THE BRONCHI THE THORACIC CAVITY 525 



The bifurcation of the trachea (Bifurcatio trachete) is situated opposite to the 

 fifth rib or intercostal space, and about four or five inches (ca. 10 to 12 cm.) ventral 

 to the vertebral column. 



The trachea is composed of— (1) the cartilaginous rings; (2) a fibro-elastic 

 membrane which encloses and connects the rings; (3) a muscular layer; (4) the 

 mucous membrane. The elastic membrane is intimately attached to the peri- 

 chondrium of the rings. In the intervals between the latter it constitutes the 

 tracheal annular ligaments (Ligamenta annularia trachealia). 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 free ends overlap, while in the thoracic part 

 they do not meet; here the deficiency is made up Ijy thin plates of variable size 

 and form, embedded in a membrana transversa. Ventrally the rings are about 

 one-half inch (ca. 1.5 cm.) wide, while dorsally they are wider and thinner. The 

 first ring is attached to the cricoid cartilage by the crico-tracheal membrane, and is 

 often fused dorsally with the second ring. Various irregularities, such as partial 

 bifurcation or partial or complete fusion of adjacent rings, are common. The 

 arrangement in the terminal part is very irregular. The trachealis muscle (M. 

 trachealis) consists of unstriped 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 

 cihated. Numerous tracheal glands (Glandule tracheales) are present; they are 

 tubulo-alveolar and mixed in type. 



Vessels and Nerves. — The arteries are derived chiefly from the common 

 carotid arteries, and the veins go mainly to the jugular veins. The lymph-vessels 

 go to the cervical and mediastinal lymph glands. The nerves come from the vagus 

 and sympathetic. 



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 larger 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, but the cartilagi- 

 nous framework consists chiefly of plates instead of rings. 



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 off very obliquely. The dorsal wall 

 or roof is formed by the thoracic vertebrae and the ligaments and muscles connected 

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

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

 ribs, and the muscles in connection therewith. It is about 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 (Apertura thoracis cranialis) is 

 relatively small, and of narrow, oval form. It is bounded dorsally by the first 

 thoracic vertebra and laterally by the first pair of ribs. It is occupied by the longus 

 colli muscles, the trachea, oesophagus, vessels, nerves, and lymph glands. 



A longitudinal septum, termed the mediastinum thoracis (Septum medias- 



