THE CAVE RAT. 



27 



An attempt to catch the rat sent it running back and forth along the ledges of 

 rock at the side of the cave. Finally the rat appeared at the ground again, and 

 despairing of catching it alive, it was killed. Its eyes seemed to be large and pro- 

 truding very much as in the common rat. Without question the rat noticed the 

 light. It had no hesitation in running from place to place. Later four of these 

 rats were sent by express. Only one arrived alive; one had been partly eaten 

 by the others. The living one was quite gentle. It permitted itself to be stroked. 

 Occasionally it pushed an object away with a sideward motion of the forefoot. 

 If provoked it snapped at the object. During daylight it sat quietly in a nest it 

 formed for itself of cotton batting, which it pulled into a fluffy mass. At night it 

 frequently moved about in its cage. Turning on an electric light near its face 

 always produced a twitching of the eyelids, so there can be no doubt that the 

 light was perceived. An object held some distance from the cage on one side or 

 another was always perceived, but just how precise its vision was has not been 

 determined. Its hearing was acute. 



FIG. 2. Retinas of Neotoma and Common Gray Rat Compared, 

 (a) Mammoth Cave Rat. (6) Common Gray Rat. 



Its eyes were as prominent as those of the gray rat. If there was any difference, 

 its eyes were larger in proportion to the size of the body weight than those of the 

 gray rat. The lens in both cases was enormously large in proportion to the eye. 

 The pupil was capable of very wide dilation. A microscopic comparison of the 

 retinas also showed little difference. Bits of retina from corresponding parts of 

 the eye of a cave rat and a gray rat were hardened by the same process, sectioned 

 the same thickness, and stained alike. The results are given in figures i and 2. 



There is little difference except in the thickness of the retina, that of the cave rat 

 being thicker. However, the difference may be due to the differences in the ages 

 of the animals, the cave rat being fully grown, the gray rat only half grown. The 

 thickness of the retinas are proportionate to the size of the eye. The increased 

 thickness is largely due to the larger size of the cells of corresponding layers of the 

 retina. For instance, the rods and cones are decidedly longer and larger in the 

 cave rat. But with the exceptions given the two retinas are nearly alike. 



