The skin is the 'vegetative nervous sense organ'. Water can account for a heat loss twenty-five times greater than possible through air. Stimuli that act on the skin travel from the skin receptors through the segmental reflex (consensual reaction) to the other side of the body, but also through the autonomic centers of the central nervous system (enlarged or distant consensual reaction).
The cold receptors are more frequent in the skin than the heat receptors; for example, in the face the ratio is 11.3 as against 3.3 per square centimeter, and even in the trunk the ratio is 9.7 for the cold as against 0.6 per square centimeter for the heat. The temperature corpuscles are nets of nerve tissue that develop action currents when there is a fall in temperature, that is, when a cold stimulus is applied to a warm skin. Even in very warm environments, such as an overheated sauna, the cold receptors are excited, with a resultant paradoxical goose flesh response.
The temperature-regulating mechanism in the hypothalamus responds to signals by an attempt to avoid cooling of the interior through reduction of the body shell, particularly the cutaneous organ. In the subcutaneous tissue, arteriovenous anastomoses are closed to reduce the amount of circulation; sympathetic innervation, both central and peripheral, produces an initial constriction. The skin cells, in turn, continue to form substances that act on vessels, since brief, cold stimuli increase cellular metabolism. With a reduced blood supply, these H-substances are not excreted in usual quantities, as in the case of other acid products of metabolism such as carbon dioxide and lactic acid. The histamine-like substances block cholinesterase, which inactivates or destroys newly formed acetylcholin, thus permitting a prologation of acetylcholine action, a dilation (Lewis' phenomenon) that increases the blood flow through arterioles and capillaries.
Reactive hyperemia occurs after a short, cold stimulus if the skin was previously well supplied with blood. When the blood, cooled during its passage in the skin, reaches the hypothalamus, the nervous center marshals the body defenses to prevent further cooling through action currents and by direct action on neighboring nerve centers, which explains the generalized effect of cold-water applications on the autonomic system. We speak of these responses as vascular training through partial hydrotherapy.
The segmental blood shift that follows cold stimulation of the skin is reflected in the other corresponding side (segmental reflex) and elsewhere in the body - brain vessels, pharynx, coronary vessels, and kidneys - a telereaction. We depend upon this reaction to obtain a slight increase in the coronary artery blood flow by stimulation of the left arm, or where this is not desirable, by a warm or contrast foot bath.
The increase in cutaneous circulation also involves the neighboring muscles with a shift from the interior (lungs and abdominal cavity), known as the Dastre-Morat law. We know of no other method that will increase the circulation so much with so mild a lasting and physiologic effect as hydrotherapy.
Brief hydrotherapy applications to a warm body segment will (a) shift blood from the interior to the skin segment (b) stimulate gastric secretion through a histamine effect (c) increase the general metabolism, particularly of the muscles and liver, through the central heat regulator (d) increase general and cardiac muscle tone and minute cardiac volume with lessened pulse rate (e) reinforce breathing by deepening respiration (f) regulate general excitability of the nerves and (g) increase resistance to the common cold.
Minor hydrotherapy also has a hardening effect, by a decreased tendency to upper respiratory infections. A deficient blood supply of the mucosa may result in stasis and even ischemia, which lowers resistance to infection. It is likely that the surface of the bacteria is coated by serum globulin, magnesium, and antibodies, the last of which kills the bacteria. Phagocytosis as well as humoral properdin depend upon adequate tissue blood supply; stasis may be followed by a perivascular edema, which is in effect a favorable nutrient for infectious organisms. The blood supply of the buccal mucosa is easily evaluated by thermocouple temperature measurement. The amount of flow of blood in the buccal cavity is influenced by distant stimuli through the telereaction of Dastre-Morat, according to which, blood vessels in the head and brain react as skin vessels do rather than as deep vessels of the thorax and abdominal cavity.
The uniform room temperatures gained by central heating or air conditioning lead to a diminution in reactivity of the cutaneous organ as opposed to the hardening in outdoor life. We believe this accounts in part for the greater frequency of colds among people with industrial, or indoor jobs.
If people with a tendency to colds immerse their legs in cool water at 15C for one to two minutes, we note that, even while still in the bath, there is telereaction and the oral temperature drops from 0.5 to 2.0C or more. A slowing of the blood flow continues for several minutes in sensitive persons. If a hardened sportsman or those who have completed a hydrotreatment are exposed similarly, there is either no change in the buccal temperature or only a transient lowering of a few seconds.
Water can be applied in any of its three physical states ( steam, liquid, or solid) or in any combination of them. Because of its high specific heat and versatility, it is an excellent medium for conductive heating or cooling since it absorbs and gives off heat slowly. While an air environmental temperature of 27C/80F is comfortable for the nude body, a water environment of the same temperature is cool. For most people, the critical level of temperature sensation is about 34C/93F - the average temperature of the skin. Hot and cold are therefore relative terms.
Hot water (up to 40C/104F) has an initial stimulating effect, but, as the body recovers from the first response to immersion at this temperature, there is a general and muscular relaxation .
Cold water, on the other hand, may cause some shivering, goose flesh, increased pulse and respiration, dilation of blood vessels, increased muscle tone and metabolism; these are the responses of most healthy individuals. This may be called 'a tonic' stimulating reaction to cold as compared with the 'atonic' response to heat. The response to hot or cold water varies with the length of application. Cold may be invigorating when used for a short period but is damaging over a longer period. Even in local applications (ice water) the hand may be able to tolerate exposure for more than a minute or two. The skin finds prolonged exposure at 40C/104F uncomfortable for more than a few minutes; water at 50C/102F may destroy mucosa in a short application (vaginal mucosa is an exception) and skin in a few minutes.
Hydrotherapy is the external application of water for therapeutic purposes. The body or any of its parts may be immersed in the water or the water may be applied to the surface with or without the intermediary of absorbent materials. In prescribing hydrotherapy, it should be specified by type, temperature, duration and frequency.
In Part 2 Dr. Bergel's article will continue with a discussion on Hydrotherapy Types.
