294 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The parietal layers of the two pleurae meet together in the middle line 

 to form, in the posterior part of the thorax, a longitudinal vertical partition, 

 the mediastinum. The two sheets of which this is composed diverge further 

 forwards, and leave between them the mediastinal space, occupied by the 

 heart in its pericardium. The bronchi enter the anterior end of the lungs, 

 and, like the trachea, are supported by cartilage. Each of them forms 

 a bronchial tree in its lung by giving off alternating right and left 

 branches. 



The diaphragm is a thin partition which forms the posterior 

 boundary of the thorax, and separates it from the abdomen, 

 towards which it is strongly concave when at rest. The centre 

 of the diaphragm is transparent and tendinous (i.e., composed of 

 white connective-tissue fibres), while its margins are muscular, 

 especially on the dorsal side, where they are continued into two 

 muscular pillars, which take origin on the under sides of the 

 lumbar vertebrae. 



Histology. The branches of the bronchial tree divide repeatedly 

 (the supporting cartilages being at the same time gradually lost), 

 to form delicate bronchial tubes. These end in clusters of dilations, 

 the infimdibula, with sacculated walls. They are lined by simple 

 squamous epithelium, beneath which is a very rich network of 

 capillaries. The trachea and larger air-passages are lined by 

 stratified columnar epithelium, ciliated in part. 



The lungs contain a large quantity of elastic connective tissue 

 and unstriated muscle. 



Respiration. Inspiration and expiration are partly effected by 

 movements of the ribs, and partly by movements of the diaphragm, 

 and respiration is hence said to be both costal and diaphragmatic. 



It must be remembered that each lung is enclosed in an air- 

 tight bag, the pleura, and is obliged to follow all the movements 

 of the thoracic walls, or else a vacuum would be formed between 

 the two layers of this. The elastic tissue in the lungs causes 

 them to be always trying to contract, but this is prevented by 

 the pressure of the air in the lung-passages. If, however, the 

 pleura is perforated, this pressure is counterbalanced and the lung 

 at once contracts. 



Inspiration. The ribs and sternum are moved downwards and 

 forwards, and thus increase the dorso-ventral capacity of the 

 thorax. At the same time the muscular parts (especially the 

 pillars) of the diaphragm contract, thus flattening it and increas- 

 ing the antero-posterior capacity of the thorax. 



