It seems to me that there would be little difficulty in harmonising the phenomenon of the inversion of irritability with the so-called principle of the “action of masses” and with the laws of certain “reversible” processes well known in chemistry. As to the simple fact of the re-establishment of irritability after stimulation has occurred, or, in certain other cases, the fact that in spite of permanent stimulation irritability seems to exist permanently also, physical analogies or even explanations might very well be found.[92]
If now we ask whether there is anything like an adaptation appearing in the general characteristics of irritation and irritability, it seems to me that we may answer the question in an affirmative manner, as far as primary regulation comes into account. We, certainly, have not studied any abnormal regulatory lines of general functioning, we only have studied general functioning itself; but, indeed, there was a certain sort of regulation in functioning. Of course, by showing that one of the most general features of all functioning is primary-regulatory in itself, we do not deny the possibility of many specific functions in which real secondary regulations actually do exist. Nothing indeed is asserted about the specific character of functioning in its different types, by proving that one of the general features of all functioning may comparatively easily be understood. It seems to me that this important logical point has not always received the attention it deserved.
The Regulation of Heat Production.[93]—Having finished our introductory remarks we now turn to the proper study of special physiological functioning with regard to its adaptive side, and begin with the most simple cases.
The so-called “regulation of heat” in warm-blooded vertebrates is an instance of a special function which can be said to be regulatory in itself. There exists a normal blood heat for each species, which is maintained no matter whether the temperature of the medium rise or fall. It might seem at first as if in this case there were a little more of an adaptive regulation than only its well-known primary type; no reversion, one might say, of the direction of one and the same process occurs in the regulation of heat production, but one kind of process is called into action if it is necessary to raise the temperature, and another whenever it is necessary to lower it. Even in the dilatation and constriction of capillary vessels there are different nerves serving for each operation respectively, and far more important are the increasing of transpiration for cooling, the increasing of combustion for heating—two radically different processes. But, nevertheless, there is a certain unity in these processes, in so far as a specific locality of the brain has been proved to be the “centre” of them all; it is to this centre of course that the analysis of heat production considered as a kind of regulation or adaptation must be directed. Such an ultimate analysis, it seems to me, would have to classify heat regulation under the primary type of adaptations in physiology without any restriction. The centre acts in one sense or in the other, if stimulated by any temperature beyond a very limited range, and it is in the action of the centre that the “regulation” of heat consists.[94]
Primary Regulations in the Transport of Materials and Certain Phenomena of Osmotic Pressure.—Very similar phenomena of regulation are present in many processes concerned in the whole of metabolism. Let us consider for a moment the migration of materials in plants. Whenever any compound is used at a certain place, a permanent afflux of this compound to that place sets in from all possible directions. No doubt this is a “regulation,” but it is also the function itself, and besides that, a very simple function based almost entirely on well-known laws of physical chemistry. And in other cases, as in the ascent of water to the highest tops of our trees, which purely physical forces are said to be insufficient to explain, we can appeal to the unknown organisation of many cells, and there is nothing to prevent our attributing to these cells certain functions which are, if you like to say so, regulatory in themselves. Among other facts of so-called regulations there is the stopping of metabolic processes by an accumulation of their products: as, for instance, the transformation of starch into sugar is stopped, if the sugar is not carried away. Of course that is a regulation, but it again is an intrinsic one, and it is one of the characteristics of reversible chemical processes to be stopped in that way. I know very well that in this particular case a certain complication is added by the fact that it is a so-called ferment, the diastase, which promotes the transformation of starch into cane-sugar, and that this ferment is actively produced by the organism: but even its production would not prove that any real kind of secondary regulation exists here, if nothing more were known about such an active production than this single case.
In a special series of experiments almost all carried out in Wilhelm Pfeffer’s botanical laboratory at Leipzig, an attempt has been made to discover in what manner the cells of plants are able to withstand very high abnormalities of the osmotic pressure of the medium—that is to say, very great changes in the amount of its salinity. That many, particularly the lower plants, are able to stand such changes had been ascertained already by the careful examinations of Eschenhagen; but recent years have given us a more profound insight into what happens. Von Mayenburg[95] has found that sundry of the species of Aspergillus, the common mould, are able to live in very highly concentrated solutions of several salts (KNO3 and Na2SO4). They were found to regulate their osmotic pressure not by taking in the salts themselves, but by raising the osmotic pressure of their own cell sap, producing a certain amount of osmotically active substances, probably carbohydrates. If in this case it were possible to assume that the osmotic pressure of the medium were the real stimulus for the production of the osmotic substances in the cell, stimulus and production both corresponding in their degree, we should be entitled to speak of a primary though physiological[96] regulation only; and it seems to me that despite the discoveries of Nathansohn that certain algae and cells of higher plants are able to change the permeability of their surfaces in a way which regulates the distribution of single salts or ions in the sap of their cells without any regard to pure osmotic equilibrium, such a simple explanation might be possible.[97]
There are many regulation phenomena connected with osmotic pressure and permeability in animal physiology also, though at present they are not worked out as fully as possible. The works of Frédéricq, J. Loeb, Overton and Sumner[98] would have to be taken into account by any one who wished to enter more deeply into these problems. We can only mention here that permeability to water itself also plays its part, and that, according to Overton’s experiments, it is a kind of solubility of the media in the very substance of the cell surface on which all permeability and its regulation depend.
Chromatic Regulations in Algae.—The phenomena of osmotic pressure and its regulation may be said to be the preliminaries of metabolism proper, conditions necessary for it to take place. Now there is another branch of such preliminaries to metabolism, in which the most interesting regulation phenomena have been lately discovered. It is well known that what is called assimilation in plants, that is, the formation of organic compounds out of carbon dioxide (CO2) and water, occurs only in the light by means of certain pigments. This pigment is in all higher plants and in many others the green chlorophyll, but it may be different in certain species of algae, and can generally be said[99] to be of the colour complementary to the colour of those rays which especially are to be absorbed and to be used for assimilation. But here we have “adaptedness,” not adaptation. It was in some species of primitive algae, the Oscillariae, that Gaidukow[100] found a very interesting instance of an active regulation in the formation of pigments. These algae always assume a colour which corresponds to the accidental colour of the rays of the medium and is complementary to it; they become green in red light, yellow in blue light, and so on—that is, they always actively take that sort of colouring which is the most suitable to the actual case.[101] There indeed occurs a sort of complementary photography in these algae; but, though adaptive, it could hardly be said to exceed the limits of “primary phenomena.”
Metabolic Regulations.—And now we enter the field of regulations in metabolism itself. There are two kinds of outside factors of fundamental importance for all metabolic processes: food is one, and oxygen is the other. And metabolism as a whole is of two different aspects also: it both serves for assimilation proper—that is, building up—and it supplies the energy for driving the functional machine. It is clear that food alone—together of course with the assimilating means of the organism, can account for the first type of metabolism, while both food and oxygen, or some sort of substitute for the latter, as in certain bacteria, supply functional energy. Of course we are not entitled to say that the importance of so-called oxidation or respiration is exhausted by its energetic rôle: it certainly is not, for if it were, the organism would only be stopped in its functions if deprived of oxygen but would not die. It seems that certain substances always arise in the metabolism, in the processes of decomposition, which have to be burnt up in order not to become poisonous. But we shall return to the phenomena of organic oxidation in another chapter of the book, and shall deal with them from a more general point of view.[102]