314 



THE ORGANS 



Small bronclii have a single layer of ciliated epithelium, a thinner muscular 

 coat, no glands, and no cartilage. (Figs. 210, 211.) 



The Lungs 



The lung is built upon the plan of a compound alveolar gland, the 

 trachea and bronchial ramifications corresponding to duct systems, 

 the air vesicles to gland alveoH. 



The surface of the lung is covered by a serous membrane — the 

 pulmonary pleura — which forms its capsule, and which at the root 

 of the lung, or hilum, is reflected upon the inner surface of the chest 



- 1 



Fig. 212. — From Lung of an Ape. The bronchi and their dependent ducts and 

 alveoH have been filled with quicksilver. Xi5. (Kolhker, after Schulze.) 6, Terminal 

 bronchus; a, alveolar duct; i, alveoli. 



wall as the parietal pleura. It consists of fibrillar connective tissue 

 containing fine elastic fibres which are more numerous in the visceral 

 than in the parietal layer. From the capsule broad connective- 

 tissue septa pass into the organ, dividing it into lohes. From the 

 capsule and interlobar septa are given off smaller septa, which sub- 

 divide the lobes into lobules. 



The human pulmonary lobule (Figs. 213 and 215) is the anatomic 

 unit of lung structure. Each lobule is complete in itself, having its 

 own bronchial system, its own vascular system, and being more or 

 less completely separated from its neighbors by connective tissue. 

 In the young and in some lower animals the lobule is quite plainly 

 outlined by connective tissue, but in the human adult the amount 

 of connective tissue is extremely small, and the lobules, especially the 

 more central, difficult of definition. The lobules are best observed 

 at the surface of the lung where their bases which lie against the pleura 

 can be seen with the naked eye. This is especially true in the aged 



