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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. With this re- 

 peated subdivision 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- 

 tion of the caliber of the tube and a decrease in the 

 thickness of its walls, there appears a layer of non- 

 striated muscle-fibers, the so-called bronchial muscle y 

 between the mucous and submucous tissues, which 

 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 diminish in thickness. 



Bronchial Innervation. The bronchial muscles are presumably in a 

 state of tonic contraction and impart to the bronchial tubes a certain average 

 caliber best adapted for respiratory purposes. Experimental investigations 

 indicate that they are innervated by efferent fibers of the vagus nerve (broncho- 

 constrictors and possibly broncho-dilatators) inasmuch as stimulation of this 

 nerve is usually followed by a contraction of the muscles and a narrowing of 

 the lumen of the bronchial system. These muscles may also be thrown into 

 increased activity by the inhalation of irritating gases and into a tetanus by 

 pathologic causes as seen in the various forms of asthma. 



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 sur- 

 face of the alveolus and of the passageway leading into it, there project thin 

 partitions which subdivide the outer portion of the general cavity or infundib- 

 ulum into small spaces, the so-called air-sacs or air-cells (Fig. 183). The 



FIG. 183. SINGLE LOB- 

 ULE OF HUMAN LUNG. a. 

 Alveolar passage, b. Cav- 

 ity of lobule or infundibu- 

 lum. c. Pulmonary sacs. 

 (Dalton.) 



