394 TEXT-BOOK OF PHYSIOLOGY 



contents of the mediastinal space: viz., the heart, the large blood-vessels, 

 the esophagus, etc. Each lung is somewhat pyramidal in shape with the 

 apex directed upward. The outer surface is convex and corresponds to the 

 general conformation of the thorax. The inner surface is concave and accom- 

 modates the contents of the mediastinal space. The under surface of the 

 lung is concave and rests on the diaphragm. The posterior border is con- 

 vex; the anterior border is thin. At about the middle of the inner surface 

 of the lung the blood-vessels which connect the heart with the interior of 

 the lung enter and leave in company with the branches of the bronchi, 

 bronchial arteries, veins, nerves, and lymphatics. 



A histologic analysis of the lung shows it to consist of the branches of 

 the bronchi, their subdivisions and ultimate terminations, blood-vessels, 

 lymphatics and nerves, imbedded in a stroma of fibrous and elastic tissue. 

 The anatomic relations which these structures bear 

 one to another is as follows: 



Within the substance of the lung the bronchi 

 divide and subdivide, giving origin to a large num- 

 ber of smaller branches, the bronchial tubes, which 

 penetrate the lung in all directions (Fig. 180) . With 

 this repeated bsudivision the tubes become narrower, 

 their walls thinner, their structure simpler. In 

 passing from the larger to the smaller tubes the 

 cartilaginous arches become shorter and thinner, 

 and finally are represented by small angular and 

 irregularly disposed plates. In the smallest tubes 

 the cartilage entirely disappears. With the diminu- 

 LOB- ti n * t ^ ie ca liker of the tube and a decrease in the 

 ULE OF HUMAN LUNG. a. thickness of its walls, there appears a layer of non- 

 Alveolar passage, b. Cav- striated muscle-fibers, the so-called bronchial muscle, 



ity of lobule or infundibu- -i , , A . , . , 



him. c. Pulmonary sacs, between the mucous and submucous tissues, which 

 (Dalton.) completely surrounds the tube and becomes especi- 



ally well developed in those tubes devoid of cartilage. 

 The fibrous and mucous coats at the same time dimmish in thickness. 



When the bronchial tube has been reduced to the diameter of about 

 one millimeter, it is known as a bronchiole or a terminal bronchus. From 

 the sides of the terminal bronchus and from its final termination there is 

 given off a series of short branches which soon expand to form lobules or 

 alveoli. The cavity of the alveolus is termed the infundibulum. From 

 the inner surface of the alveolus and of the passageway leading into it, 

 there project thin partitions which subdivide the outer portion of the 

 general cavity or infundibulum into small spaces, the so-called air-sacs 

 or air-cells (Fig. 181). The wall of the alveolus is extremely thin and 

 consists of fibro-elastic tissue, supporting a very elaborate capillary net- 

 work of blood-vessels. The bronchial system as far as the alveolar 

 passages is lined by ciliated epithelium. ,The air-sacs are lined by fla4: 

 epithelial plates of irregular shape, termed the respiratory epithelium. 

 The alveoli are united one to another by fibro-elastic tissue. 



The bronchial arteries which supply nutritive material to the pulmonic 

 structures arise from the aorta as a rule, though sometimes from an in- 

 tercostal artery. Each lung receives two arteries which accompany the 



