MANUAL OB"^ CATTLE-JB^EEDING. 



81 



ruslies into tlie lungs by virtue of the atmospheric pres- 

 sure. This constitutes the movement of inspiration^ or 

 breathing in. The reverse motion, which immediately 

 follows and expels a portion of the air, constitutes the 

 movement of eo&pircdion^ or breathing out. 



The air enters the lungs through the trachea, or wind- 

 pipe, from the mouth and nostrils. The tracliea, after 

 reaching the chest, divides into two branches, one leading 

 to the right and the other to the left lung, and each branch 

 subdivides again and again into a midtitude of fine tubes, 

 called bronchial tubes, each of 

 which finally ends in an vZtir 

 mAjbte lobule^ consisting of sev- 

 eral mhiute "Desloles, In Tig. 

 0, c represents the ultimate 

 bronchial tube, !> i the vesicles, 

 and the whole mass of vesicles 

 constitutes an ultimate lobule, a. 



The vesicles and tubes have 

 elastic walls and are siu'rounded 

 by an elastic tissue, so that the 

 whole lung constitutes a spongy 

 mass which expands or con- 

 tracts with the motions of the 

 chest, causing the air to flow into and out of all parts of it. 

 The vesicles are also surrounded by a net-work of ex- 

 tremely fine capillary blood-vessels, through which the 

 blood sent to the lungs by the contraction of the right 

 ventricle of the heart must pass, and the walls both of 

 the capillaries and of the vesicles are very thin and are 

 permeable to gases. 



Exchange of G-ases in the Lungs. — ^The venous 

 blood, as it comes to the limgs, is rich in carbonic acid, 



Pig. 6.— (Prey ) Lung Tissue. 



