Caudally blind-ending cutaneous pouch lined with skin that occupies the nasoincisive notch. It's ventral and medial walls is formed by the alar fold. It's lateral wall is formed by the external nares and it's dorsal wall is formed by the straight fold. Lined by pigmented predominantly hairless skin. It has an external opening located on the dorsolateral aspect of the "true nostril". When the horse is exercising, obliteration of the nasal diverticulum greatly widens the nostrils increasing air intake[MURDOCH]. [ https://www.worldcat.org/search?q=bn%3A3899930983 ]