STRUCTURE OF THE TRACHEA. AND BRONCHIA. 519 



their direction is longitudinal. There is some reason to doubt 

 whether these longitudinal fibres are confined altogether to 

 the spaces between the cartilaginous rings, and attache^ only 

 to their edges, because there is a fleshy substance on the inter- 

 nal surface of the rings, which appears to be continued from 

 the spaces between them. 



The internal coat of the trachea is a thin and delicate mem- 

 Fig. 129.* brane, perforated with an im- 



mense number of small foramina, 

 which are the orifices of mucous 

 ducts. 



On the surface of this mem- 

 brane there is an appearance of 

 longitudinal fibres which are not 

 distributed uniformly over it, but 

 , c run in fasciculi in some places, 

 and appear to be deficient in 

 others. These fasciculi are par- 

 ticularly conspicuous in the rami- 

 fications of the bronchia in the 

 lungs. 



TMany of the German anato- 

 mists have described these as 

 longitudinal muscular fibres, the 

 object of which is to shorten to some extent the air-passages 

 during their contraction, and to assist in loosening the mucus 

 and other matters which accumulate in their cavities. I have 

 examined them carefully in the ox and elephant, where 

 they are strongly marked ; they appeared to me to consist 

 only of longitudinal folds of mucous membrane, with a basis 

 of the fibrous contractile tissue. The same arrangement of 

 the circular fibres, and these peculiar longitudinal fibres, can 

 be traced with the microscope down the bronchia as far as 



* Fig. 129, represents the larynx, trachea, and bronchia ; on one side is seen 

 the right lung ; on the left, the lung has been destroyed to show the ramification 

 of the bronchia, a, Larynx, b, Trachea, dividing into right and left bron- 

 chium ; the left is the smaller, longer, and inclined most downwards, c, Larger 

 divisions of the left bronchium ; e, the more minute, d, Right lung. F. 



