576 A MANUAL OF VETERINARY PHYSIOLOGY 



is undoubted that some animals can recognise a smaller number of 

 vibrations than thirty per second. Galton has shown that the cat 

 is capable of recognising sounds inaudible to the human ear. 



External Ear. — The vibrations of sound are collected by a 

 freely moving funnel-shaped body or external ear ; it is composed 

 mainly of cartilage, which is curved and hollowed out in such a 

 way as to form a good collector, while several muscles enable it 

 to assume considerable changes in direction. The two chief 

 directions taken by the ears are backwards and forwards ; judging 

 from the behaviour of many horses in carrying one ear backwards 

 and the other forwards, it would appear that they are capable 

 of hearing and appreciating sound in two opposite directions at 

 one and the same time. The funnel formed by the external ear 

 leads somewhat indirectly to a canal known as the external 

 auditory meatus ; in and around this is found an unctuous 

 secretion, and above it, in the funnel of the ear, are many hairs, 

 which are for the purpose of protection. 



The movements of the ears give evidence of what is passing 

 through the mind of an animal ; this observation is as old as 

 Pliny.* The ears of the horse are turned well to the front and 

 closely pricked — viz., the points approximated, when he is 

 attentive, whether the attention be devoted to a something he is 

 alarmed at or pleased with. The ears are laid back on the poll 

 in sourness of temper and in vice ; they are moved rapidly to and 

 fro when a horse is anxious either from impending danger or 

 other cause ; one ear carried forward and the other backward, 

 or both turned backwards, is considered the sign of a good stayer 

 and willing worker ; while drooping ears are indicative of muscle- 

 fatigue or natural flabbiness. 



Whatever part those remarkable sacs, the guttural pouches 

 (confined solely to solipeds) , are intended for, it is probable, from 

 their anatomical connection, that they take some share in the 

 sense of hearing, perhaps that of supplying the needful amount of 

 air to the middle ear (Fig. 180). The actual use of the guttural 

 pouches is involved in obscurity, but they may provisionally be 

 considered as part of the middle ear. In man acuteness of hearing 

 is enhanced by listening with an open mouth ; the fact that the 

 horse cannot breathe through the mouth may explain the presence 

 of these large air-sacs beneath the skull ; in other words, they are 

 probably associated with acuteness of hearing, and in this capacity 

 act as resonators. The air enters the guttural pouches during 

 swallowing and during expiration ; in the latter respect the 

 pouches resemble the facial sinuses. 



* It is rather remarkable that the expressions of the mind in animals 

 should be conveyed through opposite poles of the body — i.e., the ears and 

 tail. In dog, cat, and horse, the movements of ears and tail each tell 

 their own distinctive story. 



