The act of breathing is so intimately associated with the continuance of life that we commonly speak of the " first" and " last" breath as terms synonymous with the beginning and end of existence. Yet that which is referred to in this sense is really only the outward movement or visible mechanism of respiration, that is to say, the means by which the introduction and expulsion of air is effected, and takes no heed of the changes that take place in the air, in the blood, and in the tissues of the animal. These, however, represent the true end and aim of the respiratory process, for experiment has shown that the persistent manifestation of life is invariably associated with the absorption of oxygen and the elimination of carbon dioxide, that the exchange of these gases is properly termed the process of respiration, and that it is accomplished in all parts of the system. Every tissue, but, above all, the muscular, develops carbon-dioxide in its substance, which, by means of the capillary circulation, is brought into relation with the blood which has taken up oxygen 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 the activity of the horse are necessarily associated with a voluminous and highly-developed respiratory apparatus, by means of which the required 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 contact 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.
Fig. 197. - The Lungs in their Natural Position. A, Oesophagus. b, Trachea. c, The Heart.
The air enters the lungs through the nostrils, which are the true commencement 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 is highly vascular. The upper part is supplied with the olfactory nerve, which is the special nerve of smell, whilst the lower part is supplied with branches of the fifth pair of nerves, which confer upon it a high degree of common sensation. Several objects are gained by the inhaled air passing over this surface. The impressions received by the sense of smell, and conducted to the brain, serve to place the animal on guard against enemies, whilst they inform it of the proximity of food, water, or other objects of desire. The acuteness of their perception in these respects is well known.
But it does more. It warms and moistens the air as it is inspired, and thus prevents cold, raw, and dry air from acting directly on the delicate tissue of the lungs. Having reached the back of the nose, the current of air crosses the pharynx and enters the larynx, shown in side view in fig. 198, and looked at from above in fig. 199. The larynx is a cartilaginous chest or box containing the organ of voice. Continuing its course the inspired air travels through the trachea or wind-pipe and the bronchial tubes, the divisions of which penetrate, and indeed form, a large proportion of the substance of the lungs. In the act of expiration the air passes through the several passages just named in the opposite direction.