DISEASES OF THE RESPIRATORY ORGANS. 111 
plied to the breast is a beneficial adjunct. The diet should be the 
most nourishing. Bulky feed should not be given. Linseed mashes, 
scalded oats, and, if in season, grass and green-blade fodder are the 
best diet. 
THE LUNGS. 
The lungs (see Pl. VII) are the essential organs of respiration. 
They consist of two (right and left) spongy masses, commonly called 
the “lights,” situated entirely within the thoracic cavity. On account 
of the space taken up by the heart, the left lung is the smaller. Ex- 
ternally, 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 be torn only 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 (pulmonary vesicles) 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 consummation 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 absorbed by the blood 
in its circulation through the different parts of the body; through 
these walls also the oxygen gas, which is the life-giving element of 
the atmosphere, is absorbed by the blood from the air in the air cells, 
CONGESTION OF THE LUNGS. 
Congestion is essentially an excess of blood in the vessels of the 
parts affected. Congestion of the lungs in the horse, when it exists 
as an independent affection, is generally caused by overexertion when 
the animal is not in a fit condition to undergo more than moderate 
exercise. Very often what is recognized as congestion of the lungs 
is but a symptom of exhaustion or dilatation of the heart. 
The methods practiced by the trainers of running and trotting 
horses will give an idea of what is termed “ putting a horse in condi- 
tion” to stand severe exertion. The animal at first gets walking 
exercises, then after some time he is made to go faster and farther 
-each day; the amount of work is daily increased until he is said to 
be “in condition.” An animal so prepared runs no risk of being 
