may be used either internally or externally. Internally it may be used simply to wash out a cavity-the nose, the mouth, the stomach, the bladder, or the rectum. In addition, it may be taken internally hot or cold and passed into the blood. It may be used hot in the rectum, as a simple enema, or for purposes of stimulation. Externally, water may be used in many conceivable ways, but essentially the use of water means with it the use of two very necessary aids, heat and cold. Hydrotherapy, apart from the use of heat and cold, is of secondary importance. The action of water in the treatment of disease is therefore largely mechanical. The psychical side is not unimportant. It has been used by enlightened physicians ever since the dawn of medicine, and its effects taught in all medical schools. Only the quack and the charlatan will claim that hydrotherapy is a cure-all. Eccentricities, like the Kneipp cure and others, are to be avoided by the sensible. Water acts physically and mechanically. It is capable of readily taking up heat and of giving it up; it thus lends itself most readily to the use of thermal agents. It may be used in gaseous form (steam), liquid form, or solid form (ice). The action of heat and cold is essentially important. Smooth muscle-fibre expands under the action of moderate heat and contracts under the influence of cold. Its contracting power may be destroyed by an excess of either. Cold and heat, therefore, act as irritants to the nervous system, and through their agency we can act upon all of the organs, to stimulate or depress their activities. The heart, lungs, kidneys, liver, spleen, skin, etc., may all be influenced; the body heat and the output of moisture and secretions may be regulated. Thus by the use of heat or cold to certain portions of the skin or internally, almost every organ in the body can be influenced through its blood-vessels, and the skilled physician can, by guiding and directing these effects, bring about changes in vascular states; can remove excess of blood from one part of the body and bring it to another; can impart tone to a flagging organ, as the heart, or to the muscles or to the spinal cord; can increase the secretions from organs, such as the skin or the kidneys, and thus assist in excreting poisonous products from the body.

The applications of water to disease are well systematized by Baruch as ablution, affusion, sheet bath, drip sheet, compresses, wet pack, tub bath, and douche.

ABLUTION. Simple application of water by the hand or moist cloth over the body. In fevers the abdomen, the back, the chest, the lower extremities as far as the knees are bathed successively every two or three hours. A temperature of 75° F. should be used at first, and gradually reduced to 60° F. Chilling is to be avoided. Reaction is to be gained, the superficial capillaries becoming suffused and the body assuming a pinkish hue. Rubbing should always be used in ablutions.

AFFUSION. The patient, with a cold wet cloth about the head, sits or stands in a tub containing about a foot of water at 100° F. A broad stream of water from a bucket or pitcher is poured with force directly over the body. The stimulation Awill depend upon the temperature of the water and its force of delivery. This is often used in states of profound prostration, in coma, and in stuporous, delirious states.

SHEET BATH. A rubber sheet is laid down over one-half of the bed. This is covered by a blanket. Then a linen sheet is dipped in water at a temperature of from 60 to 70° F., and while still wet is wrapped about the patient while he lies upon the blanket. The face is bathed in ice-water and the head covered with a wet towel. The nurse or attendant rubs the patient energetically over the body. As the body reacts and becomes warm, water at still lower temperatures may be sprayed over the sheet. Afterwards the patient may lie wrapped up in the blanket. This is an excellent method for reducing temperature in fevers, and is applicable in practically all acute diseases accompanied by high temperature.

DRIP SHEET. The patient, standing in a tub of water at 100° F., has a wet sheet, wrung out, wrapped about him. He is then firmly rubbed until a reaction occurs. He may be slapped gently instead of being rubbed. This procedure forms an excellent tonic in nervous exhaustion, in the early stages of tuberculosis, and in chlorosis or anæmia.

COMPRESSES. These consist of bands or squares of old linen to which flannel is attached and strings made to fasten the appliance about a part. The linen piece is usually wrung out of hot or cold water and applied to the part and wrapped about by the flannel to prevent evaporation. These compresses are valuable in sore throat, tonsilitis, rheumatism, pneumonia, sprains, etc. They should be renewed hourly in acute processes.

WET PACK. This is similar to the sheet bath, save that the sheet covering the body is wrung dry and then the patient is rolled up completely in thick blankets. The air should be kept out by fixing the blanket very securely about the feet and neck. The wet pack is more useful in chronic cases. Friction is essential in the acute cases, in order to bring about quicker reaction; hence in these the sheet bath is usually preferred to the wet pack.

TUB BATH. A trained nurse should give this bath. See TYPHOID FEVER.

DOUCHE. This apparatus throws a jet of water against the body from a hose, and is best given in a special institution devised for this work. Arrangement is made in the apparatus whereby the force of the jet and its temperature can be regulated.

Consult Baruch, Principles and Practice of Hydrotherapy (New York, 1900). See BATH; TYPHOID FEVER.

