142 THE STRUCTURE OF THE HORSE. 



amount of air, according to the demands of respira- 

 tion. Owing to the great length of the soft palate 

 and its relation to the upper end of the windpipe, 

 breathing takes place entirely through the nose. 

 When men, dogs, and many other animals, in con- 

 sequence of any great exertion, begin to pant, and 

 require an additional quantity of air to that which 

 is ordinarily taken in by the nose, the mouth comes 

 to the aid of that channel, and is widely opened ; but 

 the horse under the same circumstances can only ex- 

 pand the margins of the nostrils, for which action 

 there is a very efficient set of muscles, acting on the 

 cartilaginous framework which supports them and 

 determines their peculiar outline. The variations in 

 the form and amount of dilatation of the nostrils 

 give great character and expression to this part of 

 the horse's face. 



Immediately within the margin of the upper part 

 of the nostril is a structure of very considerable in- 

 terest, which is generally supposed to be peculiar to 

 the horse and its immediate allies, the use of which 

 is entirely unknown. It is a blind pouch, three to 

 four inches in depth, conical in form, though slightly 

 curved, and lying in the cleft seen in the dried skull 

 between the nasal and premaxillary bones. It is a 

 diverticulum from the nasal passage, with which it 

 freely communicates below, and is lined by a contin- 



