402 NAYIX ox THE HORSE. 



curve of the great artery leading backward. The trachea is 

 composed of about fifty or sixty rings of cartilage, so united as 

 to admit of considerable motion without interfering, to any con- 

 siderable extent, with the size of the tube. This is evident 

 when we consider the degree of motion the horse's neck is ca- 

 pable of. 



The hronchial tuhes are the two branches into which the wind- 

 pipe divides. The right goes to the right, and the left to the left 

 lung. After reaching the lungs, the right bronchial tube di- 

 vides into three principal branches, and the left into two. These 

 branches continue to send off branches which divide and sub- 

 divide in the lungs until the little branches become so small as 

 not to be visible to the naked eye. As these bronchial divi- 

 sions, or ramifications, proceed, they lose more and more of the 

 character of cartilage, and become very thin, and finally appear 

 •to be only a continuation of the inside lining mucous mem- 

 brane, which finally ends in a closed sack of less than the size 

 •of a millet-seed. These little sacks are called air-cells. 



I have just spoken oi the mucous membrane ivMch lines the air- 

 passages from the larynx down through the trachea, bronchial 

 tubes, and bronchial ramifications, or divisions, and which 

 .finally ends in the little sacks called the air-cells. This mem- 

 brane secretes mucus for the protection of the air-passages. 

 When irritated, as in colds, its secretion of mucus becomes 

 much increased. This membrane is liable to inflammation, 

 and this may begin in any of its different parts. When it 

 starts in the larynx, the inflammation is called laryngitis ; in 

 the bronchial tubes, it is called bronchitis. 



The lungs, or lights, are two large, spongy bodies of a pinkish 

 hue, one situated in the right and the other in the left side of 

 the chest. The right is the larger of the two, and is composed 

 of three divisions, or lobes ; the left is the smaller and only di- 

 vided into two lobes. They are separated from each other by 

 -the heart and a broad fold of the pleura, called the mediastinum. 

 They are held in place by blood-vessels, the division of the 



