Page:The New International Encyclopædia 1st ed. v. 10.djvu/118

HISTOLOGY. 102 HISTOLOGY. sfopic structure of plants ami nninials may be traced to the times of iMalpiglii (1028-94), who discovered the blood-corpuscles, Robert Hooke, who was the first ( Kiti" ) to describe plant cells, and Leeuwenhoek (1032-1723), who, with comparatively imperfect optical means, prac- tically laid the foundations of our knowledjte of the minute structure of the tissues, no delinile progress in histology as a science was made until the beninninj; of the nineteenth century, when the compound microscope began to assume its pres- ent improvcil form. The great work of Bichat, entitled Aiuitoniir gcncralv appliiinf d hi physio- loyie et A la mMccinc (1801), marked an epoch in the development of the science of histologj". Although he apparently did little actual work with the microscope, he brought to bear upon the achievements of other investigators the power of his generalizing mind, and was the first to clas- sify tissues .iccording to their structure. After Bichat came an epoch of histological research, during which the microscopic observations of Malpighi and Leeuwenhoek were extended in ac- cordance with the general system of Bichat. The next great step forward was made by two botanists. Hugo von ilolil and .Schleiden, who discovered the cellular basis of all plant struc- ture. The discovery of the metho<l of combining lenses so as to render them achromatic gave a new impulse to the study of histology, and a more perfect classification of the tissues of the body was the result, as it placed in the hands of Schwann an instrument which enabled him to demonstrate the law that all animal as well as ]>lant tissues are composed of and develop from cells. Schleiden had already demonstrated this, which may be called the greatest discovery in histology, and therefore he and Schwann are often called tiie founders of the science of histogenesis, or the study of the origin of tissues, more re- cently pursued with great success by Reichert, KoUiker, Remak, and others. Then the micro- scopic anatomy of diseased structures and their mode of development came to be investigated, and the science of pathological histology took its rise, tlohannes Jliiller 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. .Vfterwards Virchow published his celebrated Crlhilnr Pnthriloiji/, and later the science was still further enriched by the labors of Billroth. Rindlleiseh, Von Reckling- hausen, Cohnheim, and others. HISTOLOGY OF .NI>I.M,S. Cells. The histological basis of the body structure is the cell. In general terms all tis- sues may be said to be composed of cells of one kind or another, and these cells are always combined with more or less intercellular sub- stance. This intercellular substance may be very small in amount, as in the epithelial tissues, ■where it amounts to nothing more than n cement- ing 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. spin- dle-shaped, or irregularly stellate. The intercel- lular stibstance differs greatlv in structure, and it is upon the differences in density of the inter- cellular substance that the different degrees of hardness depend. Thus, in mucous tissue the in- tercellular suhstance is soft and gelatinous, in cartilage it is dense and firm, in bone it is in- filtrated with lime salts, and is extremely hanl. Jt was at first believed that a cell was a lillU- bag filled with tluid, hence its name. Slost ani- mal cells are, however, small masses of living matter, called protoplasm, having as a rule no cell-wall. Cells nuiy or may not liavr nucU'i. It is probable that non-nucleated cells are inca- pable of performing certain of the higher func- • lions of cells, e.g. that of reproduction. All adult tissues and organs originate in the elementary layers of the embryo. What deter- mines the lines of growth of these different cells, and why some develop to form one kind of tissue, others to fonn other kinds of tissues, is as yet beyond our knowledge. There are two modes of c-ell growth or reproduction, direct cell-divi- sion, and indirect cell-division or mitosis. TisscES. The tissues of the body fall into four great groups: (1) Kpithelial tissue: (2) connec- tive tissue: (3) nuiscular tissue: (4) nervous tissue. It is by conil)ina1ions of these tissues that the different organs of the body are formed. The most widely dislributed 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 com- bination 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 ncnous tissue held together and sup- I)ortcd 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 Delaficld and Prudden. Handbook of Palholoqirtil Analomy and Jlixloloqii (Xew York. li)01). The hi.stolog>- of the different tis- sues and organs may be found described under such titles as Connective Tissve; Miscle; Epitiiei.u'm : Xkrvois System; Liver; Kidney, etc. See also Cell: and Blood. IlISTOLOtlY OF plants. Cells. Many plants consist of only a single cell : nevertheless such plants show almost in- finite 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 eacli division a chain of cells results; when division takes place in two ])lancs a plate of cells is formed ; divisions in three ])lanes give rise to a body several cells in thickness. In the last ease considerable differentiation is likely to follow; the cells on the outside become adapted to the work of protection and absorption, some of thosp on the inside perform the nutritive functions, and others are specially modified for conducting ma- terials. Even in the alga- and fungi there is a division of labor, some cells being modified for the work of protection and absorption, others for con- duction, still others for reproduction. In the liverworts and mosses the specialization is car- ried 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