14 



A COURSE ON ZOOLOGY. 



FIG. 3. 



direct communication with the external air by a large 

 tube, called the trachea. The lungs fill almost the whole 

 of the thoracic cavity or chest, which is, as we have seen, 

 within the ribs, occupying the upper portion of the 

 trunk, and separated from the abdominal cavity by a 

 muscle called the diaphragm. 



These organs are double and of unequal size, the left 

 lung being less voluminous than the right. The smaller 

 size of the left lung is due to 

 the space occupied by the heart 

 in the left side of the thoracic 

 cavity. The lungs are enveloped 

 by a serous membrane called 

 the pleura, which separates 

 them from each other and from 

 the chest walls. 



The lungs are made up of a 

 large number of little vesicles, 

 communicating together by a 

 ramifying system of canals. 

 In each lung these canals join to 

 form a single tube, called bron- 

 chus, and the union of the two 

 bronchi forms the trachea. The 

 trachea rises along the front of the neck, and opens in 

 the back of the mouth in front of the oesophagus. It is 

 surrounded by a series of cartilaginous rings, the upper 

 of which form the larynx or organ of voice, and produce 

 exteriorly the little projection commonly called Adam's 

 apple. A mucous membrane, which is a continuation of 

 the buccal mucous membrane, covers the internal walls 

 of the trachea and bronchi. 



Through the trachea, the bronchi, and their ramifica- 



DlAGRAM OP TWO PRIMARY 



LOBULES OF THE LUNGS, mag- 

 nified. 1, bronchiole; 2, a 

 pair of primary lobules con- 

 nected by fibro-connective 

 tissue; 3, intercellular air- 

 passages; 4, air-cells; 5, 

 branches of the pulmonary 

 artery and vein. 



