are connected by their rays to each other,
or to fibers which conduct the nerve impressions, or they act as
receptacles, storehouses, and transmitters for them, as the switch-board
of a telephone system serves to connect the various wires.

All these tissues are supplied with blood in greater or less quantity.
The vascularity depends upon the function which the tissue is called
upon to perform. If this is great, as in the tongue, the lungs, or the
sensitive part of the hoof, a large quantity of blood is required; if
the labor is a passive one, as in cartilage, the membrane over the
withers, or the tendons of the legs, the vessels only reach the
periphery, and nutrition is furnished by imbibition of the fluids
brought to their surface by the blood vessels.

Blood is brought to the tissues by arterioles, or the small terminations
of the arteries, and is carried off from them by the veinlets, or the
commencement of the veins. Between these two systems are small, delicate
networks of vessels called capillaries, which subdivide into a veritable
lacework so as to reach the neighborhood of every element.

In health the blood passes through these capillaries with a regular
current, the red cells or corpuscles floating rapidly in the fluid in
the center of the channel, while the white or ameboid cells are
attracted to the walls of the vessels and move very slowly. The supply
of blood is regulated by the condition of repose or activity of the
tissue, and under normal conditions the outflow exactly compensates the
supply. The caliber of the blood vessels, and consequently the quantity
of blood which they carry, is governed by nerves of the sympathetic
system in a healthy body with unerring regularity, but in a diseased
organ the flow may cease or be greatly augmented. In health a tissue or
organ receives its proper quantity of blood; the nutritive elements are
extracted for the support of the tissue and for the product, which the
function of the organ forms. The force required in the achievement of
this is furnished by combustion of the hydrocarbons and oxygen brought
by the arterial blood, then by the veins this same fluid passes off,
less its oxygen, loaded with the waste products, which are the result of
the worn-out and disintegrated tissues, and of those which have
undergone combustion. The foregoing brief outline indicates the process
of nutrition of the tissues.

Hypernutrition, or excessive nutrition of a tissue, may b