108 



PHYSIOLOGY OF FARM ANIMALS 



[CH. 



a focus on the hinder part of the inner surface of the eye, so 

 forming an inverted image of the object in a way similar to that 

 in which the lens of a camera, when properly focussed, throws an 

 image on the ground glass focussing screen. By the contraction 

 or relaxation of a muscle, called the ciliary muscle, the crystalline 

 lens is made more or less convex and a sharp image of the object 

 looked at is thrown on the back interior surface of the eyeball. 

 Inability to focus the image sharply gives rise to the defects of 

 vision known as longsightedness (presbyopia) and shortsightedness 

 (myopia). Stretching over the posterior two-thirds of the inner 

 surface of the eyeball is a delicate nervous curtain, the retina, and 



Fig, 52. Diagrammatic section of the horse's 

 ear. 1, External auditory canal ; 2, the tym- 

 panum ; 3, chain of bones across the middle 

 ear ; 4, the Eustachian tube ; 5, the internal 

 ear ; the number is on the vestibule, above 

 which may be seen the semi-circular canals, 

 while below is the cochlea. (From Smith, 

 Messrs Bailliere, Tindall and Cox.) 



it is on this curtain that the image is thrown and focussed. The 

 retina, which is j)artially formed from the continuation of the 

 fibres of the optic nerve, is a very complex structure and contains 

 the sense end-organs, the stimulation of which by the action of 

 light give rise to the sense of perception in the brain. 



The Ear. The auditory apparatus consists of three parts — 

 the external ear, the middle ear, and the internal ear. The internal 

 ear contains the essential mechanism which converts the external 

 waves of sound into a condition suitable for transmission into the 

 brain, the middle and external ear acting as agents for the collec- 

 tion and transmission of sound to the internal ear. 



