Organs of Respiration and Circulation. 593 



The windpipe, passing down the neck, enters the chest between 

 the first pair of ribs (see fig. 8) ; and, in the ox and sheep, it 

 almost immediately afterwards sends a branch to the anterior 

 part of the right lung (b, fig. 16) : — this is called the third 

 bronchus, and does not exist in the horse. A little below this, 

 the windpipe divides into the two main bronchial tubes : one 

 of which penetrates the substance of each lung, dividing and 

 re-dividing into smaller and innumerable branches, which ulti- 

 mately communicate with the air-cells (figs. 16 and 17). 



Fig. 16. 



a. The windpipe. I. The third bronchus. c. The two principal bronchi. d, d. The rami- 

 fication of the bronchial tubes throughout the lung. 



It may here be mentioned that the chest is divided into two 

 cavities by a membranous partition, the mediastinum, extending 

 from before backwards, by which the right and left lung are 

 separated from each other. Hence an explanation in part of 

 the fact that pleuro-pneumonia, as well as other diseases, are 

 frequently confined to one lung. The air-cells are clustered 