HYDROTHORAX (Neo-Lat., from Gk. dwp, hydōr, water+pas, thorax, chest). The term applied to dropsical collections in the pleura (q.v.), a closed serous sac enveloping the lung on either side. When it exists to any extent, the pressure which it exerts on the lungs impedes the passage of the blood through them, and occasions difficulty of breathing, lividity of countenance, etc., and more or less dropsy in the face, ankles, etc., soon appears. The physical signs by which the disease can be detected are changes in the condition of the chest, as ascertained by auscultation, percussion, and succussion by the physician.

Hydrothorax may depend upon inflammation of the secreting membrane, or it may be a consequence of organic disease of the heart or lungs, liver, or kidneys. With regard to treatment, when the disease seems to depend upon inflammation

of the pleura, great advantage may often be derived from occasional cupping and repeated blistering, hydragogue catharsis, tapping of an ascitic abdomen, etc., according to the cause. HYDROTROPISM (from Gk. vdwp, hydōr, water+Tроnh, tropě, a turning, from Tpere,

FIG. 1. NEGATive hydrotroPISM.

Culture of the black mold on bread in a moist chamber. The hyphæ carrying the thread-like sporangia grown in such directions as to be as far as possible from the surfaces of bread and glass from which moisture is diffusing. Diagrammatic.

trepein, to turn). The sensitiveness of certain plant organs to the presence of unequal moisture,

3

FIG. 2. POSITIVE HYDROTROPISM.

Grains of corn, planted in wet sand in a funnel whose outer surface is covered with wet blotting-paper, have germinated. The roots, r, instead of growing straight down, as they would do in saturated air, have applied themselves to the wet paper. Hydrotropism under these conditions overpowers geotropism.

which causes them to curve toward the source of moisture. Among the fungi hydrotropism is

shown particularly by those filaments (hypha) which bear the sporangia. While most of the hyphæ ramify through the moist substance on which the fungus grows, the sporangial hyphæ rise at right angles to it. If they start from a salient angle on the substratum (e.g. the edge of a slice of bread), they may take the position shown at Fig. 1, growing equidistant from the two moist surfaces. If they arise from a reentrant angle, they place themselves likewise so as to be as far away from each moist surface as possible. The roots of higher plants are also sensitive to moisture, the stimulus of which may exceed that of gravity, as shown in Fig. 2. Here roots of corn planted in the heaped-up sand have come over the edge of the glass funnel, and have started to grow downward in the moist air. As they get farther from the surface of the wet paper covering the outside of the funnel, they find drier and drier air. A stream of water particles, however, is reaching them from one side. At a certain point the stimulus of the diffusing moisture overcomes the geotropism (q.v.) and the root, growing more rapidly on one side, is directed toward the moist surface. When it comes into the neighborhood of the moist paper again the sides are less unequally stimulated, the stimulus of gravity reasserts itself, and the tip again grows downward. The alternating prevalence of hydrotropism and geotropism is shown in the wavy course of the longer roots. It will be observed that the more rapid growth which directs the tip toward the moist surface is not due merely to the absorption of water, for it occurs on the drier side of the root. Hydrotropism is a form of chemotropism (q.v.).

HY'DROZO'A (Neo-Lat. nom. pl., from Gk. idop, hydōr, water+wov, zoon, animal). A class of Cœlenterata in which the simplest form of the body, as in the polyps, is a sac attached by its posterior end, and with a digestive cavity communicating with the exterior by a mouth only. The mouth is surrounded by hollow tentacles. The body is composed of two cellular layers, the ectoderm and endoderm, separated by a gelatinous, non-cellular 'mesogla.' The body is usually differentiated into two sorts of zooids, i.e. 'polyps' or nutritive zoöids, which are usually sexless, and 'medusæ,' or reproductive zoöids. The latter produce medusa buds, which on being set free are called medusæ, and are bisexual. In these medusæ the body is greatly more complicated than in the polyps, being bell or umbrella shaped, with a well-developed nervous system, composed of a thread-ring around the eye of the disk and with ganglia near the 'eyes' or sense organs. The digestive cavity is differentiated into a central and a peripheral portion, the latter forming radial and circular canals. The eggs and sperm are discharged externally, and fertilization of the egg takes place in the sea. Nettling organs (see NEMATOCYST) are usually present; they arise from some of the interstitial cells, and are most abundant in the tentacles. Many Hydrozoa (Hydra and Hydrac tinia excepted) exhibit the phenomenon of alternation of generations (q.v.). The asexual polyps give rise to buds which develop into medusa, and the latter lay the eggs from which are hatched the polyps. The medusæ are more or less phosphorescent. They abound in all seas, while Hydra is a minute fresh-water form. Very primitive

fresh-water forms are Protohydra and Microhydra, which are without tentacles.

The Hydrozoa are grouped in six orders: (1) Leptolina, having a fixed zoöphyte or polyp stage, and the sense organs exclusively ectodermal (Anthomedusæ and Leptomedusa); (2) Trachylina, having no fixed zoöphyte stage, all being locomotive medusa (Trachymedusa and Narcomedusæ); (3) Hydracorallina, coral-making polyps (Millepora and Stylaster); (4) Siphonophora, pelagic hydrozoans in polymorphic colonies; (5) Graptolithoidea, an extinct Paleozoic group, related to (6) the Hydromedusa. The Hydromedusæ are a very heterogeneous assemblage of polyp and medusa forms, arranged in several families. In some species we find only hydroids, in others only medusæ; but in many the hydroids and medusæ are alternate generations of a common stock.

