anatomy of diseased structures and their mode of development came to be investigated, and the science of pathological histology, took its rise. Johannes Müller is regarded as the father of this branch of histology, as he indicated the general direction in which the investigation of diseased growths should be pursued. Afterward Virchow published his celebrated Cellular Pathology, and later the science was still further enriched by the labors of Billroth, Rindfleisch, Von Recklinghausen, Cohnheim, and others.

HISTOLOGY OF ANIMALS

Cells. The histological basis of the body structure is the cell. In general terms all tissues may be said to be composed of cells of one kind or another, and these cells are always combined with more or less intercellular substance. This intercellular substance may be very small in amount, as in the epithelial tissues, where it amounts to nothing more than a cementing material holding the cells together, or it may make up the greater part of the tissue, as in some forms of connective tissue. Cells differ in shape. They may be round, oval, cuboidal, spindle-shaped, or irregularly stellate. The intercellular substance differs greatly in structure, and it is upon the differences in density of the intercellular substance that the different degrees of hardness depend. Thus, in mucous tissue the intercellular substance is soft and gelatinous, in cartilage it is dense and firm, ín bone it is infiltrated with lime salts and is extremely hard. It was at first believed that a cell was a little bag filled with fluid; hence its name. Most animal cells are, however, small masses of living matter, called protoplasm, having, as a rule, no cell wall. Cells may or may not have nuclei. It is probable that nonnucleated cells are incapable of performing certain of the higher functions of cells, e.g., that of reproduction.

All adult tissues and organs originate in the elementary layers of the embryo. What determines the lines of growth of these different cells, and why some develop to form one kind of tissue, others to form other kinds of tissues, is as yet beyond our knowledge. There are two modes of cell growth or reproduction-direct cell division and indirect cell division, or mitosis.

Tissues. The tissues of the body fall into four great groups: (1) epithelial tissue; (2) connective tissue; (3) muscular tissue; (4) nervous tissue. It is by combinations of these tissues that the different organs of the body are formed. The most widely distributed of the tissues is connective tissue, which in its various forms, as fibrous tissue, elastic tissue, cartilage, bone, etc., makes up the framework of the body. In combination with one or more of the other tissues it forms the various organs of the body, acting as their supporting framework. Thus, in the nervous system such organs as the brain and cord consist of nervous tissue held together and supported by the peculiar form of connective tissue known as neuroglia. A muscle consists of muscle tissue bound together by connective tissue; and the various glands of the body, such as the liver or pancreas, consist of a glandular epithelium peculiar to the particular organ held together by connective tissue. See CONNECTIVE TISSUE.

Consult Delafield and Prudden, A Text Book of Pathology (9th ed., New York, 1911). The

histology of the different tissues and organs may be found described under such titles as CONNECTIVE TISSUE; MUSCLE; EPITHELIUM; NERVOUS SYSTEM; LIVER; KIDNEY; ETC. See also CELL; BLOOD.

HISTOLOGY OF PLANTS

Cells. Many plants consist of only a single cell; nevertheless such plants show almost infinite variety in form. The simplest are nearly spherical, but in the desmids and diatoms almost every conceivable shape is to be found, and a high degree of differentiation is attained. When cells divide in only one plane and do not separate after each division, a chain of cells results; when division takes place in two planes, a plate of cells is formed; divisions in three planes give rise to a body several cells in thickness. In the last case considerable differentiation is likely to follow; the cells on the outside become adapted to the work of protection and absorption, some of those on the inside perform the nutritive functions, and others are specially modified for conducting materials. Even in the algæ and fungi there is a division of laborsome cells being modified for the work of protection and absorption, others for conduction, still others for reproduction. In the liverworts and mosses the specialization is carried still further, but it is in the ferns and flowering plants that difference in structure and division of labor finds its highest expression. Such differentiation of cells in form and function gives rise to tissues.

Tissues. A tissue is a group of connected cells of like origin and structure. Tissues are therefore classified according to the form of their component cells, the thickness and chemical composition of the cell walls, the character of their contents, etc. All cells of a very young plant are alike in having thin walls, abundant protoplasm, minute vacuoles or none, and relatively large nuclei. Such cells constitute embryonic, formative, or meristematic tissue, which is found in the higher plants in three places: (1) at the tips of the shoots and roots, where it constitutes the "growing points," protected in the shoot by overarching leaves which form a bud, and in the root by the root cap; (2) one or more thin layers concentric with the stem, by which new layers of wood, cork, and bast are added to it internally; (3) where wounds are made, in which cases the meristem produces tissues that heal the wound.

