RESPIRATION. 



337 



According to the investigations of Semon, the area of the air-passage 

 above and below the phonatory apparatus is about 200 sq. mm.; 

 while the area bounded by the vocal apparatus is but 155 sq. mm. 

 during quiet respiration. 



The trachea is a tube, some 1 2 centimeters in length, from one- 

 half to three-fourths of a centimeter m breaHtB^ extending from the 

 lower border of the larynx to a point opposite the fifth dorsal verte- 

 bra.)^ It consists of an external fibrous and an internal mucous 

 membrane, between which is a series of superposed C-shaped arches 

 or rings of elastic cartilage, some 18 or 20 in number. Between the 

 fibrous and mucous coats posteriorly, and occupying the space be- 

 tween and attached to the free ends of the cartilages, there is a layer 

 of transversely arranged non-striated muscle-fibers, known as the 

 tracjjgal muscle. The contraction of this muscle would approximate 

 fne ends of the arches and so di- 

 minish the caliber of the tube. 

 The surface of the mucous mem- 

 brane is covered by a layer of 

 stratified columnar ciliated epi- 

 thelium (Fig. 158). In the sub- 

 mucous tissue there are a number 

 of glands the ducts of which open 

 on the free surface. 



Opposite the fifth dorsal ver- 

 tebra the trachea divides into a 

 right and a left bronchus. Each 

 bronchus again subdivides into 

 two or three branches, which 

 penetrate the corresponding lung. 

 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; coincidently 

 there appears a layer of non-striated muscle-fibers between the 

 mucous and submucous tissues, which completely surrounds the 

 tube and is especially well developed in those tubes devoid of carti- 

 laginous rings. The fibrous and mucous coats at the same time 

 diminish 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 bron- 



FIG. 158. TRANSVERSE SECTION OF 

 THE TRACHEA or A KITTEN. 

 (Stirling.} 



