Just in proportion as the muscle becomes shorter, it becomes thicker in the middle; and this you can readily prove by grasping it lightly with your fingers when it contracts, and feeling it bulge.[22] The food tube is surrounded with muscles, as you will remember, for moving the food along it, or churning it.

These internal muscles, requiring only the presence of food to cause them to act, and not needing attention on the part of the brain or the will, are known as the _involuntary_ ("without the will") muscles.
The great group of the _voluntary_, or bone-moving muscles, which move "with the will" and are under our direct control, may be divided roughly into two divisions--those that move the trunk, or body proper, and run, for the most part, lengthwise of it; and those that move the limbs.
On the body, they may be divided into two great sheets--one running up the front, and the other up the back.

When those running up the front of the body contract, they naturally bend the back, and pull the head and shoulders forward and downward.

Or, as when you spring up and catch the branch of a tree or a horizontal bar with your hands, these same muscles will pull the lower part of the body and legs upward, so that you can climb into the tree.
The largest and thickest bands of these front body muscles are found over the abdomen, or stomach, where you can feel them thicken and harden when you bend your body forward and pull with your arms, as in hauling on a rope.

By their pressure upon the intestines, they give the bowels valuable support, assist in their movements, and help the circulation of the blood through them; so that it is of considerable importance to keep this entire group of muscles well toned up by exercises, such as swinging your arms back over your head, and then down between your legs; bending the head and shoulders backward and forward; swinging the legs up over the body, either when hanging from a bar or lying on your back.
Proper exercising and toning up of these muscles will often cure constipation and dyspepsia, by their influence upon the bowels and stomach, and also keep one from taking on fat around the waist too rapidly.
On the back of the body, the muscle-sheet has grown into great, thick ropes of muscle on each side of the backbone, which you can feel hardening and softening in the small of the back, when you stoop down or lift weights.