More definite than these, but still of internal origin, is the _muscular sense_. This, too, is of continual service to every active animal. By it information is given as to the energy of contraction of the muscles, and of the amount of movement effected--not to mention the rapidity and duration of the muscular effort. By it the position, or changes of position, of the motor-organs are indicated. It is obvious, therefore, that the sensations obtained in this way, some of which are exceedingly delicate, are an important guide to the organism in the putting forth of its activities. It is through the muscular sense that we maintain an upright position. It is through an educated and refined muscular sense that the juggler and the acrobat can perform their often surprising feats. Concerning the physiology of the muscular sense, we have at present no very definite knowledge. Some have held that we judge of muscular movements by the amount of effort required to initiate them; but it is much more probable that there are special sensory nerves, whose terminations are either in the muscles themselves or in the membranes which surround them. * * * * * We come now to the special senses. Of these we will take first the _sense of touch_. Through this sense we are made aware of bodies solid or liquid (or perhaps gaseous) which are actually in contact with the skin or its infoldings at the mouth, nostrils, etc. There are considerable differences in the sensitiveness of the skin in different parts of its surface; some parts, like the filmy membrane which covers the eye, being very sensitive, while others, like the horny skin that covers the heel of a man who is accustomed to much walking, are relatively callous. Different from this is the delicacy of the sense of touch. This delicacy is really the power of discrimination, and therefore involves some mental activity. But it is also dependent upon the distribution of the recipient end-organs of the nerve. The highest pitch of delicacy is reached in the tip of the tongue, which is about sixty times as delicate as the skin of the back. The power of discrimination is tested in the following way: The points of a pair of compasses are blunted, and with them the skin is lightly touched. When the points are close together, the sensation is of one object; 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 2-1/3 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 1/25 of an inch (1.1 millimetre) apart. For the finger-tip the distance is about 1/12 of an inch (2 millimetres); for the tip of the nose, about 1/4 of an inch (6.8 millimetres); for the forehead, a little less than an inch (22.6