This section is from the book "The Horse - Its Treatment In Health And Disease", by J. Wortley Axe. Also available from Amazon: The Horse. Its Treatment In Health And Disease.
Two peculiarities in the anatomy of the horse yet remain to be considered. The False Nostrils, as they are called, and the Guttural Pouches.
It is generally known to horsemen that the horse breathes solely through the nostrils, owing to the great depth of the soft palate, which entirely cuts off the cavity of the mouth from the opening into the breathing tubes. In compensation the nostrils are flexible, and the opening on each side is large enough to admit all the air which the animal requires for breathing under all conditions, which include violent exertion and a high rate of speed. A curious pouch, 3 or 4 inches deep, cone shaped, having its apex pointing upwards, and known as the false nostril, exists at the entrance to the nasal chambers on each side. No use can be found for this cavity. The tapir has the same structure in a more highly developed form, and it also exists in the rhinoceros.
To discover the real significance of this peculiar sac is now impossible; most probably it was an important organ in some of the earlier hoofed mammals, but unhappily the conservative earth only preserves in a fossil state the bones and like resistant structures. All that might be learned from even badly-kept soft parts has been lost to us, but the organ as we now find it in the three animals named is valuable as connecting the creatures of to-day with those of other times.
Guttural pouches (Vol. I, p. 505) are cavities at the back of the mouth, also communicating with the air-passages, and with a canal which enters the internal ear, called the Eustachian tube. The guttural pouches do not now appear to have any special function, and to the veterinary surgeon they are a source of anxiety in many cases, as they are liable to become diseased from the lodgment of foreign substances in them. Sometimes they are the seats of purulent deposits, and now and then become distended with air.
 
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