e tissue cells. Hence any food stuffs which can travel quickly
through the capillary wall to the tissue cell outside can be supplied in
proportionately greater quantity within a given time, without requiring
any very great increase in the concentration of that substance in the
blood. Conversely, any highly diffusible substance may be withdrawn
from the tissues by the blood at a similarly increased pace. These
conditions are more peculiarly of importance for the supply of oxygen
and the removal of carbonic acid-especially for the former, because the
amount of it which can be carried by the blood is small. But as the rate
at which a tissue lives, _i.e_. its activity, depends upon the rate of
its chemical reactions, and as these are fundamentally oxidative, the
more rapidly oxygen is carried to a tissue the more rapidly it can live,
and the greater the amount of work it can perform within a given time.
The rate of supply is of much less importance in the case of the other
food substances because they are far more soluble in water, so that the
supply in sufficient quantity can easily be met by a relatively slow
blood flow. Hence we find that the gradual evolution of the animal
kingdom goes hand in hand with the gradual development of a greater
oxygen-carrying capacity of the blood and an increase in the rate of its
flow.

In the groundwork of a tissue are a number of spaces--the _tissue
spaces_. They are filled with fluid and intercommunicate freely, finally
connecting with a number of fine tubes, the lymphatics, through which
excess of fluid or any solid particles present are drained away. The
contained fluid acts as an intermediary between the blood and the cell;
from it, the cell takes its various food stuffs, these having in the
first instance been derived from the blood, and into it the cell
discharges its waste products. On the course of the lymphatics a number
of typical structures, the lymphatic glands, are placed, and the lymph
has to pass through these structures where any deleterious products are
retained, and the fluid thus purified is drained away by further
lymphatics and finally returned to the blood. Thus there is a second
stream of fluid from the tissues, but one vastly slower than that of the
blood. The flow is too slow for it to act as the vehicle for the removal
of those waste products (carbonic acid, &c.) which must of necessity be
removed quickly. These must be removed by the blood. The same is true
fo