(a) Amœboid movement (in rhizopods, blood-cells, pigment-cells, etc.).
(b) A similar flow of protoplasm within enclosed cells.
(c) Vibratory motion (ciliary movements) in infusoria, spermatozoa, ciliated epithelial cells.
(d) Muscular movement (in most animals).
The elementary psychic activity that arises from the combination of sensation and movement is called reflex (in the widest sense), reflective function, or reflex action. The movement—no matter what kind it is—seems in this case to be the immediate result of the stimulus which evoked the sensation; it has, on that account, been called stimulated motion in its simplest form (in the protists). All living protoplasm has this feature of irritability. Any physical or chemical change in the environment may, in certain circumstances, act as a stimulus on the psychoplasm, and elicit or “release” a movement. We shall see later on how this important physical concept of “releasing” directly connects the simplest organic reflex actions with similar mechanical phenomena of movement in the inorganic world (for instance, in the explosion of powder by a spark, or of dynamite by a blow). We may distinguish the following seven stages in the scale of reflex action:
I. At the lowest stage of organization, in the lowest protists, the stimuli of the outer world (heat, light, electricity, etc.) cause in the indifferent protoplasm only those indispensable movements of growth and nutrition which are common to all organisms, and are absolutely necessary for their preservation. That is also the case in most of the plants.
II. In the case of many freely moving protists (especially the amœba, the heliozoon, and the rhizopod) the stimuli from without produce on every spot of the unprotected surface of the unicellular organism external movements which take the form of changes of shape, and sometimes changes of place (amœboid movement, pseudopod formation, the extension and withdrawal of what look like feet); these indefinite, variable processes of the protoplasm are not yet permanent organs. In the same way, general organic irritability takes the form of indeterminate reflex action in the sensitive plants and the lowest metazoa; in many multicellular organisms the stimuli may be conducted from one cell to another, as all the cells are connected by fine fibres.
III. Many protists, especially the more highly developed protozoa, produce on their unicellular body two little organs of the simplest character—an organ of touch and an organ of movement. Both these instruments are direct external projections of protoplasm; the stimulus, which alights on the first, is immediately conducted to the other by the psychoplasm of the unicellular body, and causes it to contract. This phenomenon is particularly easy to observe, and even produce experimentally, in many of the stationary infusoria (for instance, the poteriodendron among the flagellata, and the vorticella among the ciliata). The faintest stimulus that touches the extremely sensitive hairs, or cilia, at the free end of the cells, immediately causes a contraction of a thread-like stalk at the other, fixed end. This phenomenon is known as a “simple reflex arch.”
IV. These phenomena of the unicellular organism of the infusoria lead on to the interesting mechanism of the neuro-muscular cells, which we find in the multicellular body of many of the lower metazoa, especially in the cnidaria (polyps and corals). Each single neuro-muscular cell is a “unicellular reflex organ”; it has on its surface a sensitive spot, and a motor muscular fibre inside at the opposite end; the latter contracts as soon as the former is stimulated.
V. In other cnidaria, notably in the free swimming medusæ—which are closely related to the stationary polyps—the simple neuro-muscular cell becomes two different cells, connected by a filament; an external sense-cell (in the outer skin) and an internal muscular cell (under the skin). In this bicellular reflex organ the one cell is the rudimentary organ of sensation, the other of movement; the connecting bridge of the psychoplasmic filament conducts the stimulus from one to the other.
VI. The most important step in the gradual construction of the reflex mechanism is the division into three cells; in the place of the simple connecting bridge we spoke of there appears a third independent cell, the soul-cell, or ganglionic cell; with it appears also a new psychic function, unconscious presentation, which has its seat in this cell. The stimulus is first conducted from the sensitive cell to this intermediate presentative or psychic cell, and then issued from this to the motor muscular cell as a mandate of movement. These tricellular reflex organs are preponderantly developed in the great majority of the invertebrates.
VII. Instead of this arrangement we find in most of the vertebrates a quadricellular reflex organ, two distinct “soul-cells,” instead of one, being inserted between the sensitive cell and the motor cell. The external stimulus, in this case, is first conducted centripetally to the sensitive cell (the sensible psychic cell), from this to the will-cell (the motor psychic cell), and from this, finally, to the contractile muscular cell. When many such reflex organs combine and new psychic cells are interposed we have the intricate reflex mechanism of man and the higher vertebrates.
The important distinction which we make, in morphology and physiology, between unicellular and multicellular organisms holds good for their elementary psychic activity, reflex action. In the unicellular protists (both the plasmodomous primitive plants, or protophyta, and the plasmophagous primitive animals, or protozoa) the whole physical process of reflex action takes place in the protoplasm of one single cell; their “cell-soul” seems to be a unifying function of the psychoplasm of which the various phases only begin to be seen separately when the differentiation of special organs sets in.