18 ON THE STRUCTURE AND 



front by an annular membrane, denominated the iris, hav- 

 ing an opening in the centre, capable of contraction or 

 dilatation, according to the greater or less quantity of light 

 by which it is affected, and called the pupil. Behind the 

 ehoroid coat lies the retina, a reticulation of the finest nerves 

 and blood vessels, that take their rise from the optic nerve. 

 These coats contain what are called the humours of the eye : 

 but of these only one, the aqueous humour, which lies in 

 the anterior chamber, behind the cornea, is a true fluid. 

 The vitreous humour is soft, and included in a transparent 

 membrane. The chrystalline humour, or lens, consists of 

 concentric plates or scales, connected by cellular fibres : it 

 is also contained in a pellucid membrane. 



This delicate organ, the eye, lies embedded in a deep 

 socket, and is well fortified from injury by its surrounding 

 parts. The bones defend it on all sides, except immediately 

 in front, where there are eyelids, which are always closed 

 during sleep; and which, on the approach of any danger, 

 fly together quicker even than thought. The lachrymal 

 gland, situated in the upper part of the orbit, produces the 

 tears which wash the front of the eye, and which, by the 

 continual motion of the eyelids, keep it at all times perfectly 

 lubricated. The eye-lashes prevent the intrusion of such 

 minute objects as might otherwise injure the eye ; and these, 

 with the eye-brows, tend to break and moderate the force 

 of the light. 



Some quadrupeds, as a few of the Monkeys, the Seals, 

 the Bears, &c. have, like the birds, an internal covering to 

 their eyes, called by anatomists, membrana nictitans, or 

 the winking membrane. 



The ear consists, for 'the most part, of elastic cartilages, 

 and hard bones, about which the auditory nerve is interiorly 

 expanded. The external part is cartilaginous, and wrought 

 it the bottom into irregular bends and hollows, which col- 

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