52 



THE NATURE BOOK 



A close examination of a wild animal 

 will usually give some idea as to its 

 habits, and this is markedly the case with 

 the Long-tailed Field Mouse. His eyes 

 show that he is nocturnal, his ears show 



AN IMMATURE LONG-TAILED FIELD MOUSE 



that he is alert, his limbs show that he 

 is athletic. 



The eves of the Long-tailed Field ^louss 

 are remarkablv prominent and remark- 

 ablv large. There is no visible iris in 

 a mouse's eve. All that one sees extern- 

 all v is the fully distended pupil. This 

 in a full-grown Field Mouse measures 

 about four millimetres across, and this 

 measurement is equal to the length of 

 the longest finger in the mouse's hand. 

 A similar proportion in man would make 

 the width of his distended pupil nearly 

 four inches. We can only speculate as 

 to what effect a pupil (with corresp/onding 

 eye) of this size would have on human 

 vision ; but I think we may safely s])ecu- 

 late to this extent : that the sensation 

 of light conveyed to the brain would be 

 enormously increased, and that, conse- 

 quently, our discrimination of light and 

 shade (not necessarily of detail) in dark- 

 ness or semi-darkness would be much 

 more sensitive. 



There is little doubt, moreover, that the 

 eyes of Field Mice have a remarkable range 

 of vision. The normal human eyes embrace 

 a very narrow angle with distinctness, 

 perhaps not more than five degrees. This 

 is comj)ensated for by the fact that thev 

 move simultaneously in their sockets, and 

 by this simultaneous movement, couj^led 

 with a movement of the head, command 



a large field. The eyes of Mice are per- 

 manently fixed, but their vision, instead 

 of being parallel, is widely divergent. 

 Field Mice have a constant squint of about 

 140 degrees. Apart from the protrusion 

 of their eyes, this 

 squint means an 

 enormous field of 

 view. Dr. Lindsay 

 Johnson, whose re- 

 searches on the 

 eyes of animals are 

 extremely ^'aluable 

 and interesting, in- 

 forms me that the 

 hare and squirrel 

 can, by a combina- 

 tion of both their 

 eyes, see through 

 an angle of 420 

 degrees — that is to 

 say that, in addi- 

 tion to their ability 

 to see round themselves in a complete 

 circle, the vision of either eye over- 

 laps that of the other to the extent 

 of some 50 degrees in front and lo- 

 degrees behind. I do not think that 

 the Field Mouse is much behind them. 

 His attitude when frightened is instruc- 

 tive. If one observes a Field Mouse who 

 is not frightened from a point somewhat 

 above and behind him, one finds that his 

 eyes are hidden by his ears. If, however, 

 he is frightened, he depresses the nape 

 of his neck and flattens his ears in such 

 a way that their tips converge. Under 

 these conditions, his eyes protrude rather 

 more than usual, and are plainlv visible 

 from the same view point. To lose sight 

 of them one has to bring one's own eyes 

 very nearly to their level, when they are 

 hidden by the mouse's back. It is obvious 

 that an arrangement whereby a mouse, 

 when flattened to the earth, can look 

 behind him as well as above and on all 

 sides must be of great adv'antage to the 

 race. The same tendency to depress the 

 neck may be observed in the Meadow 

 Mice and Water Rat, and I have no 

 doubt that it is usual among the Mouse 

 family. 



As may be seen from the illustrations, 

 the Field Mouse's ear is large in proportion 

 to his head — about half the length of it, 

 in fact. At first sight it appears to be 



