THE BRONCHI—THE THORACIC CAVITY 525 
The bifurcation of the trachea (Bifurcatio trachex) is situated opposite to the 
fifth rib or intercostal space, and about four or five inches (ca. 10 to 12 em.) 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 by thin plates of variable size 
and form, embedded in a membrana transversa. Ventrally the rings are about 
one-half inch (ca. 1.5 em.) 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 iransversa 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 fibers. The epithelium is stratified columnar 
ciliated. Numerous tracheal glands (Glanduls 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 
im its anterior part, and with the base cut off very obliquely. The dorsal wall 
or roof is formed by the thoracic vertebre 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, cesophagus, vessels, nerves, and lymph glands. 
A longitudinal septum, termed the mediastinum thoracis (Septum medias- 
