60 



HUMAN PHYSIOLOGY. 



It is composed of a series of cartilaginous rings, which extend about two- 

 thirds around its circumference, the posterior third being occupied by fibrous 

 tissue and non-striated muscular fibres which are capable of diminishing its 

 calibre. 



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

 and internally by mucous membrane, 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 and 

 smaller branches, which penetrate the lung in every direction until they 

 finally terminate in the pulmonary lobules. 



As the bronchial tubes become smaller their walls become thinner ; the 

 cartilaginous rings disappear, but are replaced by irregular angular plates 

 of cartilage; when the tube becomes less than the ^ of an inch in di- 

 ameter they wholly disappear, and the fibrous and mucous coats blend 

 together, forming a delicate, elastic membrane, with circular muscular fibres. 



The Lungs occupy the cavity of the 

 thorax, are conical in shape, of a pink 

 color and a spongy texture. They are 

 composed of a great number of distract 

 lobules, the pulmonary lobules > con- 

 nected together by interlobular con- 

 nective tissue. These lobules vary in 

 size, are of an oblong shape, and are 

 composed 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, exter- 

 nally covered by elastic fibres, which 

 give the lungs their elasticity and dis- 

 tensibility. 



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 vesssls 

 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 



Diagram of the respiratory organs. 



"he windpipe leading down from th 

 larynx is seen to branch into tw 

 large bronchi, which subdivide after 

 they enter their respective lungs. 



the 

 wo 



