246 Animal Life and Intelligence. 



when they are more divergent, each point is felt as distinct 

 from the other. On the thigh and in the middle of the 

 back, two distinct points of contact are not felt unless the 

 compass-tips are about 2| inches (67'7 millimetres) apart. 

 When the divergence is 2 inches, they are felt as one. 

 "With the tip of the tongue, however, we can distinguish the 

 two separate points when they are only -^ of an inch (1*1 

 millimetre) apart. For the finger-tip the distance is 

 about y^ of an inch (2 millimetres) ; for the tip of the nose, 

 about \ of an inch (6*8 millimetres) ; for the forehead, a 

 little less than an inch (22*6 millimetres) ; and so on. 

 Shut your eyes, and allow a friend to draw the compass 

 with the points about \ an inch apart, from the forehead 

 to the tip of your nose, or (setting the points about \ of an 

 inch apart) from the ball of your thumb to the finger-tip. 

 The increasing delicacy and power of discrimination is 

 readily felt, and it is difficult to believe that the compasses 

 are not being slowly opened. 



It is beyond the purpose of this chapter to describe 

 minutely the nature and structure of the nerve-ends in 

 the sense-organs. This is a matter of minute anatomy, or 

 histology. A full description of them as they occur in 

 man will be found in any standard text-book of physiology ; 

 while Sir John Lubbock's " Senses of Animals " gives 

 much information concerning, and many illustrations of, 

 the minute structure of the sense-organs in the inverte- 

 brates. Here I can only touch very briefly on some of the 

 more important points. 



One of the larger nerves of the body (e.g. the sciatic 

 nerve), consists of a bundle of nerve-threads collected from 

 a considerable area ; some of these (motor threads) end in 

 muscles, others (sensory threads) in the skin or its neigh- 

 bourhood. Each nerve-thread has a central axis-fibre, 

 which is surrounded by a fatty, insulating medullary sheath, 

 and this by a delicate primitive sheath. In some parts 

 of the skin the sensory nerve-threads lose their medullary 

 sheath, and end in very fine branches between the cells of 

 the tissue. In other cases the cells near their termination 



