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FUNCTIONS OF CELLS. CELL-CONTENTS. 289
nutrition of the part is induced without there necessarily being any excitation of its functions; or a formative process may set in, giving rise to a greater or less number of new elements. These differences manifest themselves with greater or less distinctness, in proportion as the individual tissues of the body are more or less capable of responding to the one or other kind of excitation. When, namely, we speak of the functions of parts—in the case of a considerable number of tissues the real functions shrink into a very small compass; we are on the whole able to say but very little concerning the real functions, in the higher sense of the word, of nearly all the connective tissues, and of the great majority of epithelial cells. We are no doubt able to say what their use under particular circumstances is, still they always rather appear to be relatively inert masses, which scarcely perform any real functions in the ordinary meaning of the word, but rather serve as supports to the body, or as coverings to the different surfaces, or, in other localities, according to circumstances, act as media of union, intervention, or separation.
The case is different, on the other hand, with those parts, which, owing to the peculiar nature of their internal arrangement, are liable to a more rapid change, such as the nerves, muscles and muscular organs, glands and a few other structures, as, for example, among the epithelia, ciliated epithelium. In all these tissues, which are subservient to important functions, we find that these functions are chiefly due to very delicate changes of arrangement, or if you wish it expressed in more precise terms, to minute changes of place, in the minute particles of the internal matter, the cell-contents. In these cases therefore it is not so much the real cell in its pure form which decides the question, as the specific matters with which it is provided internally; the chief agent is not so much the membrane or the nucleus of the cell, as the contents. It is these which, when exposed to certain influences, become comparatively rapidly changed, without our being always able morphologically to detect any trace of a change in the arrangement of the contained particles. The utmost that we can observe in the shape of a palpable result is a real locomotion of small, visible particles, but we cannot push our analysis to such an extent, as to enable us to form any opinion as to the internal cause, in virtue of which this locomotion is effected by the ultimate particles which compose the cell-contents. When an excitation takes place in a nerve, we now know that a change in its electrical state is connected with it, a change which, from all that is known to us concerning electrical excitation in other bodies, must of necessity be referred to a change in the position which the individual molecules assume to one another. If we
'. conceive the axis-cylinder to be made
>I§l!tlll<f^ UP of electrical molecules, we can easily B imagine that every two of these molecules
®®«©®J® take up an altered position with regard to one another at the moment the stimulus is applied. Of these processes we see nothing. The axis-cylinder looks just as usual. If we watch a muscle during its contraction, we remark, it is true, that the intervals which separate the individual so-called discs (p. 54) become shorter; and as we now know that the substance of the muscle consists of a series of minute fibrils, which in their turn contain little granules at certain intervals corresponding to these discs, we conclude therefrom with some degree of assurance that really local changes take place in the minutest elements, though they cannot be further referred to any visible or directly recognizable cause. We cannot perceive any definite che
Fig. 97- Ideal diagram of the condition of the molecules of a nerve when it is at rest (in a peripolar state, A), or in an electrotonic (dipolar) state, B. From Ludwig, * Physiolog.,' I, p. 103.
FUNCTIONAL IRRITABILITY. 291
mical change, or any alteration in the state of nutrition of the parts; we only see a displacement, a dislocation of the particles, which, however, probably depends upon some slight chemical change in the molecules composing them.
In the case of ciliated epithelium you see how the fine cilia which are seated upon the surface of the cells, move in a certain direction, and in this direction exercise a loconiotory effect upon the little particles which come near to them. If we isolate the individual cells, we see that every one of them has at its upper end a border of a certain thickness, from which little hair-shaped prolongations run out. These all move in such a way that a cilium which, whilst quiet, stands quite upright, bends forwards and then throws itself backwards. But we are unable to perceive any changes within the individual cilia, by means of which the movement is effected.
Just the same is the case with gland-cells, concerning which we cannot entertain the least doubt that they produce a definite locomotory effect. For since Ludwig has shown in his researches on the salivary glands, that the pressure of the outward current of saliva is greater than that of the inward stream of blood, the only conclusion that is left us is, that the gland-cells exercise a definite motor influence upon the fluid; and that the secretion is driven out with a definite force, which is not due to the pressure of the blood, or any special muscular action, but to the specific energy of the cells as such. Still we are just as little able to discern in a gland-cell, whilst performing its functions, that its constituent particles are engaged in any peculiar material process, as we were in the case of the nerves, or ciliated epithelium.
These facts derive great support from the circumstance that we are able to perceive, that the functional activity of individual parts does experience a certain amount of impairment, if it is continued for too long a time. In all parts certain states of fatigue manifest themselves, states, during which the part is no longer able to originate the same amount of movement, that up to that time could be perceived in it. But, in order that they may again become competent to perform their functions, these parts by no means always require a new supply of nutriment, a fresh absorption of nutritive material; rest alone is sufficient to enable them to resume their activity in a short space of time. A nerve, which has been cut out of the body, and used for experiment, after a certain lapse of time becomes incapable of discharging its functions; but if it be allowed to repose under favourable circumstances which prevent it from drying up, it gradually regains its powers. This restitution of functional power (functional restitution), which takes place without any proper nutritive action, and in all probability depends upon the circumstance, that the molecules which had quitted their usual position gradually revert to it —we can produce in different parts by means of certain stimuli. According to the views of the neuro-pathologists these stimuli would only act upon the nerves, and through the medium of the nerves upon the other parts; but with reference to this very point we have some facts which cannot well be explained in any other way than by the assumption, that an influence is really exercised upon the parts themselves. If we take a single ciliated cell, and, after entirely isolating it from the body, allow it to swim about, and wait until a state of complete repose has declared itself, we can again call forth the peculiar movements of its cilia by adding a small quantity of potash or soda to the fluid, a quantity not large enough to produce corrosive effects upon the cell, but sufficient, upon penetration into it, to induce a certain change in its contents. A peculiarly interesting fact, however, is that the number of substances which will act, as stimuli, upon ciliated epithelium, is limited to these two. This explains how it happened that Purkinje
FUNCTIONAL IRRITATION. 293
and Valentin (who, as is well known, first made experiments, and those upon a very extensive scale, upon ciliary movement), although they experimented with a very large number of substances, at last, after they had tried all sorts of things—mechanical, chemical and electrical stimuli—came to the conclusion that there was no stimulus whatever, which could provoke the ciliary movement. I had the good fortune incidentally to stumble upon the peculiar fact, that potash and soda are such stimuli. Here we certainly cannot call in any nervous influence to our aid, and such influence appears to be the less admissible for the reason that, in accordance with well-known experiments, the ciliary movement is maintained in the dead body at a time when other parts have already begun to putrefy. The ciliated epithelium of the frontal sinuses and the trachea is found in human corpses in a state of perfect excitability thirty-six to forty-eight hours after death, when every trace of irritability has long vanished from the remainder of the body.
Much the same is the case with all other excitable parts. We see nearly everywhere that certain excitants act more readily than others, and that many are totally incapable of producing any particular effect. Nearly everywhere do we find specific relations or affinities to exist. If we cast our eyes upon the glands, it is a well-known fact that there are specific substances, by which we are enabled to act upon one gland, and not upon another; to rouse the specific energy of one gland, whilst all the rest remain unaffected. In the case of glands it is certainly much more difficult to exclude the influence of the nerves, than in that of ciliated epithelium, still certain experiments are recorded, in which, after the section of all the nerves, say of the liver (G. Harting), it was found possible, by means of the injection of irritating substances into the blood (these being such as experience had shewn to bear some intimate relation to the organ), to provoke an increased secretion in the organ.