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.
The wing of the bat is very sensitive to touch; and it is supposed that it is through this sense that the bat is able to direct its course in the darkness of caves. Miss Caroline Bolton thus describes an experimental trial of this power of the bat at which she was herself present. A room, about twenty feet by sixteen, was arranged with strings crossing each other in all directions so as to form a network with about sixteen inches space between the strands. To each string was attached a bell in such a way that the slightest touch would make it ring. One corner of the room was left free for those who were present at the experiment. A bat, measuring about one foot from the tip of one wing to that of the other, was let loose in the room when it was quite dark, "and it was distinctly heard flying about all over the room, but never once did it touch a string or stop flying. It several times came quite near to the spectators, so that they could feel the vibration of the air in their faces. The experiment was continued for half an hour. Then, when the door was opened and light let in, the bat stopped flying, and settled down in the darkest corner." Now, here it may be said that, although the room was dark to human spectators, there may have been light enough for a bat to see his way. The cruel experiments of Spalanzani, however, who put out the eyes of bats and obtained a similar result, seem to show that the animal is guided by some sense other than that of sight.
Fig. 24.—Touch-Hair of insect.
t.h., touch-hair; cu., cuticle; h.y., hypodermis; g., ganglion-cell connected with nerve passing into the cavity of the touch-hair (after Miall). The ganglion is often surrounded by several—eight or less—accessory cells, which are not figured here.]
The crustaceans and many insects are covered with a dense armour, and it might be supposed that in them there could be no sense of touch. But this sense is by no means absent. Seated on the tough integument are delicate little hairs, to the base of which a nerve-fibril passes through a perforation in the integument. These are specially numerous in the antennæ of insects.
In yet lower organisms we know in some cases the manner in which they are sensitive to touch; but in a great number of cases, although observation shows that they are thus sensitive, we know nothing definite as to how the surface is specially fitted to receive the stimuli. Even the primitive amœba, however, is sensitive in the sense spoken of on [p. 8]; that is to say, it reacts under the influence of a stimulus.
Closely associated with the sense of touch is the temperature-sense. Goldschneider and others have shown that on the skin of the human hand, for example, there are special points that are sensitive to heat and cold. Some of these little specialized areas are sensitive to cold; others are sensitive to heat; and neither of these seem to be sensitive to pressure. It therefore seems probable that special nerve-fibrils are set apart for the temperature-sense; but of the manner in which these fibrils terminate little or nothing is known.