the path by which such messengers travel is the blood stream.
A further and most important manner in which the circulating fluid is
utilized in the life of an animal is seen in the way in which it is
employed in protecting the body should it be invaded by micro-organisms.

Hence it is clear that the blood is of the most vital importance to the
healthy life of the body. But the fact that it is present as a
circulating medium exposes the animal to a great danger, viz. that it
may be lost should any vessel carrying it become ruptured. This is
constantly liable to happen, but to minimize as far as possible any such
loss, the blood is endowed with the peculiar property of _clotting_,
i.e. of setting to a solid or stiff jelly by means of which the orifices
of the torn vessels become plugged and the bleeding stayed.

The performance of these essential functions depends upon the
maintenance of a continuous flow past all tissue cells, and this is
attained by the circulatory mechanism, consisting of a central pump, the
heart, and a system of ramifying tubes, the arteries, through which the
blood is forced from the heart to every tissue (see VASCULAR SYSTEM). A
second set of tubes, the veins, collects the blood and returns it to the
heart. In many invertebrates the circulating fluid is actually poured
into the tissue spaces from the open terminals of the arteries. From
these spaces it is in turn drained away by the veins. Such a system is
termed a _haemolymph system_ and the circulating fluid the haemolymph.
Here the essential point gained is that the fluid is brought into direct
contact with the tissue cells. In all vertebrates, the ends of the
arteries are united to the commencements of the veins by a plexus of
extremely minute tubes, the capillaries, consequently the blood is
always retained within closed tubes and never comes into contact with
the tissue cells. It is while passing through the capillaries that the
blood performs its work; here the blood stream is at its slowest and is
brought nearest to the tissue cell, only being separated from it by the
extremely thin wall of the capillary and by an equally thin layer of
fluid. Through this narrow barrier the interchanges between cell and
blood take place.

The advantage gained in the vertebrate animal by retaining the blood in
a closed system of tubes lies in the great diminution of resistance to
the flow of blood, and the consequent great increase in rate of flow
past th