oblongata and pons. The changes consist in a varying degree of inflammation, marked by small round-cell infiltration of the blood-vessel walls, exudation into the pericellular lymph spaces, small hemorrhages, and sometimes thrombosis of the small blood vessels. More recently microscopical study has demonstrated extensive degenerative changes in the nerve cells. Lesions have also been noted in the sympathetic, consisting in degeneration of the nerve cells and increase in the thickness of their endothelial capsules. In 1903 Negri announced the discovery of certain bodies in the nerve cells of rabid animals. These, the so-called Negri bodies, are found very constantly and are described as consisting of a homogeneous, nongranular substance, resembling coagulated albumin. They are from 1 to 23 micromillimeters in diameter, oval, round, or triangular in shape, and within them are clear shining areas. The nature of these bodies is unknown. Negri believes them to be protozoa and considers them the causative agents of the disease. Williams, in 1906, was convinced that these cell inclusions were of animal nature and called them Neuroryctes hydrophobia. All attempts to cultivate this alleged organism, however, were unsuccessful, and the late "complement fixation test" is likewise negative. In addition to the changes in the nervous system there is usually congestion of the mucous membrane of the gastrointestinal tract and of the pharynx, larynx, and bronchi. Despite the fact that innumerable attempts to discover the cause of the disease have been made without success, it still seems probable that hydrophobia is due to an ultramicroscopic microorganism. This organism is not apparently widely distributed throughout the body, but confined mainly to the saliva and the central nervous system. An emulsion made of the medulla of a rabid animal, injected into dogs, cats, rabbits, guinea pigs, etc., produces symptoms characteristic of the disease, although in rodents there is little or none of the stage of excitement.

Although we have no knowledge as to the specific germ of the disease, hydrophobia furnishes our most remarkable example of the success of artificial immunization by means of protective inoculation. To Pasteur (q.v.) is due the credit not only of the discovery of the preventive treatment of hydrophobia, but of demonstrating through it a principle in thera. peutics which is of constantly widening applica tion. Pasteur found that he could induce the disease in rabbits by inoculations with portions of the spinal cords of rabid animals, and that the spinal cords of these rabbits possessed a high degree of virulence. Drying in air reduced the virulence in direct proportion to the length of the drying. It was found that, while inoculation of man or animals from the fresh rabbit cords was invariably fatal, if the man or animal was first inoculated from one of the cords the virulence of which had been greatly reduced by the drying, and then from cords of gradually increasing virulence, he could become so accustomed to the virus that injection of the fresh cord would no longer be fatal, and such a series of inoculations made sufficiently soon after the bite of a rabid animal was found to prevent entirely the development of the hydrophobia. Advantage is taken of the long period usually elapsing between the bite and the onset of the disease to practice these preventive inoculations, and the result has been a marked

decrease in the mortality from the bites of rabid animals. In view of the uniformly fatal results of untreated hydrophobia and the success of the Pasteur treatment, the importance of determining at the earliest possible moment whether the animal by whom a person has been bitten had rabies can be readily appreciated. The animal should not be killed, for, as rabies is invariably fatal to canines, the recovery of a sick animal definitely disproves rabies. On the contrary, the animal should be carefully watched and, if it dies, should be sent to a laboratory where examination can be made and the question of rabies definitely settled by inoculation experiments on animals or microscopical examination of the brain and cord. The treatment should be cauterization in every case, less than one hour after the bite, by means of the actual cautery, strong acid, or acid nitrate of mercury. Wounds must be opened by the surgeon, and even amputation may be necessary. Washing and syringing wounds with water at 130° F. is desirable. Sucking the wound to draw out the poison has been practiced and may be safely done if there are no breaks of the membrane of the lips or mouth. A method recommended by Keirle is as follows. The wound is washed with bichloride solution and dried; a strong solution of carbolic acid is then applied to the depths of the wound or wounds with a suitable applicator; this is followed immediately by pure nitric acid, and this again by alcohol. A surgical dressing is then applied. Under thorough cauterization rabies develops in 33 per cent of cases bitten; without cauterization, in 83 per cent. Administration of morphine or alcohol does harm. Immunizing by the Pasteur method should be practiced in all cases. By this method the patient is inoculated with attenuated virus by the injection hypodermically of emulsion made from the brain of a rabid animal repeated in stronger and stronger concentration during 21 days. The results of the Pasteur method are now indisputable, but it is of the utmost importance to begin the treatment as soon as possible after the injury, as the prospect of success grows less and less day by day of delay. Following are the results of the antirabic treatment at the Institut Pasteur, Paris, from 1886 to 1908 inclusive: 31,330 cases were treated with 238 deaths, a mortality rate of 0.759 per cent. During treatment 50 deaths occurred, or 0.159 per cent; within a short time after treatment 188 died, or 0.601 per cent. Reinlinger found a mortality rate of 0.41 per cent in 131,579 cases. The treatment is not entirely harmless. Besides the local and constitutional reaction there sometimes occurs paralysis or paraplegias, which occasionally prove fatal. Pasteur institutes are distributed well over the civilized world, most large cities having one. In New York City the virus is now made and distributed by the department of health. The New York State Department of Health was the first to send out treatment packets by mail. The practice of State treatment and distribution, as well as the examination of dogs, is now well organized throughout the United States.