In all the higher plants tissues are grouped in such a way as to form tissue systems. (See MORPHOLOGY IN PLANTS.) Those there described are reduced to three by some: viz., (1) the tegumentary (equivalent to the protective); (2) the vascular (equivalent to the conducting and in part the mechanical); (3) the fundamental, a sort of limbo to which are assigned all the tissues not included in the other two systems. Of all tissues it may be remarked that the cell walls are subject to thickening as they grow older. This thickening consists of material added to the surface of the primary wall. The successive additions are frequently unlike and in the mature wall are apt to be distinctly stratified. The thickening is seldom uniform. Sometimes minute regions escape thickening and remain as small pits in the added layers. These pits occur at points where the protoplasm of one cell has not been com

4. Trachea and tracheids. Tracheids are usually elongated cells, whose walls have become lignified (by which they are made very

1, longitudinal section; 2, transverse section. plasm disappears at maturity, and the tissue is of use to the plant only by its mechanical strength. The short-celled sclerenchyma is common in the stone fruits (peach and cherry), in the shell of various nuts, in the gritty parts of the flesh of pears, quinces, etc., and in the hard portions of bark, many dry fruits, and seeds. Elongated sclerenchyma cells are most abundant in stems and leaves, in which they form continuous strands or bands, closely associated with the vascular bundles. They are often called bast fibres, but do not always belong to the bast or phloem bundles. The individual cells taper

1, irregular form (idioblast) from leaf of tea; 2, surface view of a fibre showing tapering end and oblique pits; 3, transverse section of a fibre showing concentric structure of wall with pore pits.

at each end, the ends of adjacent cells above and below overlap, and thus give to the strands great flexibility and a tensile strength which in many cases exceeds that of the best steel. The cells are relatively very long, 1 to 4 millimeters in jute, 10 millimeters or more in hemp, 20 to

FIG. 7. TRACHEIDS.

a, from stem of pine; b, forming the termination of a xylem bundle in a leaf of Impatiens parviflora. pervious to water) and thickened in spiral or At maturity the protoplasm disappears, leaving annular lines or in reticulate patterns (Fig. 1). only the cell wall of service to the plants. Trachea are similar to tracheids in the sculpturing of their walls, but instead of being single cells they are formed by the fusion of a row of cells lying originally end to end, the end walls being resorbed as the lateral walls thicken and the protoplasm disappears. At maturity the long empty tubes thus formed show little trace of the cells from which they originated. angiosperms they constitute the greater part of the xylem bundles, changing to tracheids as the bundles grow smaller and come to an end (Fig. 7, b). But in gymnosperms (pines and their allies) trachea are formed only in the primary xylem, almost all the secondary xylem being tracheids with characteristic circular-bordered pits (Fig. 7, a). Trachea and tracheids are the most efficient tissues for the transport of water in the larger plants.

In

5. Sieve tubes are cell fusions formed by the partial resorption of the end walls of a row of young cells. The end partitions and sometimes the lateral walls which adjoin other sieve tubes become perforated, forming a so-called "sieve plate," through which the contents of the sieve tubes (a slimy mixture of soluble proteids, carbohydrates, and other foods) pass freely. Sieve tubes are found in the phloem bundles, in which they constitute the most efficient tissue for the transport of foods (Fig. 8). See SIEVE VESSELS.

6. Latex tubes are long, much-branched tubes,

with free or anastomosing branches, which contain the milky or colored sap in certain plants. There are two sorts, articulated and nonarticulated. A nonarticulated tube arises by early differentiation of certain cells in the embryo, which push their way among the other develop

2

FIG. 8. SIEVE TUBE AND COMPANION CELL.

The first step is fixation. By this is meant a rapid killing of the tissue in such a way as to allow it to retain as nearly as possible the same form and relation as does the living tissue. Among the more common fixatives may be mentioned alcohol of various strengths, formalin in from 5 to 20 per cent solutions, osmic acid, usually in 1 per cent solution, Müller's fluid (2.5 grams of potassium bichromate and 1 gram of sodium sulphate, dissolved in 100 cubic centimeters of water), Fleming's fluid (a mixture of osmic, chromic, and acetic acids in water), chromacetic acid (1 per cent solution in water), and corrosive sublimate in saturated aqueous solution. Many other fixatives are used for special purposes; thus, osmic acid is employed for the demonstration of fat and myelin. The small slices of tissue employed are allowed to remain from 12 to 48 hours in a large amount of fixative. Tissues which have been fixed by any other agent than alcohol are then subjected to prolonged washing with running water. The next step is hardening. Fixing solutions are also hardening agents if allowed to act sufficiently long. Many fixatives, however, have a detrimental effect upon the tissues if their action is too prolonged. It is, therefore, quite common to transfer the tissues after proper fixation to some new fluid for the purpose of