Of the orders above mentioned, the Hydrocorallina and Graptolithoidea are of importance to the geologist, for they contain fossils of considerable interest. (See STROMATOPORA, and GRAPTOLITE.) Fossil representatives of the Scyphomedusæ are rare, but have been described from Cambrian, Jurassic, and other formations. Consult: Parker and Haswell, Text-Book of Zoology (London and New York, 1897); Huxley, The Oceanic Hydrozoa (London, 1859); Fowler, "The Hydromedusidæ and the Scyphomedusidæ," in Lankester's Treatise on Zoology, part ii. (London, 1900). See CORAL; CTENOPHORA; HYDRA; HYDROID; JELLYFISH; MEDUSE; POLYP; SIPHONOPHORA; etc.

HYENA, hi-e'nå (Lat. hyana, from Gk. vaiva, hyaina, hyena, from us, hys, hog, Lat. sus, AS., OHG. su, Ger. Sau, Eng. sow, Av. hu, pig; connected either with Skt. su, to bear, or with Skt. sukara, swine, su, maker, so called from the grunt). One of a family (Hyænida) of carnivorous animals remarkable for their generalized structure, which unites features characteristic of the cats, the civets, and the dogs. Their dentition is cal

Appa

DENTITION OF HYENA.

i, incisors; c, canine; p, premolars; m, molars.

culated for great tearing and crushing power, and includes six incisors and two canine teeth in each jaw, five molars on each side of the upper jaw, and four in the under. This dentition and other features place them between the dogs and the civets, with the latter of which they are connected by Proteles. (See AARD-WOLF.) They seize an object with so firm a hold that among Arabs they are proverbial for obstinacy. The vertebræ of the neck sometimes become anchylosed in old hyenas. In size they equal the largest dogs. The hind quarters are lower and weaker than the fore quarters of the body, so that hyenas move with a shambling gait. The body is covered with long coarse hair, forming a mane along the neck and back. The feet have each four toes. The claws are strong, fit for digging, and not retractile. The tail is short. Beneath the anus is a deep glandular pouch, contributing much to the offensive odor which is one of their

many disagreeable characteristics. Hyenas eat carrion as well as newly killed prey, and are of much use, like vultures, as scavengers, clearing away the last remnants of carcasses. They sometimes attack cattle, especially if they flee, but rarely man, though they sometimes seize children. Selous gives surprising instances of their boldness and strength in East Africa. During the day they hide themselves in caves, old rock tombs, and ruined edifices; by night they roam singly or in packs. They prowl about towns and villages, and often dig up corpses that have not been deeply buried. This, together with their aspect and manners, has caused them to be generally regarded with horror, and very exaggerated accounts of their fierceness have been prevalent. Instead of being untamable, as was long the popular belief, they are capable of being completely tamed, and show an attachment to man similar to that of the dog; they have even been used as watch-dogs. Hyenas are found only in Africa and in South-central Asia.

The most typical and at the same time familiar form is the striped hyena (Hyæna striata), which ranges from Abyssinia and the Libyan Desert eastward to India, where it is common throughout all the more open country, roaming widely at night in search of carrion, or living sheep and dogs. It is dirty gray, with narrow tawny or zebra-like stripes, a coarse mane along the back, and a bushy tail. It is cowardly, silent, and both hated and feared by the rural people. Two other species are exclusively African, and differ much from the striped, so that some naturalists put them in another genus (Crocuta). The spotted hyena (Hyæna crocuta) is larger than either the striped or the brown, and is yellowish, thickly spotted with black, with nose and feet dark. It inhabits nearly all Africa south of the Sahara, hunts in packs, is courageous, and really does most of the ravaging it is accused of. earthly coughing cry is one of the most terrifying of animal utterances. Owing to the peculiar arrangement of the reproductive organs in this species, it is extremely difficult, except by careful examination, to distinguish the sexes by external characters, and from this fact the old myth that hyenas are hermaphrodites has doubtless arisen.

Its un

Median in size stands the brown hyena (Hyæna brunnea), found on both sides of southern Africa near the coast, and often on the mountains. It is a less repulsive looking animal than the others; has a dog-like mantle of long brown hair which half conceals its barred legs, and becomes white on the sides of the head and chest, giving a quaint appearance to the front view, since the face itself is black, while the tall ears are gray.

The hyena family, of comparatively recent origin, appears to have evolved from the Viverridæ, through such intermediate genus as Ictitherium of the Lower Pliocene of Southern Europe. That Ictitherium ate bones in the same manner as do the modern hyenas has been proved by the nature of the coprolites found associated with their skeletons. True hyenas are common fossils in the Pliocene and Pleistocene deposits of Europe, during which periods they roamed in abundance as far north as France and England, as is exhibited by their remains in cave floors; and they are known in deposits of similar age in Asia. None have been found in America.