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.