Fes., 1912. Mammats oF ILLINoIS AND WISCONSIN — Cory. 25 
of an inch. On the shoulder, however, unless the points are more 
than 14 of an inch apart, they can not be distinguished; and portions 
of the back are so insensitive as to require the points to be separated 
as much as 2 inches to give the impression of more than a single 
point of contact. Many mammals are provided with long hairs or 
bristles in more or less specialized regions, such as eyebrows, cheeks 
and lips, which are connected by their basal papillee with sensory 
nerve filaments and on which the sense of touch is very highly devel- 
oped. The whiskers (vzbrisse@) of the common House Cat belong to 
this category. In Bats the wing membrane is well supplied with 
nerves and is so sensitive as to enable the animals to avoid objects 
in their flight. 
The Sense of Sight — While eyes are present in all mammals, in some, 
like the Moles, they are small and practically rudimentary, in a few 
cases being covered by the skin; but the sense of sight is highly devel- 
oped in the majority of the class. There is considerable variation 
in the color and shape of the eye in different species, and the pupil 
varies from circular to elliptical. In many mammals there is a 
modification of the choroid known as the tapetum lucidum, which is 
a membrane exhibiting green and blue reflections, and which is 
the cause of the striking appearance of the eyes of many animals 
in the dark. In some of the smaller Cats the pupil contracts 
to a vertical slit, while in the larger felines it remains cir- 
cular. In most of the mammals the eye is protected by an upper 
and a lower lid which close over the front, meeting in a nearly 
horizontal slit. In the Sirenia, however, the lids are not distinct 
and the aperture is circular, drawing together at a central point. 
In the eyes of all mammals excepting those of the Primates and the 
Cete, a third eyelid is present called a nictitating membrane, which 
is placed at the inner corner of the eye and passes horizontally over 
the eyeball under the true eyelids. This is apparently for the pur- 
pose of cleaning the cornea and: is represented in a rudimentary 
form in Man and other Primates in the shape of the so-called semi- 
lunar fold at the inner angle of the eye. In all mammals excepting 
some of the aquatic species, the eye is kept moist by the secretions 
of the lachrymal gland situated in the upper lid at the outer side. 
In Man, in whom the gland is highly developed, this secretion is 
most susceptible of being secreted to excess and a consequent over- 
flow in the shape of tears. The eyelids are lubricated by other 
glands. In some mammals, like the Hares and Jerboas, the eyes are 
large and prominent, and from their lateral position it is claimed 
they are susceptible of perceiving the image of a pursuer. 
