rganic." And as it
is universally characteristic of them, this term "organic" has been very
conveniently employed to denote the whole of living nature,--the whole
of the plant world, and the whole of the animal world.

Few animals can be more familiar to you than that whose skeleton is
shown on our diagram. You need not bother yourselves with this "Equus
caballus" written under it; that is only the Latin name of it, and does
not make it any better. It simply means the common Horse. Suppose we
wish to understand all about the Horse. Our first object must be to
study the structure of the animal. The whole of his body is inclosed
within a hide, a skin covered with hair; and if that hide or skin be
taken off, we find a great mass of flesh, or what is technically called
muscle, being the substance which by its power of contraction enables
the animal to move. These muscles move the hard parts one upon the
other, and so give that strength and power of motion which renders the
Horse so useful to us in the performance of those services in which we
employ him.

And then, on separating and removing the whole of this skin and flesh,
you have a great series of bones, hard structures, bound together with
ligaments, and forming the skeleton which is represented here.

[Illustration: FIGURE 1. (Section through a horse.)]

[Illustration: FIGURE 2. (Section through a cell.)]

In that skeleton there are a number of parts to be recognized. The long
series of bones, beginning from the skull and ending in the tail, is
called the spine, and those in front are the ribs; and then there are
two pairs of limbs, one before and one behind; and there are what we
all know as the fore-legs and the hind-legs. If we pursue our researches
into the interior of this animal, we find within the framework of
the skeleton a great cavity, or rather, I should say, two great
cavities,--one cavity beginning in the skull and running through the
neck-bones, along the spine, and ending in the tail, containing the
brain and the spinal marrow, which are extremely important organs. The
second great cavity, commencing with the mouth, contains the gullet,
the stomach, the long intestine, and all the rest of those internal
apparatus which are essential for digestion; and then in the same great
cavity, there are lodged the heart and all the great vessels going from
it; and, besides that, the organs of respiration--the lungs: and then
the kidneys, and the organs of repr