age adult, and these vary in
character and in relative numbers (Fig. 12). The most numerous of
these are round and slightly larger than the red cells; they have a
nucleus of peculiar shape and contain granules of a definite
character. These cells serve an important part in infectious diseases
in devouring and destroying parasites. They have power of active
independent motion and somewhat resemble certain of the free living
unicellular organisms. The blood plasma, when taken from the vessels,
clots or passes from a fluid into a gelatinous or semi-solid
condition, which is due to the formation within it of a network of
fine threads termed fibrin. It is by means of the clotting of the
blood that the escape of blood from ruptured vessels is arrested.

Several of the organs of the body, in addition to the formation of
secretions which are discharged on the surfaces by means of their
ducts, produce also substances which pass directly into the blood or
lymph, and have an influence in stimulating or otherwise regulating
the activity of other organs. There are also certain organs of
glandular structure which are called the _ductless glands_; these
are not connected with the surface and all their secretion passes into
the blood. It is a part of recent knowledge that the substances
produced in these glands are of great importance for the body, some of
them even essential for the maintenance of life. In front of the neck
is such an organ, the thyroid gland (Fig. 8, 14). Imperfect
development or absence of this organ, or an inactive condition of it,
produces in the child arrested growth and deficient mental development
known as cretinism, and in the adult the same condition gives rise to
mental deterioration, swelling of the skin, due to a greater content
of water, and loss of hair. This deficiency in the production of
thyroid secretion can be made good and the symptoms removed by feeding
the patient with similar glands removed from animals. The very complex
disease known as exophthalmic goitre, and shown by irregular and rapid
action of the heart, protruding eyeballs and a variety of mental
symptoms, is also associated with this gland, and occasioned not by a
deficiency but by an excess or perversion of its secretion.

Adjoining the thyroid there are four small glands, the parathyroids,
each about the size of a split pea. The removal of these glands in
animals produces a condition resembling acute poisoning accompanied by
spasmod