THE EYE— THE EAR. 289 



absent, and the iris is either partially or entirely white, in which 

 case it is called a "wall eye;" but though this is considered 

 unsightly, it does not interfere with vision. The iris is stretched 

 across the chamber of the aqueous humor, and is thus enabled to 

 act freely. There are many other delicate structures worthy of 

 being examined, but want of space must prevent any further allu- 

 sion to them. 



The appendages of the eye are 1st. The conjunctiva or mem- 

 brane protecting the exposed surface of the eye. 2d. The eyelids. 

 3d. The membrana nictitans or haw. 4th. The muscles of the 

 eye. 5th. The lacrymal apparatus. The conjunctiva covers the 

 whole front of the eye, being thin, and perfectly transparent in a 

 healthy state, but on the occurrence of inflammation speedily be- 

 coming red and puffy. It is reflected from this face to the inside 

 of the eyelids, and the whole membrane is extremely liable to in- 

 flammation from any external irritation. The eyelids have nothing 

 very remarkable about them, being merely cartilaginous shutters 

 covered with fine skin, and lined with conjunctiva, and raised and 

 lowered by muscles peculiar to them. The membrana nictitans or 

 haw is a cartilage lying just within the inner corner of the eye, 

 but capable of being thrust outwards so as to partially cover it 

 when the muscles retract the eye, and for want of space drive it 

 forward. This happens whenever the eye is irritated either by 

 an insect, or by the dust or hayseeds which are so often deposi- 

 ted upon the conjunctiva, and which, causing the eye to be drawn 

 back, displace the fat deposited in the back of the orbit, and this 

 again pushes forward the haw. For this reason, in all irritable 

 states of the eye, the haw is prominent, but it by no means follows 

 that its removal will diminish the irritation ; on the contrary, the 

 usual effect is to increase it, and the operation is not only useless, 

 but injurious. The muscles move the eye in all directions, and 

 have the peculiar property of keeping the long diameter of the 

 pupil always nearly in a line parallel with the horizon. Practi- 

 cally they are not of any great importance. The lacrymal appa- 

 ratus consists of the lacrymal gland, situated beneath the outer 

 wall of the orbit, and secreting the tears, which are intended to 

 wash the conjunctiva, clear of any foreign body. The secretion is 

 thrown out upon its surface through a number of small ducts, and, 

 traversing from the outer angle to the inner, is conducted through 

 two small openings in the lids to the lacrymal sac, and from that 

 by the nasal duct to the nose. 



THE EAR. 



This organ is divided into the external ear for collecting the 

 waves of sound, and conveying them inwards, and the internal ear 

 which is situated within the petrous part of the temporal bone. 

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