THE ORGANS OF RESPIRATION 483 



at the lungs. In obedience to physical laws, an exchange of gases is 

 immediately effected; the blood rendering up the oxygen it contains to 

 the tissues, which in turn give up the carbon dioxide they have formed 

 in their substance to the blood. This process constitutes " internal or 

 tissue respiration". The blood, now deprived of part of its oxygen and 

 charged with carbon dioxide, passes by the veins to the right heart, and 

 from thence to the lungs, where it surrenders its carbon dioxide to the 

 air, as well as some watery vapour, and in return takes up oxygen from it. 

 This constitutes " external respiration". The great muscular vigour and 



Fig. 197. The Lungs in their Natural Position 

 A, (Esophagus. B, Trachea. c, The Heart. 



the activity of the horse are necessarily associated with a voluminous 

 and highly-developed respiratory apparatus, by means of which the re- 

 quired large exchange of gases can be effected. Accordingly the thorax, 

 or chest, which is of great size, is chiefly occupied with the lungs, which 

 are composed of a spongy tissue presenting an enormous surface of con- 

 tact for the air, a surface that even in man has been estimated at 81 square 

 metres, or more than 54 times the superficial area of the skin, and that 

 must be many times greater in the horse. 



The air enters the lungs through the nostrils, which are the true com- 

 mencement of the respiratory tube, and then traverses successively the 

 larynx, trachea, large and small bronchial tubes, reaching ultimately the 

 air-cells of the lungs. The nasal cavities present a large and very irregular 

 surface, covered with a mucous membrane that is constantly moist and 



