millimetres); and so on. Shut your eyes, and allow a friend to draw the compass with the points about 1/2 an inch apart, from the forehead to the tip of your nose, or (setting the points about 1/4 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 invertebrates. 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 neighbourhood. 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 are specially modified to form tactile cells, or tactile corpuscles, in contact with or surrounding the axis-fibre or its expansion (Fig. 23). [Illustration: Fig. 23.--Tactile corpuscles. 1. In the beak of a goose. 2. In the finger of a man. 3. In the mesentery of a cat.] Hairs are delicate organs of touch, though, of course, this is not their only function. They act as little levers embedded in the skin. Turning now to the vertebrate animals other than man, we find in them a sense of touch closely analogous to our own. As in us, so in them, the specially mobile parts are eminently sensitive and delicate; for instance, the lips in many animals, such as the horse, and the finger-like organ at the end of the elephant's trunk. In some of them special hairs are largely developed as organs of touch, as in the whiskers of the cat and the long hairs on the rabbit's lip. With the aid of these the rabbit finds its way in the darkness of its burrow; and it is said that, deprived of these organs, the poor animal blunders about, and is unable to steer its course in the dark.