VISION. 223 



four hours. Besides this, there is an ointment pre- 

 pared in the beautiful little glands, about thirty in 

 number, interspersed within the fine skin of the in- 

 ner eyelid near to the roots of the eyelashes, ap- 

 pearing when magnified like studs of minute pearls. 

 This ointment, which, according to M. Magendie, is 

 of a glairy consistence, like white of egg, is dis- 

 solved and diluted by the tears, and the whole is 

 constantly spread over the eyeball by the sweep of 

 the eyelids, which act like valves, and are compo- 

 sed of semi-transparent muscular substance, attach- 

 ed to a ring of gristle or cartilage, which is hinged 

 on the adjacent bone, and gives the eyelids firm- 

 ness and preserves their shape. Our eyes are, be- 

 sides, elegantly fringed with short hairs, either to 

 defend the eye with a gratework from anything 

 falling into it, or to perform some unknown opera- 

 tion on light. The use of the eyelids is strikingly 

 demonstrated from what takes place when they are 

 cut off, a savage punishment sometimes practised 

 in barbarous countries. This prevents sleep, and, 

 from the constant irritation of the light, the eyes 

 inflame, the inflammation spreads to the brain, and 

 the victim of torture expires in the most dreadful 

 agony. 



Birds differ considerably from other animals with 

 respect to the eyelids, which are indeed formed 

 horizontally and very distinct ; but the under lid is 

 in most birds much larger and more moveable than 

 the upper, as was observed by Aristotle, in many 

 cases forming on the inner surface a smooth, pol- 

 ished plate (lamina). 



The part, however, which we have more particu- 

 larly alluded to, we may with some propriety call 

 the eyebrush (membrana nictitans). In our own 

 eye we may examine this eyebrush at a looking-- 

 glass by turning the eye away as far as possible 

 from the nose. It is a little red fleshy membrane, 

 in form of a crescent, which in such a position of 



