FUNDAMENTAL PRINCIPLES OF DISEASE. 29 
When the membrane serves for secreting or excreting purposes, as 
in the salivary glands or the kidneys, it is usually simple; when it 
serves the mechanical purpose of protecting a part, as over the tongue 
or skin, it is invariably multiple and stratified, the surface wearing 
away while new cells replace it from beneath. 
(7) In nerves, stellate cells 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 quan- 
tity. 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 periph- 
ery, 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 termina- 
tions 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 tle 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 sup- 
ply. 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 achieve- 
ment 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 indi- 
cates the process of nutrition of the tissues. 
Hypernutrition, or excessive nutrition of a tissue, may be normal 
or morbid. If the latter, the tissue becomes congested or inflamed. 
