RESPIRATION. 379 



the mucous membrane is covered by a layer of stratified columnar ciliated 

 epithelium (Fig. 179). In the submucous tissue there are a number of glands 

 the ducts of which open on the free surface. 



Opposite the fifth dorsal vertebra the trachea divides into a right and a 

 left bronchus. Each bronchus again subdivides into two or three branches, 

 which penetrate the corresponding lung. 



The lungs, in the physiologic condition, occupy the greater part of the 

 cavity o'f the thorax. They are separated from each other by the contents 

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

 agus, 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 arter- 

 ies, 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 



, . , . , ,, -, FIG. 179. TRANSVERSE SECTION OF 



relations which these structures bear one THE TRACHEA OF A KITTEN. (Stirling.) 

 to another is as follows: 



Within the substance of the lung the bronchi divide and subdivide, 

 giving origin to a large number of smaller branches, the bronchial tubes, 

 which penetrate the lung in all directions. With this repeated 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 diminution 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, between the mucous and submucous 

 tissues, which completely surrounds the tube and becomes especially 

 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 



