Before we leave the subject of the skeletal muscles it will be interesting to say a word about the different kinds of motions that they bring about. We have already seen that they work by pulling at the joints, and we have no intention of enlarging on that topic. What we want to do here is to group the bodily motions into a few classes, regardless of what joints are actually moved. First, and most important, comes _locomotion_; by that we mean any motions that move the body from one place to another. Under that head we have walking, running, swimming, jumping; in birds, flying. Next in order comes _grasping_; this includes all motions by which we take hold of anything. We can realize the importance of this group of movements when we think that our fore limbs are specifically grasping organs, while in the great majority of animals they are organs of locomotion along with the hind limbs. Originally grasping had to do, undoubtedly, with the taking of food and not much else. In civilized man we have in addition the use of all kinds of tools from the coarsest to the finest. In most four-legged animals the chief organ of grasping is the mouth. We still use our mouths to some extent as grasping organs, and could probably learn to make even more use of them in that direction if forced to it. _Chewing_ and _swallowing_ make up a group of movements concerned primarily with the handling of food after it has been grasped. Not much need be said about them. Of small extent but great importance are the motions connected with _sense perception_; these include chiefly the motions of the body, neck, and eyes in _vision_; we are constantly turning to look at something; in such animals as horses movements of the ears help greatly in _hearing_; and both man and animals make sniffing motions to increase the keenness of _smelling_. There is a group of motions devoted to _voice production_ (including breathing). In man the vocal cords, tongue, and muscles of the cheeks are the chief muscles that have to do with the voice, not including the muscles of breathing, which, of course, are essential. The interesting things about the vocal cords are the excessive fineness of their operation, enabling expert singers to produce tones that vary by only a few vibrations a second, and the amazing exactitude of the control that the nerves have over them, so that good singers can set them at the tension needed for producing a particular tone with absolute certainty. The tongue is not a single muscle, but a mass of several muscles working one upon the other. It plays a part both in voice production and in the chewing of food. As an organ of voice production it helps by changing the shape of the mouth cavity. Speech depends very largely on this, since not the tension of the vocal cords but the shape of the mouth and throat determines the making of letters and syllables.