16 .-- PHAGOCYTOSIS.

_a_, _b_, _c_ are the microphages or the bacterial phagocytes.

(_a_) Contains a number of round bacteria, and (_b_) similar bacteria arranged in chains, and (_c_) a number of rod-shaped bacteria (_d_) Is a cell phagocyte or macrophage which contains five red blood corpuscles.] All life in the tissues depends upon the circulation of the blood.
There is definite relation between the activity of cells and the blood supply; a part, for instance, which is in active function receives a greater supply of blood by means of dilatation of the arteries which supply it.

If the body be exactly balanced longitudinally on a platform, reading or any exercise of the brain causes the head end to sink owing to the relatively greater amount of blood which the brain receives when in active function.

The regulation of the blood supply is effected by means of nerves which act upon the muscular walls of the arteries causing, by the contraction or the relaxation of the muscle, diminution or dilatation of the calibre of the vessel.