If the number of platelets is greatly reduced, the person bleeds easily under the skin. In some infectious diseases the number of platelets is promptly reduced, and one of the most significant manifestations of these diseases is the collection of large black and blue spots over the body.

The medical profession knows a good deal about the blood platelets, but there is far more unknown than known. It knows a great deal about the clotting of blood, but it needs to know much more. Tests have been devised which indicate that the blood clots normally in from one to three minutes after bleeding commences. The clotting can be hastened by squeezing the bleeding spot, by pressing on the blood vessel from which the blood comes, by applying hot applications, by applying substances which break down the cells, by rubbing, and by other manipulations with which almost everyone is familiar.

If there is a deficiency of calcium in the blood, this will delay the clotting. However, calcium itself is associated with many other conditions of health. Of these more will be said later.

The white blood cells are certainly as significant as the red blood cells for human health and life. When inflammation occurs anywhere in the human body, the blood brings enormous numbers of white blood cells to the spot. They come there and remove the degenerated or broken down material. They are of many varieties. In times of infection, for instance in appendicitis, the number of white blood cells will be raised promptly to 15,000 or even to 50,000 as compared with the 5,000 to 7,000 that is normal. One variety will be increased greatly in percentage as compared with the other varieties. This variety is called the polymorphonuclear white blood cell because it is a white cell with many nuclei. Whereas normally it composes from 55 to 65 per cent of all the white blood cells, the number may increase so rapidly in time of infection that this form will constitute 75 to 90 per cent. Another of the forms is mononuclear. In many diseases this form is greatly reduced, and in other diseases increased. A third form of white blood cell, called eosinophile because it stains readily with eosin, is increased in number in some diseases - unquestionably in infestation with the pork worm known as trichina.

The medical profession and workers in research laboratories throughout the world are spending vast amounts of time and energy to identify the various forms of white blood cells and to discover their exact significance in different kinds of diseases. Experts on the blood can tell even now whether a person is suffering from one type of infection or another, whether he has been poisoned by some chemical element, or whether his blood-forming organs are functioning properly. But though it is known that in the blood-forming and destroying system the bone marrow and the spleen are especially significant and that the liver and the gall bladder are seriously concerned, the whole mechanism is so complicated and intricate that it will require the best brains of thousands of scientists for many years before it is fully understood.

Blood Pressure. When a volume of fluid is forced through a tube, the pressure of the fluid in the tube varies according to the amount of pressure behind the fluid as it goes through the tube, and the size of tube through which the fluid flows. It varies also according to the ability of the tube to stretch. If the pressure behind the fluid is temporarily lessened, the pressure in the tube is lessened. If the tube is rigid, the amount of pressure will increase over that of a tube which can relax. These are some of the considerations which govern the blood pressure of man.

An increase in the amount of blood flowing through the blood vessels will raise the blood pressure; a decrease in the quantity will lower it. If the blood vessels become hardened by arteriosclerosis, the blood pressure increases. If the blood vessels are constricted by the use of certain drugs, or by certain reactions in the nervous system, the blood pressure increases.

The amount of blood pressure can be measured in numerous ways, including the method usually seen of putting a cuff around the arm, inflating it with air and reading the amount of pressure on a mercury column or on a spring device. Recently a machine has been developed that will record with a moving finger or moving screen the variations in the blood pressure over a certain period. By listening with his stethoscope while the blood pressure is being measured, the physician is able to determine two phases of the blood pressure, while the heart is contracted and while the heart is relaxed.

At first everyone was concerned largely with the question of high blood pressure, but now it is recognized that either high or low blood pressure may be of diagnostic importance. Life insurance companies have given particular consideration to this question, and to-day the determination as to the life expectancy of an individual may rest largely on an exact determination of his blood pressure. The majority of medical directors of insurance companies are inclined to think that pressure somewhat below the average predisposes the individual to increased longevity. Pressure very much above the average is serious. These observations represent the fundamentals of our knowledge of the blood pressure, but do not begin to represent all of the knowledge that scientific medicine must have before it is able to render unequivocal judgments in this field.

Since the factors that control blood pressure are numerous and include not only purely physical factors but also mental and emotional conditions; since the part played by heredity is not certainly known; and since the influence of proper elimination from the kidneys, of the amount of fluid and of salt taken into the body, and of alcohol and tobacco, is not certainly established, there is a wide field of research on this frontier of medicine still available to the explorer.