THE RESPIRATORY SYSTEM 255 



bronchioli into alveolar ducts, the alveolar ducts into atria, alveolar 

 sacs, and alveoli which form the ultimate subdivisions (Fig. 236). 

 Exchange of gases occurs chiefly in the alveoU, although the thin respira- 

 tory epithelium is found also in the atria and alveolar sacs and may extend 

 even into the bronchioli, which in general are lined with a simple cuboidal 

 non-respiratory epithelium. There is an elaborate network of capillaries 

 in the walls of the alveoli, so that only two extremely thin membranes 

 separate the blood in the capillaries from the air in the alveoK. 



Lungs are very elastic, and their elasticity is increased by the smooth 

 muscle fibers which extend into the connective tissue of the lungs as far 

 as the alveolar sacs but not into the walls of the alveoli. 



The respiratory blood-vessels of the lung are branches of the pulmonary 

 arteries and veins. The bronchial artery and vein supply the connective 

 tissues of the lungs. The innervation of the lung is through branches 

 of the vagus and of the sympathetic. 



On the outside of the lung the pleura, corresponding to the peritoneal 

 lining of the abdominal cavity, consists of a subserous connective tissue 

 which extends into the walls of the lobules of the lung, and an external 

 epithelial serosa. The pulmonary pleura is reflected back on the inside 

 of the chest as the parietal pleura. 



Larynx. The larynx or voice-box lies between the root of the tongue 

 and the trachea, and opens into the pharynx by the glottis. Nine carti- 

 lages support it, the unpaired epiglottic, thyroid, and cricoid cartilages, 

 and the paired arytenoids, corniculate, and cuneiform cartilages. Small 

 paired triticeous cartilages also sometimes are found. Numerous muscles 

 are attached, some extrinsic and some intrinsic. The extrinsic muscles 

 are chiefly to lift the larynx in swallowing. Among the intrinsic muscles 

 are the thyro-arytenoid or vocalis and the cricothyroid, which affect the 

 pitch of the voice. At puberty in the male, the larynx becomes enlarged 

 and the vocal cords within it elongated so that the voice is deepened. The 

 epiglottis and vocal cords are covered with the same kind of squamous 

 stratified epitheUum as that which lines the pharynx, but the rest of the 

 larynx is lined with ciliated columnar epithelium similar to that of the 

 trachea. The action of the cilia is such as to carry the secretions of 

 the mucous glands of the lungs, together with particles of dust, out into the 

 pharynx. Mucous glands are numerous. The nerve supply is from 

 the vagus and the sympathetic. 



Trachea and Bronchi. The human trachea or wind-pipe is a membra- 

 nous tube, four to five inches long, supported by fibrous connective tissue 

 and incomplete U-shaped rings of cartilage. It carries air to and from the 

 lungs. The cartilages vary in number from sixteen to twenty and are 

 incomplete on the side next to the esophagus. The trachea divides to 

 form the right and left bronchi. The lining of the trachea is a mucous 



