r the main number of other waste products, which, however, being of
small molecular size are readily absorbed into the blood stream.

But in addition to fluid, the tissue spaces may at times be found to
contain solid matter in the form of particles, which may represent the
debris of destroyed cells, or which are, as is quite commonly the case,
micro-organisms. Apparently such material cannot be removed from a
tissue by absorption into the blood stream--indeed in the case of living
organisms such an absorption would in many instances rapidly prove
fatal, and special provision is made to prevent such an accident. These,
therefore, are made to travel along the lymphatic channels, and so,
before gaining access to the blood stream and thus to the body
generally, have to run the gauntlet of the protective mechanism provided
by the lymphatic glands, where in the major number of cases they are
readily destroyed.

Hence we see that first and foremost we have to regard the blood as a
food-carrier to all the cells of the body; in the second place as the
vehicle carrying away most if not all the waste products; in a third
direction, it is acting as a means for transmitting chemical substances
manufactured in one tissue to distant cells of the body for whose
nutrition or excitation they may be essential; and in addition to these
important functions there is yet another whose value it is almost
impossible to overestimate, for it plays the essential role in rendering
the animal immune to the attacks of invading organisms. The question of
immunity is discussed elsewhere, and it is sufficient merely to indicate
the chief means by which the blood subserves this essential protective
mechanism. Should living organisms find their way into the surface cells
or within the tissue spaces, the body fights them in a number of ways,
(1) It may produce one or more chemical substances capable of
neutralizing the toxic material produced by the organism. (2) It may
produce chemical substances which act as poisons to the micro-organism,
either paralysing it or actually killing it. Or (3) the organism may be
attacked and taken up into the body of wandering cells, _e.g_. certain
of the leucocytes, and then digested by them. Such cells are therefore
called phagocytes ([Greek: Phagein], to eat). Thus, by its power of
reacting in these ways the body has become capable of withstanding the
attacks of many different varieties of micro-organisms, of both animal