"The morphological character depends upon the presence of the intercellular substance.
"According to the quantity and the structure of the intercellular substance the connective substances are divided into (1) cellular (with little intercellular substance); (2) homogeneous; (3) fibrillar connective tissue; (4) cartilage; (5) bone.
"The physiological character of muscular tissue is contained in the increased capacity for contraction.
"The morphological character is found in the fact that the cells have secreted muscle-substance.
"According to the nature of the muscle-substance are distinguished smooth and cross-striated muscle-fibres.
"According to the character and derivation of the cells (muscle-corpuscles) the musculature is divided into epithelial (epithelial muscle-cells, primary bundles) and connective-tissue muscle cells (contractile fibre-cells).
"The physiological character of nervous tissue rests upon the transmission of sensory stimuli and voluntary impulses, and upon the co-ordination of these into unified psychic activity.
"The conduction takes place by means of nerve-fibres (non-medullated and medullated fibrils and bundles of fibrils); the co-ordination of stimuli by means of ganglion-cells (bipolar, multipolar ganglion-cells)." (General Principles of Zoology, pp. 117-8.)
But now concerning cells out of which, variously modified, obscured, and sometimes obliterated, tissues are formed, we have to note a fact of much significance. Along with the cell-doctrine as at first held, when attention was given to the cell itself rather than to its contents, there went the belief that each of these morphological units is structurally separate from its neighbours. But since establishment of the modern view that the essential element is the contained protoplasm, histologists have discovered that there are protoplasmic connexions between the contents of adjacent cells. Though cursorily observed at earlier dates, it was not until some twenty years ago that in plant-tissues these were clearly shown to pass through openings in the cell-walls. It is said that in some cases the openings are made, and the junctions established, by a secondary process; but the implication is that usually these living links are left between multiplying protoplasts; so that from the outset the protoplasm pervading the whole plant maintains its continuity. More recently sundry zoologists have alleged that a like continuity exists in animals. Especially has this been maintained by Mr. Adam Sedgwick. Numerous observations made on developing ova of fishes have led him to assert that in no case do the multiplying cells so-called—blastomeres and their progeny—become entirely separate. Their fission is in all cases incomplete. A like continuity has been found in the embryos of many Arthropods, and more recently in the segmenting eggs and blastulæ of Echinoderms. The syncytium thus formed is held by Mr. Sedgwick to be maintained in adult life, and in this belief he is in agreement with sundry others. Bridges of protoplasm have been seen between epithelium-cells, and it is maintained that cartilage-cells, connective tissue cells, the cells forming muscle-fibres, as well as nerve-cells, have protoplasmic unions. Nay, some even assert that an ovum preserves a protoplasmic connexion with the matrix in which it develops.
A corollary of great significance may here be drawn. It has been observed that within a vegetal cell the strands of protoplasm stretched in this or that direction contain moving granules, showing that the strands carry currents. It has also been observed that when the fission of a protozoon is so nearly complete that its two halves remain connected only by a thread, currents of protoplasm move through this thread, now one way now the other. The inference fairly to be drawn is that such currents pass also through the strands which unite the protoplasts forming a tissue. What must happen? So long as adjacent cells with their contents are subject to equal pressures no tendency to redistribution of the protoplasm exists, and there may then occur the action sometimes observed inside the strands within a cell: currents with their contained granules moving in opposite directions. But if the cells forming a portion of tissue are subject to greater pressure than the cells around, their contained protoplasm must be forced through the connecting threads into these surrounding cells. Every change of pressure at every point must cause movements and counter-movements of this kind. Now in the Metazoa at large, or at least in all exhibiting relative motions of parts, and especially in all which are capable of rapid locomotion, such changes of pressure are everywhere and always taking place. The contraction of a muscle, besides compressing its components, compresses neighbouring tissues; and every instant contractions and relaxations of muscles go on throughout the limbs and body during active exertion. Moreover, each attitude—standing, sitting, lying down, turning over—entails a different set of pressures, both of the parts on one another and on the ground; and those partial arrests of motion which result from sitting down the feet alternately when running, send jolts or waves of varying pressure through the body. The vital actions, too, have kindred effects. An inspiration alters the stress on the tissues throughout a considerable part of the trunk, and a heart-beat propels, down to the smallest arteries, waves which slightly strain the tissues at large. The component cells, thus subject to mechanical disturbances, small and great, perpetual and occasional, are ever having protoplasm forced into them and forced out of them. There are gurgitations and regurgitations which, if they do not constitute a circulation properly so called, at least imply an unceasing redistribution. And the implication is that in the course of days, weeks, months, years, each portion of protoplasm visits every part of the body.