128 



HUMAN PHYSIOLOGY. 



its circumference, the posterior third being occupied by fibrous tissue and 

 non-striated muscle-fibers, which are capable of diminishing its caliber. 



The trachea is covered externally by a tough, fibro-elastic membrane, 

 and internally by mucous membiane, lined by columnar, ciliated, epithelial 

 cells. The cilia are always waving from within outward. When the two 

 bronchi enter the lungs, they divide and subdivide into numerous smaller 

 branches, which penetrate the lungs in every direction until they finally 

 terminate in the pulmonary lobules. 



As the bronchial tubes become smaller 

 their walls become thinner; the cartilagin- 

 ous rings disappear, but are replaced by 

 g\ fy irregular angular plates of cartilage; when 



the tube becomes less than -fa of an inch 

 in diameter, they wholly disappear, and the 

 fibrous and mucous coats blend, forming a 

 delicate elastic membrane, with circular 

 muscle-fibers. 



The lungs occupy the cavity of the 

 thorax, are conic in shape, of a pink color 

 and a spongy texture. They are composed 

 of a great number of distinct lobules (the 

 pulmonary lobules), connected by interlob- 

 ular connective tissue. These lobules vary 

 in size, are of an oblong shape, and are com- 

 posed of the ultimate ramifications of the 

 bronchial tubes, within which are contained 

 the air-vesicles or cells. The walls of the 

 air-vesicles, exceedingly thin and delicate, 

 are lined internally by a layer of tessellated 

 epithelium, externally covered by elastic 

 fibers, which give the lungs their elasticity 

 and distensibility. 



The venous blood is distributed to the lungs for aeration by the pulmonary 

 artery, the terminal branches of which form a rich plexus of capillary vessels 

 surrounding the air-cells; the air and blood are thus brought into intimate 

 relationship, being separated only by the delicate walls of the air-cells and 

 capillaries. 



The Thorax. The thorax in which the respiratory organs are lodged, is 

 of a conic shape, having its apex directed upward, its base downward. Its 

 framework is formed posteriorly by the spinal column, anteriorly by the ster- 



FIG. 18. DIAGRAM OP THE RES- 

 PIRATORY ORGANS. 

 The windpipe, leading down 

 from the larynx, is seen to branch 

 into two large bronchi, which sub- 

 divide after they enter their re- 

 spective lungs. 



