RESPIRATION. 



55 



FIG. 



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 distinct 

 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 vessels 

 surrounding the air cells; the air and 

 blood are thus brought into intimate relationship, bein 

 by the delicate walls of the air cells and capillaries. 



The thoracic cavity in which the respiratory organs are lodged is of a 

 conical shape, having its apex directed upward, its base downward. Its 

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

 sternum, and laterally by the ribs and costal cartilages. Between and over 

 the ribs lie muscles, fascia and skin; above the thorax is completely closed 

 by the structures passing into it and by the cervical fascia and skin; below 

 it is closed by the diaphragm. It is therefore an air-tight cavity. 



The Pleura. Each lung is surrounded by a closed serous membrane, 

 the pleura, one layer of which, the visceral, is reflected over the lung, the 

 other, \\-\t parietal, reflected over the wall of the thorax; between the two 

 layers is a small amount of fluid which prevents friction during the play of 

 the lungs in respiration. 



Diagram of the respiratory organs. 

 The windpipe leading down from the 

 larynx is seen to branch into two 

 large bronchi, which subdivide after 

 they enter their respective lungs. < 



separated only 