ing tissues by independent growth. The articulated tubes arise by the fusion of rather indefinite rows of cells, either longitudinal or transverse, which form a network of irregular tubes. Latex tubes are found in the phloem bundles, or just outside these bundles in the cortex of stems, and accompany them into the leaves, where they ramify widely. Their terminals come into close relation with the nutritive cells. (See Illustration under LATEX.) They seem to constitute a transportation system for foods. See GROWTH, IN PLANTS; ANATOMY OF PLANTS; CELL, IN PLANTS; ROOT; STEM; CONDUCTION; LATEX.

HISTOLOGICAL TECHNIQUE

The methods of histological observation were at first extremely crude, consisting merely in tearing apart the tissues and examining them under the microscope. Such handling, of course, largely destroyed the relations of the different elements of the tissues to one another. The cutting of thin sections of tissue with a razor was soon introduced, this being much facilitated by the previous hardening of the tissues in some suitable solution. Such sections were, however, difficult to study, owing to their transparency and the fact that the different tissue elements possess nearly the same index of refraction. The staining of sections was the next great improvement in technique. At first a single stain was used, the denser elements taking a darker shade than those of less dense structure. The discovery of what is known as differential staining, whereby the different tissue elements are stained different colors, and the introduction of an instrument for section cutting, known as the microtome (see further below), together with improvements in the microscope, have been the main factors in the recent rapid development of the art. At present the most commonly used technical procedure in the examination of tissues and organs is as follows: VOL. XI.-22

versal hardening and preserving agent is alcohol, but formalin, too, in from 5 to 10 per cent aqueous solution, is now extensively used. The first alcohol bath for plant tissues should be 35 per cent or weaker, while many animal tissues will stand 50 per cent. This should be followed consecutively by 50, 70, 90, and 97 per cent alcohol. For long preservation 80 per cent alcohol is the most satisfactory. Next in the process is embedding. By this is meant the impregnation of the tissues with a liquid which afterward hardens, thus holding the tissues in a firm mass, which can be easily cut. For this purpose paraffin and celloidin are most commonly used. In paraffin embedding the tissue is first immersed in any pure solvent of paraffin, then passed to a warm solution of paraffin in the solvent, and finally left in pure melted paraffin until thoroughly impregnated. In celloidin embedding the tissue is transferred from alcohol to a mixture of alcohol and ether and then placed in a solution of celloidin in a mixture of equal parts of alcohol and ether. After impregnation the paraffin is allowed to harden by cooling, or the celloidin to thicken by exposure to the air and consequent evaporation of the alcohol and ether, after which it is immersed in chloroform for hardening. The operator then proceeds to cut sections by means of the microtome. This instrument consists essentially of a knife carrier, which can be made to slide back and forth past a clamp to which the specimen is attached. The embedded specimen is fastened to a block, usually of wood, clamped in the microtome. The clamp is so arranged that the blocked specimen can be raised any desired fraction of a millimeter, thus bringing any thickness of it above the knife. In paraffin cutting the knife is kept dry; in celloidin section cutting it is kept flooded with alcohol. The sections are then stained for the purpose of bringing out sharply the different tissue elements. For staining the nuclei, carmine, hæmatoxylin, and various aniline dyes are commonly used. For demonstrating the other tissue ele

ments other dyes may be used, eosin being much employed. The procedure in staining celloidin sections with hæmatoxylin and eosin is as follows: The sections are first allowed to remain for several minutes in an aqueous solution of hæmatoxylin; then they are thoroughly washed with 97 per cent alcohol and placed for several minutes in an alcoholic solution of eosin; they are then again washed in alcohol and cleared in oil of origanum or bergamot containing a little eosin. From the clearing bath the specimen is lifted to a glass slide, the excess of oil is removed by means of blotting paper, a drop of a solution of Canada balsam is placed upon the specimen, and the whole is covered with a thin glass, called the cover glass. By the drying and hardening of the balsam a permanent "mount" of the specimen is secured. The above methods of procedure are illustrative of those applicable to general histological material. The examination of special tissues and organs requires the use of special methods of technique. This is especially true in regard to the nervous tissues, for the study of which some very elaborate methods have been devised, some of which will be found described in the article NERVOUS SYSTEM.

