Fig. 27.
Scheme of the foam structure of living substance. A—In undifferentiated protoplasm. B—In fibrillae protoplasm.
On the basis of investigation in the physical chemistry on the properties of semi-permeable membranes, we know that such membranes produce an elective effect on the diffusion of dissolved substances. This is in the way that the two different solutions, separated by a semi-permeable surface, do not follow the known laws of diffusion, but are altered in that certain substances in contrast to their rapidity of diffusion pass through the membrane or are prevented from entering by the latter. This applies likewise to the two kinds of ions, which are dissociated in diluted substances. If the surface exercises a selection in the way, for instance, that the positive kations are allowed to pass through, whilst the negative anions are held back, a difference of potential must exist between the two. In this manner, wherever two different solutions are separated from each other by a semi-permeable surface, an opportunity occurs for the taking place of galvanic currents. As we know, living protoplasm by reason of its colloidal components possesses, in common with all colloidal substances, on its surface the properties of semi-permeable membranes. Between the cell and the medium, therefore, there is always the opportunity for the occurrence of differences of electric potential. But more. We likewise know that protoplasm itself represents a mixture of colloid substances and actual solutions. Frequently, if not always, living structure presents a morphological differentiation of two types, when seen under the microscope, in the form of a foam structure described by Bütschli. (Figures [27] and [28].) If we suppose that with the disintegration of complex molecules, which we must assume as taking place in the material of the walls of the protoplasm network, substances are formed which are subjected to electrolytic dissociation, the anions and kations hereby liberated must be diffused from the place of their separation into the surroundings. Their diffusion, however, is restricted by the protoplasmic network. The positive ions may pass through, but the negative ions may not. As a result: the reticulated substance is the seat of electric discharge, which in turn gives the impact to the breaking down of new molecules and with this to the occurrence of new potential differences, and so on, consequently the disintegration is extended further and further through the connected masses of the protoplasmic framework.
Fig. 28.
Protoplasm of different cells, showing foam structures. A—Pseudopod of a marine rhizopod. The protoplasm only shows foam structure at the point of stimulation. B—Epidermic cell of lumbricus. C—Nerve fiber. D—Part of the cell body of a ganglia cell. (A-C after Bütschli, D after Held.)
This theory, founded on facts gained entirely from investigation, would involve those forms of energy which play the rôle of activator in the extension of the breaking down of the molecule from cross section to cross section, namely, the osmotic and the electrical energy. Based on the general properties of physical chemistry and those of morphology of the living substances, they would be applicable to all vital systems. It would be premature to attempt to extend this assumption and further develop its specific details, above all to make it responsible for the specific differences in the process of the conduction of excitation in various forms of living substance. For this our knowledge of the properties of living substance is still far too incomplete. Nevertheless, it furnishes us even now with various points of view for the further analysis of a series of vital manifestations, as, for instance, the facts concerning the production of electricity, of galvanotaxis, chemotaxis and so on. This, however, exceeds the limits of the task we have here mapped out. We are concerned here solely with the general principle on which the conductivity of excitation in the living substance is founded.
CHAPTER VII
THE REFRACTORY PERIOD AND FATIGUE
Contents: Conception of specific irritability. Alteration of specific irritability during and after excitation. Refractory period in various forms of living substance. Absolute and relative refractory period. Curve of irritability during refractory period. Dependence of the duration of the refractory period on the rapidity of the course of the metabolic processes in the living substance. Dependence on temperature. Dependence on supply of oxygen. Theory of refractory period. Refractory period as basis of fatigue. Fatigue as a form of asphyxiation. Alterations of irritability and the course of excitation in fatigue. Recovery from fatigue. The rôle played by oxygen in recovery. Fatigue as an expression of the prolongation of the refractory period conditioned by the relative want of oxygen. Fatigue of the nerve.
Every living system possesses, as we know, a peculiar and characteristic manner of reacting to stimulation. The muscle responds with a contraction, the salivary cell with production of saliva, the luminous cell with the emission of light. This is the specific energy in the sense of Johannes Müller. Every living system is likewise characterized by a certain degree of irritability, which can be expressed by the threshold value of the stimulus at which the specific reaction is just perceptible. This degree of irritability, by which the system concerned is distinguished, may be termed its specific irritability.