Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/22

Rh 12 PHYSIOLOGY the hope arose that the functions of the cell might be deduced from the mutual relations of cell-membrane, cell- contents, and nucleus, and that the functions of an organ might be deduced from the modified functions of the con stituent modified cells. Continued investigation, however, proved destructive of this physiological cell-theory. It soon became evident that the possession of an investing envelope or cell-membrane was no essential feature of a cell, and that even the central kernel or nucleus might at times be absent. It was seen in fact that the anatomical unit need have no visible parts at all, but might be simply a minute mass, limited in various ways, of the material spoken of as cell -contents. Under the cell -theory, the cell was supposed to be the first step in organization, the step by which a quantity of formless unorganized plasm became an organized unit ; this plasm was further supposed still to form the chief part of the cell-contents, and soon Proto- became recognized under the name of protoplasm. Hence plasm. t ue destructive anatomical researches which deprived the cell of its cell -membrane, and even of its nucleus, left nothing except a mass of protoplasm to constitute an anatomical unit. For such a unit the word &quot; cell &quot; was a misnomer, since all the ideas of organization denoted by the word had thus vanished ; nevertheless it was retained with the new meaning, and up to the present time the definition of a cell is that of a limited mass of proto plasm, generally but not always containing a modified kernel or nucleus. Proto- With this anatomical change of front the physiological c cell-theory was utterly destroyed. The cell was no longer a unit of organization ; it was merely a limited mass of protoplasm, in which, beyond the presence of a nucleus, there was no visible distinction of parts. It was no longer possible to refer the physiological phenomena of the cell to its organization ; it became evident that the work done by a &quot; cell &quot; was the result not of its form and cellular structure but simply of the nature and properties of the apparently structureless protoplasm which formed its body. A new idea pressed itself on men s minds, that organization was a concomitant and result of vital action, not its condition and cause ; as Huxley in one of his earliest writings put it, &quot;They [cells] are no more the producers of the vital phenomena than the shells scattered in orderly lines along the sea-beach are the instruments by which the gravitative force of the moon acts upon the ocean. Like these, the cells mark only where the vital tides have been, and how they have acted.&quot; l Hence arose the second of the two movements mentioned above, that which may be called the &quot;protoplasmic&quot; movement, a movement which, throwing overboard altogether all conceptions of life as the outcome of organization, as the mechanical result of structural conditions, attempts to put physiology on the same footing as physics and chemistry, and regards all vital phenomena as the com plex products of certain fundamental properties exhibited by matter, which, either from its intrinsic nature or from its existing in peculiar conditions, is known as living matter, mechanical contrivances in the form of organs serving only to modify in special ways the results of the exercise of these fundamental activities and in no sense determining their initial development. Tissues. Long before the cell-theory had reduced to an absurdity the &quot; organic &quot; conception of physiology, the insight of the brilliant Bichat, so early lost to science, had led him to prepare the way for modern views by developing his doctrine of &quot; tissues.&quot; That doctrine regarded the body as made up of a number of different kinds of living material, each kind of material having certain innate qualities proper to itself 1 &quot;The Cell-Theory,&quot; in Brit, and For. Med. Chir. Rev., vol. xii. (1853) p. 314. as well as certain structural features, and the several kinds of material being variously arranged in the body. Each of these body-components was spoken of as a tissue, muscular tissue, nervous tissue, and the like; and the varied actions of the body were regarded as the result of the activities of the several tissues modified and directed by the circum stance that the tissues were to a great extent arranged in mechanical contrivances or organs which largely deter mined the character and scope of their actions. The imperfection of microscopic methods in Bichat s time, and, we may perhaps add, his early death, prevented him from carrying out an adequate analysis of the qualities or properties of the tissues themselves. During the middle portion of this century, however, histological investigation, i.e., inquiry into the minute structure of the tissues, made enormous progress, and laid the basis for a physiological analysis of the properties of tissues. In a short time it became possible to lay down the generalization that all the several tissues arise, as far as structure is concerned, by a differentiation of a simple primitive living matter, and that the respective properties of each tissue are nothing more than certain of the fundamental properties of the primordial substance thrown into prominence by a division of labour running to a certain extent parallel to the differ entiation of structure. Developed in a fuller manner, this modern doctrine may be expounded somewhat as follows. In its simplest form, a living being, as illustrated by some of the forms often spoken of as amoebae, consists of a mass of substance in which there is no obvious distinc tion of parts. In the body of such a creature even the highest available powers of the microscope reveal nothing more than a fairly uniform network of material, a network sometimes compressed, with narrow meshes, sometimes more open, with wider meshes, the intervals of the mesh- work being filled, now with a fluid, now with a more solid substance or with a finer and more delicate network, and minute particles or granules of variable size being some times lodged in the open meshes, sometimes deposited in the strands of the network. Sometimes, however, the net work is so close, or the meshes filled up with material so identical in refractive power with the bars or films of the network, and at the same time so free from granules, that the whole substance appears absolutely homogeneous, glassy or hyaline. Analysis with various staining and other re agents leads to the conclusion that the substance of the network is of a different character from the substance filling up the meshes. Similar analysis shows that at times the bars or films of the network are not homogeneous, but composed of different kinds of stuff; yet even in these cases it is difficult if not impossible to recognize any de finite relation of the components to each other such as might deserve the name of structure ; and certainly in what may be taken as the more typical instance, where the network seems homogeneous, no microscopic search is able to reveal to us a distinct structural arrangement in its substance. In all probability optical analysis, with all its aids, has here nearly reached its limits ; and, though not wholly justified, we may perhaps claim the right to con clude that the network in such case is made up of a substance in which no distinction of parts will ever be visible, though it may vary in places or at times in what may be spoken of as molecular construction, and may carry, lodged in its own substance, a variety of matters foreign to its real self. This remarkable network is often spoken of as consisting of protoplasm, and, though that word has come to be used in several different meanings, we may for the present retain the term. The body of an amoeba, then, or of a similar organism consists of a network or framework which we may speak of as protoplasm, filled up with other matters. In most cases it is true that in Evolu tion ai eharac teristii of pro plasm.