ANIMAL MECHANISM. 119 



Every species of animal with which naturalists are con- 

 versant, possess this defensive transparent membrane. 



MEMBRANA NICTITANS. 



A third eye-lid is given such animals as are destitute 

 of hands, or are incapacitated, by the arrangement of their 

 limbs, from reaching their eyes. This is called mem- 

 brana nictitans, and a more striking piece of mechan- 

 ism, there is not in existence. It slides from one angle 

 of the eye to the opposite one, under the first pair of lids, 

 and that, too, whether the others are open or shut, 

 being totally independent of them in muscular action. 

 Its use cannot be mistaken : it is on purpose for clearing 

 away matter that may be irritating to the eye. Any ex- 

 traneous substance is brushed from the cornea in an 

 instant, by the broad sweep of the night lid. Birds that 

 seek their food in the night, as owls, defend their irrita- 

 ble organs, through the glare of daylight, by drawing over 

 this singular curtain. Dogs, cats, foxes, wolves, bears, 

 lions, tigers, &c, can each of them, by this brush, remove 

 the minutest mote from the cornea, more expeditiously 

 than any occulist on the globe. 



Perfection is everywhere observed in animal mechanics. 

 The eye would soon become a useless instrument, not- 

 withstanding the nice adjustment of its several parts, 

 were it not for the external apparatus of eye-lids, glands 

 and tears, whose combined action keeps it always in a 

 condition to be useful. Were not the cornea frequently 

 moistened, it would become dry and shrivelled. To ob- 

 viate this, a sack of fluid is fixed just under the edge of 

 the orbit, above the eye-ball, which is continually pouring 

 out its contents by the pressure and rolling of the eye. 

 Flowing through numberless apertures, it washes the 

 crystal, and finally passing into grooves, on the inner 

 margin of both eye-lids, runs to their terminations in a 

 small pin like orifice, at the inner angles. To keep them 

 open, a hoop is set in the mouth of this lachrymal duct. 

 This, too, can be shown by turning the lid outward by 

 the finger. Finally, the tears are conveyed into the nose 



