450 RESPIRATORY SYSTEM OF THE HORSE 



of the rings. In the intervals between the latter it constitutes the annular liga- 

 ments (Ligamenta annularia). The rings of the trachea (Cartilagines tracheales) 

 are composed of hyaline cartilage. They are incomplete dorsally, and when their 

 free ends are drawn apart, resemble somewhat the letter C. In the cervical part 

 the thin wide free ends overlap, while in the thoracic part they do not meet; here 

 the deficiency is made up by thin plates of variable size and form, embedded in a 

 membrana transversa. Ventrally the rings are about one-half inch (ca. 1 to 1 .2 cm.) 

 wide and much thicker than dorsally. The first ring is attached to the cricoid 

 cartilage l)y the crico-tracheal membrane, and is usually fused dorsally with the 

 second ring. Various irregularities, such as partial bifurcation or partial or com- 

 plete fusion with an adjacent ring, are common. The arrangement in the terminal 

 part is very irregular. The muscular layer (Musculus trachealis) consists of un- 

 striped fibers which stretch across the dorsal part of the tube. It is separated from 

 the ends of the rings and the membrana transversa by a quantity of areolar tissue. 

 When it contracts the caliber of the tube is diminished. The mucous membrane 

 is pale, normally, and presents numerous fine longitudinal folds, in which are 

 bundles of elastic fillers. The epithelium is stratified columnar ciliated. Numer- 

 ous tubular mucous glands (Glandulse tracheales) are present. 



Blood-supply. — Common carotid arteries. 



Nerve-supply. — Vagus and sympathetic nerves. 



THE BRONCHI 

 The two bronchi, right and left (Bronchus dexter, sinister), are formed by the 

 bifurcation of the trachea. Each passes backward and outward to the hilus of 

 the corresponding lung. The right bronchus is a little wider and less oblique in 

 direction than the left. They are related ventrally to the divisions of the pulmon- 

 ary artery, and dorsally to the branches of the bronchial artery and the bronchial 

 lymph glands. Their structure is similar to that of the trachea. 



THE THORACIC CAVITY 



The thoracic cavity (Cavum thoracis) is the second in point of size of the body 

 cavities. In form it is somewhat like a truncated cone, much compressed laterally 

 in its anterior part, and with the base cut ofT very obliquely. 



The dorsal wall or roof is formed ])y the thoracic vertebrae and the ligaments 

 and muscles connectetl with them. 



The lateral walls are formed by the ribs and the intercostal muscles. 



The ventral wall or floor is formed by th(» sternum, the cartilages of the sternal 

 ribs, and the muscles in connection therewith. It is al^out one-half as long as the 

 dorsal wall. 



The posterior wall, formed by the diaphragm, is very oblique and is strongly 

 convex. 



The anterior aperture or inlet (Ai)ertura thoracis cranialis) is relatively small, 

 and of narrow, oval form. It is liounded dorsally l)y the first thoracic vertel)ra 

 and laterally by the first pair of ribs. It is occupied by the longus colli muscles, 

 the trachea, oesophagus, vessels, nerves, and the prepectoral lymph glands. 



The cavity is lined ])v the endothoracic fascia and ])v the pleurae. 



A longitudinal septum, termed the mediastinum (Septum mediastinale), ex- 

 tends from the dorsal wall to the ventral and posterior walls, and subdivides the 

 cavity into two lateral chambers which contain the lungs. Each of these chambers 

 is lined by a serous membrane called the pleura, and is called a pleural cavity 

 (Cavum pleurae). The mediastinum is, for the most part, not median in position; 

 this is largely due to the fact that the largest organ contained in it, the heart, is 



