The senses are five in number, namely, common sensation and the four special senses; and the special senses are naturally arranged in two pairs, taste and smell giving sensations of a simple kind, while the sensations of sight and hearing are of a far more complex description, and produced by the action of exceedingly complex organs.

159. Common Sensation

Common Sensation includes feeling and touch. In feeling, the mind has simply the idea of a condition of the body; while from touch it receives an idea of properties of external objects. Pain, tickling, and a sense of warmth or cold are instances of feeling which are not necessarily accompanied with touch.

Although the variety of common sensation constituting touch is confined to the surface of the body and to the mouth, and the skin, with its nerve terminations, already described (p. 68), is the principal organ of this sense, feeling is not confined to the surface. Pain, as we all know, may be felt in the deep parts; and there is another form of feeling in deep tissues which has excited a good deal of attention, namely, muscular sense. Although the mind is unconscious of the particular muscles which exist in any part, it is yet able to regulate their contraction, so as to produce and direct every movement with precision, which it could not do without a knowledge of the position of the parts. No doubt this knowledge is only partly due to a sense of the state of the muscles, as may be illustrated by the consideration that in moving one's fingers there is little sense of action save in the fingers themselves, although we know that the muscles which flex and extend the fingers are really in the forearm; but we can make the muscles of a limb rigid by an effort of the will without changing the position of the limb, and this rigidity is accompanied with a peculiar sensation. Also, the sense of muscular exhaustion or weariness is derived, in part at least, from the muscles themselves; and the sense of resistance consists partly in a feeling of the expenditure of a certain amount of muscular exertion which meets with opposition. That, however, is only one element in the sense of resistance, and one which comes into play only when there is muscular action; but suppose that the head is reclined, and that something comes in contact with it, the force of the contact and the hardness or softness of the object are both appreciated, although no muscles are brought into play, and although there are none in the part which has been touched. Obviously, in such circumstances, what is appreciated is the character and degree of the pressure exercised against the skin.

And now, if the student, having considered these different varieties of feeling, will attempt to analyse what touch consists in, he will find that it is not a different sense from feeling, but only an application of it, accompanied with a judgment of the mind. The hardness or softness, roughness or smoothness, of the object touched, exercise different kinds of pressure on the skin, and can be examined by means of more or less muscular exertion; while the sense of the position of the touching organ enables us to determine the form and extent of the object.

The sense of temperature has some claim to be considered as distinct from all others; for rare instances are on record of the loss of this sense in a limb, leading to the exposure to severe injury from burning, while the other varieties of common sensation remained.