CELLS. The histological basis of the body structure is the (p11. In general terms all tis sues may he said to be composed 44 veils of one kind or another. and these cell: are always eombined with more or I,— int,•reelinlar sub stance. This intercellular substance may be very small in amount. as in the epithelial tissues, where it amounts to nothing more than a cement ing material holding the cells 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. enhoidal. spin dle-shaped, or irregularly stellate. The intercel lular sulistance differs greatly in structure. and it is upon the difforenees in density of the inter cellular substance that the different degrees of hardness depend. Thus, in mucous tissue the in tereenular substance is soft and gelatinous. in cartilage it is dense and firm, in bone it is in filtrated with lime salts, and is extremely hard. It was at first believed that a cell was a little hag tilled with fluid, hence its mine. Most ani mal cells are. however, small masses of living matter, called protoplasm. having as a rule no (ell-wall. cell, may or not have nuclei. It is probable that noll-illieleated cells are illea pahle of performing certain ot the higher funs-' Lions of cells. e.g. that of reproduction.
.111 adult tissues and organn originate in the elementary layers of the embryo. What deter mines the lilies of growth of these different cell:, and why some develop to form one kind of tissue, others to form other kinds of tissues, is as yet lwyonol our knowledge. 'Chore are two tuodes
of cell growth or reproduction, direct tell-divi sion, and indireet cell-division or mitosis.
Tisst.Es. The tissue: of the body fall into four great groups; I 1) Epithelial tissue; (2) eomiee tiNe tissue; (3) muscular tissue; (-1) 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 emu bination with one or inert- of the other tissues it forms the various organs of the body, acting as their supporting framework. Thus, in flue nervous system such organs as the brain and cord consist. of nervous tissue held together and sup ported by the peculiar form of eonneetive tissue kiumm as neuroglia. A muscle cr.nsists of inusele tissue bound together by connective tissue; and the various glands of the body, sileh as the liver or pancreas. consist of a glandular epithelium peculiar to the particular organ held together by connective tissue. See CONNEcTIVE TIssUE.
Consult Delatield and l'rudden. Handbook of l'ailiolonira 1 A na I omy and IliNtoloay (New York. 1901). The histology of the dit•erent tis sues and organs may be found described under such titles as CONNECTIVE TISSUE: NUsCLE; EPITIIF.LIUll ; NERVOUS ••;'N'S.TENI; LIN•ER; KIDNEY, etc. See also CELL; and 11.1.0ou.