EYES AND EARS. 63 



The humors which occupy the cavity of the eye are of 

 different densities. The fluid which fills the spaces in the 

 front part of the eye is called the aqueous humor it is 

 clear and watery. Behind the pupil there is a somewhat 

 firm and transparent body called the crystalline lens. Be- 

 hind this lens, the whole of the remainder of the globe is 

 filled with the vitreous humor, a perfectly transparent 

 gelatinous liquid. 



The eyes of Mammals are protected by two lids fur- 

 nished with eyelashes, and the muscles of the eyes are so 

 arranged that these organs can be rolled so as to look in 

 different directions without moving the head. 



The ears of the Mammals are very complicated. These 

 are always two in number, and situated in the hinder part 

 of the head. In Man, and in most other mammals the ear 

 is divided into three portions the external ear, the middle 

 ear, and the internal ear. 



The external ear consists of the conch and the auditory tube ; it is in 

 the latter that the ear-wax is formed. 



The middle ear is an irregular cavity, separated from the auditory 

 tube by a partition called the tympanal membrane or drum; this 

 cavity has several openings, one of which is through the Eustachiaii 

 tube, which opens into the back part of the mouth. In the middle ear 

 there is a chain of four small bones called the malleus, the incus, the 

 stapes, and the os orbiculare. 



The internal ear is situated within the temporal bone, and is com- 

 posed of three parts the vestibule, the semi-circular canals, and the 

 cochlea. 



The vestibule is in the middle, and communicates with the tympanum 

 by an opening called the fenestra ovalis. The semi-circular canals are 

 three in number, and are rounded, bony, and membranous tubes. The 

 cochlea is half bony and half membranous, and resembles a snail shell 

 in shape, and it is divided into two parts by a longitudinal partition ; 

 and it communicates with the vestibule and with the tympanum by an 

 opening called the fenestra rotunda. 



The internal ear contains a watery liquid, and in this 



