ORGANS OF BREATHING. 



351 



number of small lobvles that are connected together and kept separate by 

 cellular tissue. Each of these lobules is composed of many minute air- 

 cells, and is supplied with a small bronchial tube, which conveys air to these 

 cells. The capillary blood-vessels are distributed through the walls of the 

 air-cells, so that while the air in one cell acts on one side of a capillary, the 

 air in the adjoining cells acts on the other side. The blood thus brought into 

 extremely close proximity with the air contained in the cells, w^hile traversing 

 their walls, takes up the necessary supply of oxygen for the requirements of 

 the system, and, on its return to the lungs, gives off the carbonic acid which 



Larynjc 

 Gullet 



Posterior aorta 



Luiigs 

 Sumuuh 



Kidiie^s 



' Ureteis 



Internal orjTans of the horse. 



The skin, to a small extent, also excretes 



it absorbed from the various tissues, 

 carbonic acid. 



The pleurce are two smooth and glistening membranes which line the cavity 

 of the chest and cover the lungs, thus forming two closed sacs; their office 

 being to prevent friction between the lungs and the walls of the chest. They 

 secrete serous fluid (p. 13) with which to lubricate their inner surface. 



General Remarks on Pneumonia, Pleurisy, and 



Bronchitis. 



The hmgs are so intimately connected with the bronchial tubes 

 and the pleurae, that they are rarely affected without either of 

 the other two structures being also implicated; and, if either of 



