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 
