142 HUMAN PHYSIOLOGY. 



RESPIRATION. 



Respiration is the function by which oxygen is absorbed into the 

 blood and carbonic acid exhaled. The assimilation of the oxygen 

 and the evolution of carbonic acid takes places in the tissues as a 

 part of the general nutritive process, the blood and respiratory ap- 

 paratus constituting the media by means of which the interchange 

 of gases is accomplished. 



The respiratory apparatus consists of a larynx, trachea, and lungs. 



The larynx is composed of firm cartilages, united by ligaments and 

 muscles. Running anteroposteriorly across the upper opening are 

 four ligamentous bands the two superior or false vocal cords, and 

 the two inferior or true vocal cords, formed by folds of the mucous 

 membrane. They are attached anteriorly to the thyroid cartilages 

 and posteriorly to the arytenoid cartilages, and are capable of being 

 separated by the contraction of the posterior crico-arytenoid muscles, 

 so as to admit the passage of air into and from the lungs. 



The trachea is a tube from four to five inches in length, ^ of an 

 inch in diameter, extending from the cricoid cartilage of the larynx 

 to the third dorsal vertebra, where it divides into the right and left 

 bronchi. 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 muscle-fibers, 

 which are capable of diminishing its caliber. 



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

 brane, 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 smaller branches, which penetrate the 

 lungs in every direction until they finally terminate in the pul- 

 monary lobules. 



As the bronchial tubes become smaller their walls become thinner ; 

 the cartilaginous rings disappear, but are replaced by irregular an- 

 gular plates of cartilage ; when the tube becomes less than J^ 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 



