CHAPTER XX. 



THE LUNGS. 

 BY FKANZ EILHARD SCHULZE. 



I. THE LUNGS OF MAMMALS. 



FROM the bronchus, or free branch of the trachea, that enters each 

 lung a system of tubes with solid and smooth walls is deve- 

 loped. These tubes ramify in an arborescent manner through- 

 out the substance of the organ, and are termed the bronchia. 

 From the primary trunk there originate in the first place, by a 

 process of dichotomous division at acute angles, a number of 

 diverging and successively smaller branches, which, after 

 having attained a certain calibre, amounting in Man to about 

 four millimeters (l-6th inch), almost completely cease to 

 divide in this manner, but, constantly diminishing in size, con- 

 tinue in nearly a straight line to the surface of the lung, and 

 give off spirally from their sides a series of small branches at 

 an angle of 45, which likewise pursue a straight course. The 

 lateral branches springing in a similar manner from these 

 last again branch with dichotomous, but almost perfect rectan- 

 gular, ultimate subdivision. In consequence of this, as well as 

 of the circumstance that one of the two branches at each 

 division maintains the direction of the original trunk, whilst 

 the divisions for the most part take place in planes alternately 

 at right angles to each other, a peculiar zigzag course is pre- 

 sented by the smallest bronchi, which ultimately having a 

 diameter of O3 to 0'2 of a millimeter, and even in the smallest 

 Mammals, as in the Mouse, Bat, etc., not less than O'l of 

 a millimeter, open into the respiratory cavities. These are 

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