Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/897

Rh TEXTURES.] A N A T O M Y 843* hardness and strength, and the bones, of which it forms the essential texture, are organs of protection and support. But the study of the textures embraces an inquiry not only into the special, structural, and functional properties of each tissue, and organ into the special part which each plays in the animal economy but the consideration of their properties as living structures. It would be out of place in this article to enter into a discussion of the mean ing of the term LIFE, or LIVING, or to attempt an analysis of the various definitions of the term which have been suggested from time to time by different philosophers, which will naturally find a place in the article PHYSIOLOGY. It will suffice for our present purpose to adopt the old Aristotelian definition, and to speak of Life as the faculties of self -nourishment, self-growth, and self-decay. All the tissues, over and above the special properties which they possess, have the power of growing and of maintaining themselves in full structural perfection and functional activity for a given period of time. After a time they begin to exhibit signs of diminished perfection and activity, they degenerate or decay ; ultimately they die, and the entire organism of which they form the constituent parts is resolved by the putrefactive process into more simple forms of matter. GENERAL CONSIDERATIONS ON CELLS. The simplest form of organic matter capable of ex hibiting the phenomena of life is called Cyto-blastenia or Protoplasm. It possesses a viscous or jelly-like con sistency. Under the highest powers of the microscope it seems to be homogeneous, or dimly granulated, like a sheet of ground glass. Not only can it assimilate nutri ment and increase in size, but it possesses the power of spontaneous movement and contractility. It enters in a very important manner into the structure of the bodies of the lower animals. The elongated processes, or pseudo- podia, to which Dujardin applied the name of sarcode, which the llhizopoda can project from their surface into the surrounding medium, and again with draw into their substance, consist of protoplasm, and may be cited as fur nishing excellent examples of its motive and contractile power. From the recent researches of Haeckel it would appear that protoplasm is capable of forming without the super- FlG, 5 _ UluUfTerontIiltcd addition of any other structure, inde- cytodo mass of proto- pendent organisms, which stand at P lasm - the lowest grade of organisation, and from their extreme simplicity are named by him Monera. To the group Monera belong the genera Protamoeba, Protogeues, and Bathybius. Of these, Bathybius is that which has attracted most attention. It has been regarded as a layer of soft slimy un- differentiated protoplasm covering the bot tom of the deep sea, and capable of exhibit ing the phenomena of contractility, growth, assimilation of food, and reproduction, Doubts, however, have been expressed re garding the nature of this Bathybius, so that it cannot now be cited as so definite an organism as the freely-swimming Pro- F JG. 26.-A simple tamoeba and Protogenes. Haeckel has re- chScdceii &quot;/&amp;gt;&quot; ferred these simple organisms to a sub- protoplasm ceii- L ingdoni of PROTISTA, which he considers SdmSTih, in to He on the confines of both the animal cicoius. and vegetable kingdoms. To a mass of protoplasm, whether it forms, as in one of these PROTISTA, an independent organism, or is merely a portion of the substance of the body of a higher organism, he has given the general name of a Cytode. Sometimes a cytode is a naked clump of Cytode. soft protoplasm, without a trace of differentiation either on its surface or in its substance, as in the freely-moving Monera ; at others the peripheral part of the cytode. hardens, and differentiates into a more or less perfect envelope, as in the genera Protomonas and Protomyxa. So far back as 1861, Lionel Beale had described, under the name of germinal matter (Bioplasm), minute living particles of vegetable protoplasm, and in 1863 he demon strated the presence of extremely minute particles of living matter in the blood. More recently Strieker has also called attention, in the bodies of the higher animals, to minute detached clumps of protoplasm which exhibited the phenomena of life. As a rule, however, in both vegetable and animal Cell, organisms the specks or clumps of protoplasm assume definite shapes, and show evidence of an internal dif ferentiation. In the midst of a minute clump of this substance a sharply-defined body called a nucleus is found, which differs from the surrounding protoplasm in not being contractile; and sometimes a minute speck, or nucleolus, exists within the nucleus. When a definite clump of protoplasm contains a nucleus in its interior, whether a nucleolus be present or not, it is called a Nucleated Cell. Cells are definite anatomical and physiological units, and exhibit all the phenomena of life. Some of the lowest organisms consist merely of a single cell, others of two or more cells united together, and these are called uni- or multi-cellular organisms. Cells also enter in the most material manner into the constitution of the textures of all the higher forms of plants and animals. Not unfrequently the peripheral part of the protoplasm of the cell differentiates into a distinct investing envelope, technically named a cell wall or cell membrane. In the earlier periods of investigation into the minute structure of cells it was believed that a cell wall was con stantly present, and that each cell was a minute micro scopic vesicle or bladder, which in its typical shape was globular or ovoid, but capable of undergoing various modi fications both in form, and chemical composition. The material enclosed by the cell wall was termed the cell con tents, and either in the midst of these contents or in con tact with the cell wall was the nucleus, which might or might not contain a nucleolus. Schwann believed that the cell wall was the most active constituent of the cell, i.e., pos sessed the power not only of producing chemical and physical changes in its own substance and in the cell contents, but of separating materials from the surrounding media, of secreting them, as it were, into the interior of the cell. In this manner he accounted for the formation in some cells of fat, in others of pigment, in others of the characteristic secretion of glands, and so on. It was then maintained by John Goodsir that the nucleus was the part of a cell which in all probability was concerned in separating and preparing its characteristic cell contents, and in its nutrition. Martin Barry and Goodsir also contended that the reproduction and multi plication of cells were due to self-division of the nucleus, which was thus the source of successive broods of young cells. They gave to the nucleus, therefore, an importance in the economy of the cell greater than had previously been assigned to it. As the investigations into cell structure became more extended, it was ascertained that a cell wall was by no means always present; that in many of the cells in which it had been supposed to exist it could not satisfactorily be demonstrated, and that in others, more especially in young actively-growing cells, no trace of an investing envelope