For histological methods as applied to plant tissues, consult C. J. Chamberlain, Methods in Plant Histology (2d ed., Chicago, 1905), and Eduard Strasburger, Handbook of Practical Botany (7th ed., New York, 1911); for methods in animal histology, consult A. B. Lee, Microtomist's Vademecum (6th ed., Philadelphia, 1905), and F. R. Bailey, Text-Book of Histology (4th ed., New York, 1913).

HISTO'NIUM. See FRENTANI; VASTO. HISTORIA AUGUSTA. See AUGUSTAN HISTORY.

HISTORICAL ASSOCIATION, AMERICAN. A society of historical students and writers, founded at Saratoga, N. Y., in September, 1884, at the suggestion of Herbert B. Adams, of Johns Hopkins University. Its original membership was 40, but within a year it had increased to 250, and in 1889 it received a definite standing and recognition by an Act of Congress incorporating it in the District of Columbia. The society has exerted wide influence in directing and stimulating historical research, and its publications and monographs have covered a broad field of historical study. The American Society of Church History, founded in March, 1888, became in 1896 the Church History Section of the American Historical Association, but regained its independence in 1904. Important committees of the society are the Historical Manuscripts Commission, which prepares valuable manuscripts for publication, and the Public Archives Commission, concerned in the preservation of public records. The society holds annual meetings, publishes an annual report through the Smithsonian Institution, and appoints a board of editors for the American Historical Review, published quarterly. The society has published five volumes of Papers, some 40 volumes of Annual Reports, a series of prize essays, two volumes on The Study of His tory in Schools, secondary and elementary, and a series of reprints of Original Narratives of American History in 20 volumes.

HISTORIC GEOLOGY. See GEOLOGY. HISTORY (Lat. historia, Gk. loropía, history, from forwp, histor, learned, from eidévai, eidenai, Skt. vid, OHG. wizzan, Ger. wissen, to

know). A systematic narrative of past events, or, in the light of modern historical scholarship, the science of the progressive development of human society. The social and economic conditions of peoples, their racial affinities and physical environment, exercise determining influences upon their history and find expression in their thought, their art, and their politics. History deals with the social structure in its successive forms and recognizes as inseparable allies all sciences which contribute to a knowledge of man as a social being and in his relation with the physical world.

As a science, history is primarily inductive. It proceeds from a body of concrete facts, which critical study links together according to the sequences of time and causation. Afterward deductive processes may be used, but always sparingly and cautiously.

Historical method comprises four processes: the collation of facts; the arrangement of these facts according to the sequences of time and causation; criticism, by which the value of the facts is determined; and their interpretation in accordance with the results of arrangement and criticism. These processes are all simple; they are the ordinary processes of scientific research, but in carrying them out the human equation becomes so large an element of the problem as to make it essentially different from the problems of the physical world, and for this reason history can never be an exact science.

History depends upon human evidence, and its investigation must follow the laws governing the reception of human evidence. These are found to a considerable extent in the body of principles developed by jurisprudence for the reception of evidence in the courts. The historical material is contained in several categories: (a) Remains, such as buildings, walls, roads, statues, pictures, medals, coins, implements-whatever, indeed, man has made and used and which may thus throw light upon his civilization and his deeds. These may be studied directly, when accessible, or through the reproductions easily obtainable by means of modern processes.

(b) Documents. Under this head are included official and business papers and letters written with an immediate practical purpose.

(c) Literatures. This class includes a great body of material of the highest value the writings through which are expressed the ideas of the peoples, their philosophy, poetry, science, and religion.

(d) Traditions. Much of this class is preserved in the literatures; much of it must be gathered from other sources.

(e) Laws. These, especially public law, are found in codes and treatises and are of great value in determining many questions.

(f) Contemporary writings with historical purpose annals, chronicles, biographies. Under these heads can be classified the original material on which secondary historical work, the written history of a nation or an age, is based and by examination of which its accuracy must be tested.

This material may also be divided into two great classes of evidence-conscious and unconscious. It will be seen that some of this material must have been prepared consciously to influence the opinion of contemporary or succeeding generations. In this class are proclamations, statements, writings, narratives, told with intention concerning events within the narrator's own knowledge, or reported to him by others.