

1087 



at the margins. Two or more of the cartilages often unite, partially or completely, and they 

 are sometimes bifurcated at their extremities. They are highly elastic, but may become calcified 

 in advanced life. In the right bronchus the cartilages vary in number from six to eight; in the 

 left, from nine to twelve. They are shorter and narrower than those of the trachea, but have 

 the same shape and arrangement. The peculiar tracheal cartilages are the first and the last 

 (Fig. 961). 



The first cartilage is broader than the rest, and often divided at one end; it is connected by 

 the cricotracheal ligament with the lower border of the cricoid cartilage, with which, or with 

 the succeeding cartilage, it is sometimes blended. 



The last cartilage is thick and broad in the middle, in consequence of its lower border being 

 prolonged into a triangular hook-shaped process, which curves downward and backward between 

 the two bronchi. It ends on each side in 

 an imperfect ring, which encloses the com- 

 mencement of the bronchus. The cartilage 

 above the last is somewhat broader than 

 the others at its center. 



The Fibrous Membrane. The cartilages 

 are enclosed in an elastic fibrous mem- 

 brane, which consists of two layers; one, 

 the thicker, passing over the outer surface 

 of the ring, the other over the inner sur- 

 face: at the upper and lower margins of 

 the cartilages the two layers blend together 

 to form a single membrane, which connects 

 the rings one with another. They are thus 

 invested by the membrane. In the space 

 behind, between the ends of the rings, the 

 membrane forms a single layer. 



The muscular tissue consists of two 

 layers of non-striated muscle, longitudinal 

 and transverse. The longitudinal fibers 

 are external, and consist of a few scattered 

 bundles. The transverse fibers (Trachealis 

 muscle) are internal, and form a thin layer 

 which extends transversely between the 

 ends of the cartilages. 



Mucous Membrane. The mucous mem- 

 brane is continuous above with that of the 

 larynx, and below with that of the bron- 

 chi. It consists of areolar and lymphoid 

 tissue, and presents a well-marked base- 

 ment membrane, supporting a stratified 

 epithelium, the surface layer of which is 



Stratified 

 ciliated 

 epithelium 

 Longitudinal 

 elastic fibers 



Submucous 

 \ layer 



Mucous 



Fibrous 

 membrane 



Hyaline 

 cartilage 



Fibrous 

 membrane 



Fia. 964. Transverse section of trachea. 





columnar and ciliated, while the deeper 

 layers are composed of oval or rounded 

 cells. Beneath the basement membrane 

 there is a distinct layer of longitudinal 



elastic fibers with a small amount of intervening areolar tissue. The submucous layer is com- 

 posed of a loose mesh-work of connective tissue, containing large bloodvessels, nerves, and 

 mucous glands; the ducts of the latter pierce the overlying layers and open on the surface 

 (Fig. 964). 



Vessels and Nerves. The trachea is supplied with blood by the inferior thyroid arteries. 

 The veins end in the thyroid venous plexus. The nerves are derived from the vagus and the 

 recurrent nerves, and from the sympathetic; they are distributed to the Trachealis muscles and 

 between the epithelial cells. 



THE PLEURA. 



Each lung is invested by an exceedingly delicate serous membrane, the pleura, 

 which is arranged in the form of a closed invaginated sac. A portion of the serous 

 membrane covers the surface of the lung and dips into the fissures between its 

 lobes; it is called the pulmonary pleura. The rest of the membrane lines the inner 

 surface of the chest wall, covers the diaphragm, and is reflected over the structures 

 occupying the middle of the thorax; this portion is termed the parietal pleura. The 

 two layers are continuous with one another around and below the root of the lung; 





