The arteries have rings of plain muscular fibre in their walls; when these contract they narrow the artery, and when they relax they allow it to widen under the pressure of the blood in its interior. The vessel then carries more blood to the capillaries of the organ which it supplies. Blushing is due to a relaxation of the muscular layer of the arteries of the face and neck, allowing more blood to flow to the skin.

Why The Arteries Have Muscles

The amount of blood in the body is not sufficient to allow of a full stream of blood through all its organs at one time: the muscular fibres controlling the diameter of the arteries are used to regulate the blood-flow in such a manner that parts hard at work shall get an abundant supply, and parts at rest shall only get just enough to keep them nourished. Usually when one set of organs is at work and its arteries dilated, others are at rest and their arteries contracted. Few persons, for example, feel inclined to do brain-work after a heavy meal; for then a great part of the blood of the whole body is led off into the dilated vessels of the digestive organs, and the brain gets but a small supply. On the other hand, when the brain is at work its vessels are dilated, and often the whole head flushed; and when the muscles are exercised, a great portion of the blood of the body is carried off to them; therefore, hard thought or violent exercise soon after a meal is very apt to produce an attack of indigestion by diverting the blood from the abdominal organs, where it ought to be at that time. Young persons whose organs have a superabundance of energy, enabling them to work under unfavorable conditions, are less apt to suffer in such ways than their elders. One sees boys running actively about after eating, when older people feel a desire to sit quiet or even to go to sleep.

What would be apt to happen if blood were sent into them in sudden jets?

How are the muscles of the arteries arranged? What results from their contraction? From their relaxation? To what is blushing due?

Why cannot all the organs have a full blood stream through them at the same time? For what purpose are the muscular fibres in the walls of arteries used? What is the usual condition of the arteries of a resting organ?

Taking Cold

When the skin is chilled its arteries contract, as shown by the pallor of the surface. This throws an undue amount of blood into internal parts, whose vessels become gorged with blood or "congested," and congestion very easily passes into inflammation. Consequently, prolonged exposure of the surface to cold is very apt to be followed by inflammation of parts inside the body, and give rise to a so-called "cold" (which is really an inflammation) of the mucous membranes of the head, or throat, or lungs; or of the intestines, causing diarrhoea. In fact, the common summer diarrhoea is far more often due to a chill of the surface leading to intestinal inflammation than to the fruits eaten in that season, which are so often blamed for it. The best preventive is to wear when exposed to sudden changes of temperature, a woollen or at least a cotton garment over the trunk of the body; linen permits any change in the external temperature to act almost at once upon the skin. After an unavoidable exposure to cold or wet, the thing to be done is of course to maintain the cutaneous circulation; movement should be persisted in, and a thick dry outer covering put on until warm and dry underclothing can be obtained.

Why is it not wise to take hard exercise or do severe mental work soon after eating? Why do young persons suffer less from exercise soon after dinner than do their elders?

What happens to its arteries when the skin is chilled? How does this manifest itself? What is its result on the blood-supply of internal parts? What is congestion? Into what diseased state does it often pass?

What diseases are apt to follow a surface chill? What is the most frequent cause of summer diarrhoea? What should be worn when liable to exposure to considerable changes of temperature?

In healthy persons, a temporary exposure to cold, as a plunge in a bath, is good, since in them the sudden contraction of the cutaneous arteries soon passes off, and is succeeded by a dilatation causing a warm healthy glow on the surface. If the bather remain too long in cold water, however, this reaction passes off, and is succeeded by a more persistent chilliness of the surface, which may last all day. The bath should therefore be left before this occurs; but no absolute time can be stated, as the reaction is more marked and lasts longer in strong persons and in those used to cold bathing than in others.

By partially paralyzing certain nerves of the heart which keep its beating in check, wines and spirits quicken the beat of the heart, leaving it less time for repair between its strokes. This causes the heart's walls to be at first thickened (hypertrophied) and then its cavities dilated by the excessive work (p. 111) which alcoholic drinks cause it to perform. If, as is usually the case, fatty degeneration ensues, the organ gradually becomes too feeble to pump the blood around the body, and death results. A heart which has undergone this change is commonly spoken of by pathologists as a " whiskey heart;" for although fatty degeneration of the heart may occur from other causes, alcoholic indulgence is the most frequent one. Fatty liver or fatty heart is rarely if ever curable ; either will ultimately cause death.

Why does linen not form a good inner garment under such circumstances ? What should be done after unavoidable exposure to cold or wet? How does alcohol injure the heart?

Why is a plunge in a cold bath useful to healthy persons? What results if a person remains too long in cold water ? When should a cold bath be left? What persons may remain longest with safety in a cold bath?