ELEMENTS OF HIPPOLOGY. 157 



the lights, situated entirely within the thoracic cavity. On 

 account of the space taken up by the heart, the left lung is the 

 smaller. Externally, they are completely covered by the pleura. 

 The structure of the lung consists of a light, soft, but very 

 strong and remarkably elastic tissue, which can only be torn 

 with difficulty. Each lung is divided into a certain number of 

 lobes, which are subdivided into numberless lobules (little lobes). 

 A little bronchial tube terminates in every one of these lobules. 

 The little tube then divides into minute branches, which open 

 into the air-cells of the lungs. The air-cells are little sacs, having 

 a diameter varying from one-seventieth to one-two-hundredth 

 of an inch; they have but one opening, the communication with 

 the branches of the little bronchial tubes. Small blood-vessels 

 ramify in the walls of the air-cells. The air-cells are the con- 

 summation of the intricate structures forming the respiratory 

 apparatus. They are of prime importance, all the rest being 

 complementary. It is here that the exchange of gases takes 

 place. As before stated, the walls of the cells are very thin; 

 so also are the walls of the blood-vessels. Through these walls 

 escapes from the blood the carbonic acid gas that has been ab- 

 sorbed by the blood in its circulation through the different parts 

 of the body; and through these walls is absorbed, by the blood 

 from the air in the air-cells, the oxygen-gas, which is the life- 

 giving element of the atmosphere."* 



The nasal passages, or nostrils, unite above the back of the 

 mouth, forming the pharynx. The pharj-nx is a box-like 

 organ, through which the food passes into the gullet, as well 

 as the air into the lungs. The nostrils and mouth open into it 

 in front, nostrils above, and the gullet and windpipe lead from 

 it in rear, the windpipe below. To guard against the introduc- 

 tion of food into the windpipe, a large valve, 2 (Figure 98), 

 closes it, that is hinged below and that lies, when the horse is 



*Pages"3ll9 and 120, "Diseases of the Horse," Bureau of Animal 

 Industry, 1903. 



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