THE EYE. 179 



which nature preserves throughout the higher order of her 

 creatures. 



The fenestra ovalis is an opening between the tympanum 

 and the vestibule. The fenestra rotunda is situated in- 

 feriorly to this, and is the entrance to the mastoid cells. 

 The vestibule is a cavity immediately beyond the tympanum. 

 The semicircular canals are three bony canals, which com- 

 municate with the vestibule ; and the cochlea is a partially 

 double spiral canal, within the opening into the vestibule 

 and into the tympanum. These spiral convoluted canals, 

 partially divided from each other by a fine membrane, are 

 filled with a thin fluid, to perfect the undulations com- 

 municated by the tympanum. The nerves of the inner and 

 outer ear are furnished by the portio mollis of the seventh 

 pair. Blood is furnished to the organ, both internal and 

 external, by means of the carotids ; which blood is returned 

 by the jugulars. 



SENSE OF HEARING, 



The collision of elastic bodies produces phenomena which 

 are called sounds ; thus a tremulous motion communi- 

 cated to the surrounding medium extends in all directions, 

 and at length reaches the external ear whose form is 

 admirably adapted to receive a large portion of these sono- 

 rous waves ; which are then reflected from the cartilaginous 

 sides of the concha, till they reach the bottom of the outer 

 ear. Impinging upon the ear-drum, they force it into similar 

 oscillations ; which being communicated to the fluids in 

 the spiral channels of the inner ear, finally act on the 

 acutely sensitive expansion of the auditory nerve, and pro- 

 duce those sensations we call hearing. 



THE EYE. 



The eyes of the horse are not situated, as in the human, 

 directly in front of the face ; but have a more lateral aspect, 

 to increase the field of view. Each eye may be divided into 

 eye and appendages. 



The eye, which we shall first describe, is situated in a 

 cavity formed by several bones, and called the orbit. Within 

 this cavity, the globe, surrounded by its moving agents or 

 muscles, rests on a quantity of fatty matter, serving to give 



n2 



