Every organism, whether vegetable or animal, must be regarded as a progressive total, made up of a larger or smaller number of similar or dissimilar cells. Just as a tree constitutes a mass arranged in a definite manner in which, in every single part, in the leaves as in the root, in the trunk as in the blossom, cells are discovered to be the ultimate elements, so it is with the forms of animal life. Every animal presents itself as a sum of vital unities, every one of which manifests all the characteristics of life. The characteristics and unity of life cannot be limited to any one particular spot in an organism (for instance, to the brain of a man) but are to be found only in the definite, constantly recurring structure, which every individual element displays. A so-called individual always represents an arrangement of a social kind, in which a number of individual existences are mutually dependent, but in such a way that every element has its own special action, and even though it derive its stimulus to activity from other parts, yet alone affects the actual performance of its duties.
Between cells there is a greater or less amount of a homogeneous substance—the intercellular substance. According to Schwann, the intercellular substance was cyto-blastema destined for the development of new cells; I believe this is not so, I believe that the intercellular substance is dependent in a certain definite manner upon the cells, and that certain parts of it belong to one cell and parts to another.
At various times, fibres, globules, and elementary granules, have been regarded as histological starting-points. Now, however, we have established the general principle that no development of any kind begins de novo and that as spontaneous generation is impossible in the case of entire organisms, so also it is impossible in the case of individual parts. No cell can build itself up out of non-cellular material. Where a cell arises, there a cell must have previously existed (omnis cellula e cellula), just as an animal can spring only from an animal, and a plant only from a plant. No developed tissues can be traced back to anything but a cell.
If we wish to classify tissues, a very simple division offers itself. We have (a) tissues which consist exclusively of cells, where cell lies close to cell. (b) Tissues in which the cells are separated by a certain amount of intercellular substance. (c) Tissues of a high or peculiar type, such as the nervous and muscular systems and vessels. An example of the first class is seen in the epithelial tissues. In these, cell lies close to cell, with nothing between.
The second class is exemplified in the connective tissues—tissues composed of intercellular substance in which at certain intervals cells lie embedded.
Muscles, nerves, and vessels form a somewhat heterogeneous group. The idea suggests itself that we have in all three structures to deal with real tubes filled with more or less movable contents. This view is, however, inadequate, since we cannot regard the blood as analogous to the medullary substance of the nerve, or contractile substance of a muscular fasciculus.
The elements of muscle have generally been regarded as the most simple. If we examine an ordinary red muscle, we find it to be composed of a number of cylindrical fibres, marked with transverse and longitudinal striæ. If, now, we add acetic acid, we discover also tolerably large nuclei with nucleoli. Thus we obtain an appearance like an elongated cell, and there is a tendency to regard the primitive fasciculus as having sprung from a single cell. To this view I am much inclined.
Pathological tissues arise from normal tissues; and there is no form of morbid growth which cannot in its elements be traced back to some model which had previously maintained an independent existence in the economy. A classification, also, of pathological growths may be made on exactly the same plan as that which we have suggested in the case of the normal tissues.
Nutrition, Blood, and Lymph. Pus