and the inner surfaces of the arytenoid cartilages. The epithelium is of the strati- 

 fied squamous type from the aditus to the glottis, beyond which it is columnar 

 ciliated in character. There are numerous mucous glands (Glandulse larjaigeae), 

 except over the glottis and the pharyngeal surface of the epiglottis, in which situa- 

 tions they are scanty. 



Blood-supply. — Thyro-laryngeal artery. 



Nerve-supply. — Superior laryngeal and recurrent laryngeal nerves (from the 

 vagus). 



THE TRACHEA 



The trachea extends from the larynx to the roots of the lungs, where it divides 

 into the right and left l)ronchi. It is kept permanently open by a series of about 

 fifty to fifty-five incomplete cartilaginous rings embedded in its wall. It occupies 

 a median position, except near its termination, where it is pushed a little to the 

 right by the arch of the aorta. It is approximately cylindrical, but its cervical 

 portion is for the most part depressed dorso-ventrally by contact with the longus 

 colli muscle, so that the dorsal surface is flattened. The average caliber is al^out 

 two to two and one-half inches (ca. 5 to 6 cm.), but in the greater part of the neck 

 the transverse diameter is greater and the dorso-ventral smaller. It is inclosed 

 by a fascia propria. 



In its cervical part the trachea is related dorsally to the oesophagus for a short 

 distance, but chiefly to the longus colli muscle. Laterally it is related to the 

 thyroid gland, the carotid artery, the jugular vein, the vagus, sympathetic, and 

 recurrent laryngeal nerves, and the tracheal lymph ducts and cervical lymph glands. 

 The oesophagus lies on its left face from the third cervical vertebra backward. 

 The sterno-cephalicus muscles cross it very obliquely, passing from the ventral 

 surface forward over its sides, and diverging to reach the angles of the jaw. The 

 omo-hyoiclei also cross it very obliquely, passing over the lateral surfaces of the 

 tube, and converging ventrally to the body of the hyoid bone. The sterno-thyro- 



1 In so-called oodcma fj:lotti(lis this loose submucous tissue becomes infiltrated with fluid, 

 in some cases causing occlusion of the vestibule. - - 



