FEB., 1912. MAMMALS OF ILLINOIS AND WISCONSIN CORY. 25 



of an inch. On the shoulder, however, unless the points are more 

 than % of an inch apart, they cannot 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 papillae with sensory 

 nerve filaments and on which the sense of touch is very highly devel- 

 oped. The whiskers (vibHssa) 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. 