Pseudorabies, or Lyssophobia, is a hysterical condition, in which the patient, who imagines that he has rabies, in his morbid condition of mind enacts some of the symptoms and may even be frightened to death.

Bibliography. Youatt, On Canine Madness (London, 1830); Pasteur, in Comptes rendus de

l'Académie des Sciences (Paris, 1881 and 1889) and in Annales de l'Institut Pasteur (ib., 188788); Keirle, Studies in Rabies (Baltimore, 1909); Kerr and Stimson, The Prevalence of Rabies in the United States (Public Health Bulletin, No. 29, ib., 1909); Stimson, Facts and Problems of Rabies, in Hygiene Laboratory Bulletin, No. 65 (Washington, 1910); Babes, Traité de la rage (Paris, 1912).

HY'DROPHYTES (from Gk. towp, hydōr, water + OUTóv, phyton, plant). Plants which grow naturally either in water or in very wet soil. This term is contrasted with "mesophytes" and "xerophytes" (qq.v.). Common illustrations of hydrophytic plant societies are swamps of all kinds, pond societies, and ocean plants. Various classes of hydrophytes are taken up under separate heads, where the main features of the various hydrophytic societies will be discussed. It will be desirable, however, to give a short account of the characteristic hydrophytic structures. The roots of hydrophytic plants are in most cases very greatly reduced and in some cases altogether lost, as in some of the duckweeds. Root hairs are also commonly lacking in water plants. The stems and leaves are not, as a whole, conspicuously reduced in water plants, but they show peculiarities of structure that are quite interesting. The leaves of hydrophytes are frequently finely divided, as in the water milfoil and mermaid weed. In many cases where the leaves are not finely divided, they are very thin, consisting of one or two layers of cells only; e.g., in tape grass. An examination of the structure of the leaves shows the entire absence of stomata in the submerged parts, an absence of air spaces in most of the ribbon-like forms, complete or almost complete absence of palisade cells, and a very thin epidermis which contains chlorophyll. In contrast with submerged leaves those floating upon the surface of the water usually have well-developed palisade cells, many stomata upon the upper side, and an abundance of intercellular air spaces. The stems suffer a noteworthy reduction in the size and development of the water-conducting vessels and mechanical tissues and a great increase in air spaces. The structures just described are found in their highest development in submerged water plants. Hydrophytes whose leaves are aërial show no conspicuous differences from ordinary land plants in most respects. One class of hydrophytes, which may perhaps be called amphibious plants, shows some remarkable variations, especially in structure. Leaves which are developed under the water show the characteristic structures outlined above, including the fine leaf division, whereas leaves of the same plant developed in the air show typical aërial leaves without these divisions and with palisade cells, stomata, and a thick cuticle. The stimulus or stimuli which cause these wide variations are not certainly known, but they are discussed to some extent in the article LEAF. Common American plants, which show variations to a high degree, are the aquatic buttercups, the mermaid weed, some of the cresses, and water hemlocks.

