of low fuel value, such as lettuce, watercress, etc., though
practically all foods yield at least a small mineral constituent. When
fuel values alone are considered, fruits have a low value, but because
of the flavor they impart to other foods, and because of the healthful
influence they exercise in digestion, they cannot be excluded from the
diet.

Care should be constantly exercised to provide substantial foods of
high fuel value. But the nutritive foods should be wisely supplemented
by such foods as fruits, whose real value is one of indirect rather
then direct service.

58. Our Bodies. Somewhat as a house is composed of a group of
bricks, or a sand heap of grains of sand, the human body is composed
of small divisions called cells. Ordinarily we cannot see these cells
because of their minuteness, but if we examine a piece of skin, or a
hair of the head, or a tiny sliver of bone under the microscope, we
see that each of these is composed of a group of different cells. A
merchant, watchful about the fineness of the wool which he is
purchasing, counts with his lens the number of threads to the inch; a
physician, when he wishes, can, with the aid of the microscope,
examine the cells in a muscle, or in a piece of fat, or in a nerve
fiber. Not only is the human body composed of cells, but so also are
the bodies of all animals from the tiny gnat which annoys us, and the
fly which buzzes around us, to the mammoth creatures of the tropics.
These cells do the work of the body, the bone cells build up the
skeleton, the nail cells form the finger and toe nails, the lung cells
take care of breathing, the muscle cells control motion, and the brain
cells are responsible for thought.