Lectures and Essays | Page 4

all has a minute structure, visible only
under the microscope. All these parts constitute microscopic anatomy
or 'Histology.' These parts are constantly being changed; every part is
constantly growing, decaying, and being replaced during the life of the
animal. The tissue is constantly replaced by new material; and if you go
back to the young state of the tissue in the case of muscle, or in the case
of skin, or any of the organs I have mentioned, you will find that they
all come under the same condition. Every one of these microscopic
filaments and fibres (I now speak merely of the general character of the
whole process)--every one of these parts--could be traced down to
some modification of a tissue which can be readily divided into little
particles of fleshy matter, of that substance which is composed of the
chemical elements, carbon, hydrogen, oxygen, and nitrogen, having
such a shape as this (Figure 2). These particles, into which all primitive
tissues break up, are called cells. If I were to make a section of a piece
of the skin of my hand, I should find that it was made up of these cells.
If I examine the fibres which form the various organs of all living
animals, I should find that all of them, at one time or other, had been
formed out of a substance consisting of similar elements; so that you
see, just as we reduced the whole body in the gross to that sort of
simple expression given in Figure 1, so we may reduce the whole of the
microscopic structural elements to a form of even greater simplicity;

just as the plan of the whole body may be so represented in a sense
(Figure 1), so the primary structure of every tissue may be represented
by a mass of cells (Figure 2).
Having thus, in this sort of general way, sketched to you what I may
call, perhaps, the architecture of the body of the Horse (what we term
technically its Morphology), I must now turn to another aspect. A horse
is not a mere dead structure: it is an active, living, working machine.
Hitherto we have, as it were, been looking at a steam-engine with the
fires out, and nothing in the boiler; but the body of the living animal is
a beautifully-formed active machine, and every part has its different
work to do in the working of that machine, which is what we call its
life. The Horse, if you see him after his day's work is done, is cropping
the grass in the fields, as it may be, or munching the oats in his stable.
What is he doing? His jaws are working as a mill--and a very complex
mill too--grinding the corn, or crushing the grass to a pulp. As soon as
that operation has taken place, the food is passed down to the stomach,
and there it is mixed with the chemical fluid called the gastric juice, a
substance which has the peculiar property of making soluble and
dissolving out the nutritious matter in the grass, and leaving behind
those parts which are not nutritious; so that you have, first, the mill,
then a sort of chemical digester; and then the food, thus partially
dissolved, is carried back by the muscular contractions of the intestines
into the hinder parts of the body, while the soluble portions are taken
up into the blood. The blood is contained in a vast system of pipes,
spreading through the whole body, connected with a force pump,--the
heart,--which, by its position and by the contractions of its valves,
keeps the blood constantly circulating in one direction, never allowing
it to rest; and then, by means of this circulation of the blood, laden as it
is with the products of digestion, the skin, the flesh, the hair, and every
other part of the body, draws from it that which it wants, and every one
of these organs derives those materials which are necessary to enable it
to do its work.
The action of each of these organs, the performance of each of these
various duties, involve in their operation a continual absorption of the
matters necessary for their support, from the blood, and a constant
formation of waste products, which are returned to the blood, and
conveyed by it to the lungs and the kidneys, which are organs that have

allotted to them the office of extracting, separating, and getting rid of
these waste products; and thus the general nourishment, labour, and
repair of the whole machine is kept up with order and regularity. But
not only is it a machine which feeds and appropriates to its own support
the nourishment necessary to its existence--it is an engine for
locomotive purposes. The Horse desires to go from