However the hydrophytic structures that have been described in the preceding paragraph may have arisen, certain advantages can be clearly seen, at least in some cases. The thin walls of the epidermis, which are in striking contrast to the thick cutinized walls of many aërial leaves, permit the easy entrance of water and substances dissolved in the water. On this account many

submerged plants are practically independent of soil relations; they take in most of their material directly from the water. It is easily possible to grow cultures of many of these plants without having any root connection whatsoever. While certain forms, such as the water milfoil and the water weed (Elodea), develop roots in ordinary aquarium cultures, other forms, such as hornwort (Ceratophyllum), never develop roots and yet grow quite as vigorously as in their natural rooted condition. A few forms, such as the bladderwort (Utricularia) and some of the duckweeds, have no roots in nature. Of course, in such cases the entire absorption of nutriment must take place through the leaf epidermis. The fact that the leaf epidermis contains chlorophyll is also a matter of advantage, since water very soon destroys the efficiency of rays of light. At a comparatively shallow depth there is a cessation of the development of green plants. A reduction in the water-conducting tissues, while not necessarily an advantage, is, nevertheless, not harmful, inasmuch as the absorption is so largely through the leaf, instead of the root as in land plants. The reduction in root development is not so easy to understand, since it would seem that holdfast organs would ordinarily be of advantage; then, again, any absorption which the roots might make would so much the more increase the capacity of the plant. A high development of air cavities is a distinct advantage, not only to help float the plant, but probably to a much higher degree to act as a sort of air storage. It can readily be seen that the conditions for obtaining air underneath the water are not of the best, and that any additional means for obtaining or for storing air would increase the plant's efficiency. The reduction in the development of mechanical tissues, of palisade cells, and of stomata is not necessarily an advantage to water plants; but since these tissues are not actually needed, the plant loses nothing by its failure to develop these structures. The peculiar leaf forms that have been noted above are, perhaps, not necessarily of any exceptional advantage. It must not be supposed that everything in a plant can be explained in accordance with the need of the plant. It is much more likely that the explanations should be referred to definite chemical and physical causes. However, in the case of finely divided leaves, it can be seen that a much larger proportion of cells comes in contact with the material than is true with the more compact air leaves. Thus, the absorption capacity of the leaf is increased. Finely divided leaves are also doubtless more able to escape the dangers coming from currents of water than leaves which are more compact.

The hydrophytic plant societies are essentially all edaphic, i.e., they are conditioned by local causes. In this respect there is a wide contrast as compared with the xerophytic plant societies. Perhaps some of the ocean formations may be conditioned by climatic causes to some extent, but the ordinary hydrophytic societies of swamps and ponds are due to essentially local conditions. Perhaps no plants have such a wide distribution as certain of the hydrophytes. This is particularly true of ocean plants, where it can easily be accounted for by the almost universal distribution of the oceans themselves. It is true, to a striking degree, as well of the pond and swamp plants. Such plants as the pond weeds, cat-tails, and bulrushes are found almost throughout the world where the habitats are favorable. Perhaps

the chief reason for the wide distribution of hydrophytic species is the great ease of dispersal by means of the water itself; but a reason, almost if not quite as important, is furnished by the wide degree of uniformity of hydrophytic conditions. Since water is colder in summer and warmer in winter than adjoining portions of the land, it is obvious that water plants can, for reasons of temperature, have a much wider distribution than land plants.

The hydrophytic plant societies may be roughly subdivided into those associated with salt water and those with fresh water. The former are treated under the heads PLANKTON, BENTHOS, MANGROVE SWAMP, and HALOPHYTE; the latter under the head of SWAMP, where it will be convenient, not only to treat the swamps proper, but also to some extent the development of the swamps from ponds and lakes. See AERATION; DISTRIBUTION OF PLANTS.

HY'DROPLANE. See MOTOR BOATING. HYDROSPHERE. See GEOGRAPHY. HYDROSTATIC PRESS. See HYDRAULIC

PRESS.

HYDROSTATICS (from Gk. dwp, hydōr, waterσTATIKós, statikos, causing to stand, from lorával, histanai, to stand). That branch of mechanics which treats of the properties of liquids in equilibrium and of solids either totally or in part immersed in liquids. Many of the laws and phenomena of hydrostatics apply equally well to both liquids and gases, i.e., to fluids. A fluid may be defined to be such a form of matter that it yields to any force, however small, which acts to make one layer of the substance move over another; thus, the shape of a liquid or gas depends entirely on the forces acting on it, however small, and not on the body itself, as in the case of a solid. A portion of liquid left to itself as a falling drop-assumes a spherical shape owing to the contraction of the surface layer. (See CAPILLARITY.) Under the action of gravity a liquid contained in an open vessel takes the shape of the vessel so far as all the surface is concerned, except that portion in contact with the air and the portions near the edges of this "free surface." This surface is horizontal, being perpendicular to the vertical force of gravity if the liquid is at rest; because, if it were inclined to this, there would be a component of gravity tending to make the higher portion of the liquid slide down. When a fluid is said to be at rest, it is not implied that there is no motion of the molecules, but simply that there is no flowing, i.e., no currents, no wind. In the case of the open vessel there is a force pressing down on the free surface due to the weight of the atmosphere, and, since the liquid presses against the solid walls, they have a force of reaction against the liquid; thus, it is exactly as if the liquid were contained in a vessel and a tight-fitting piston were pressing down on its top surface. If a gas is contained in a balloon or in a room, it expands and is uniformly distributed throughout the space open to it; it presses against the containing walls, and they have an equal reaction on the gas. If a small quantity of a certain liquid is poured into a tall cylindrical vessel, then another liquid with which the first does not mix is poured carefully on top of this, etc.; the equilibrium-if there is anywill be stable only if the density of any one liquid is less than that of the liquid below it and greater than that of the one above it. For, if the density of any layer is greater than that of

the one below it, the potential energy of the two will be decreased if the heavier liquid gets to the bottom and so comes closer to the earth.

Fluid Pressure. As a result of the reaction

of the containing walls on a liquid or a gas, there is always a "pressure" at each point throughout the fluid, i.e., there is a force acting over any surface immersed in the fluid; the numerical value of the pressure over any area is by definition the force per square centimeter, and the "pressure at a point" is the limiting value of the force acting on any surface at that point divided by the area of the surface, as the area is supposed to be taken smaller and smaller until it becomes practically a point. This pressure due to the reaction of the walls is the same for all points in the fluid. There is also an additional pressure at each point of a fluid on the surface of the earth owing to the fact that any horizontal plane passing through that point has to support the weight of the column of fluid vertically above it. If the area of this plane is A; the vertical height above it to the top of the fluid, h; the average density of the fluid, p; the acceleration due to gravity of a falling body g; the upward force will be phgA, and therefore the pressure is pgh. These two pressures are the only ones which affect our senses or produce mechanical effects in general; but there is also, of course, at any point in a liquid what may be called "cohesion," or pressure due to the action of the molecules on each other. Some idea of the magnitude of this may be obtained by separating the molecules, e.g., by boiling the liquid. It is greatly affected by dissolving substances in the liquid. The pressure against any surface immersed in a fluid at rest is always at right angles to it, otherwise there would be produced a flowing owing to the component of the pressure along the surface. Further, the pressure at any point in a fluid at rest is the same in all directions, because, if it were greater in one direction than in another, the fluid would flow. Therefore the pressure at any point in a fluid at rest is the sum of the pressure due to the reaction of the walls, P, and that due to gravity, pgh. As noted above, the former is the same for all points in the fluid. As a consequence, if a fluid is inclosed in a cylinder into which fit two pistons of different areas, A, and A2, the forces which must be applied to these pistons from without to prevent the fluid from pressing them outward are PA, and PA, omitting any action of gravity. Therefore, if A, is small compared with A, the force on the former piston is small compared with the balancing force on the latter; so a small force may produce a large one. In a liquid which is almost incompressible, P may be very great, and so the force produced may be enormous; but in a gas, which is easily compressed, P is never very large, and so the force produced is small. This principle is that of the hydrostatic press. See HYDRAULIC PRESS.

The total pressure, i.e., P + pgh, at all points in the same horizontal level in any one fluid, regardless of the shape or size of the containing vessel, is the same; for imagine a vessel with a horizontal bottom, all points of the fluid along this must have the same pressure, otherwise the fluid would flow; the pressure at any point at a level h centimeters above this bottom plane is less than that at the bottom by an amount pghthe same for all points in the plane. If, now, portions of the vessel are imagined removed so as to leave a vessel of any shape, the pressure at