Popular Science Monthly/Volume 11/August 1877/The Import of Protoplasm
By MICHAEL FOSTER, M. D., F. R. S.
AMONG the simpler organisms known to biologists, perhaps the most simple as well as the most common is that which has received the name of Amœba. There are many varieties of amœba, and probably many of the forms which have been described are, in reality, merely amœbiform phases in the lives of certain animals or plants; but they all possess the same general characters. Closely resembling the white corpuscles of vertebrate blood, they are wholly or almost wholly composed of undifferentiated protoplasm, in the midst of which lies a nucleus, though this is sometimes absent. In many a distinction may be observed between a more solid external layer, or ectosarc and a more fluid granular interior, or endosarc; but in others even this primary differentiation is wanting. By means of a continually occurring flux of its protoplasmic substance, the amœba is enabled from moment to moment not only to change its form, but also to shift its position. By flowing round the substances which it meets, it, in a way, swallows them; and, having digested and absorbed such parts as are suitable for food, ejects or rather flows away from the useless remnants. It thus lives, moves, eats, grows, and after a time dies, having been during its whole life hardly anything more than a minute lump of protoplasm. Hence to the physiologist it is of the greatest interest, since in its life the problems of physiology are reduced to their simplest forms.
Now, the study of an amœba, with the help of knowledge gained by the examination of more complex bodies, enables us to state that the undifferentiated protoplasm, of which its body is so largely composed possesses certain fundamental vital properties:
1. It is contractile.—There can be little doubt that the changes in the protoplasm of an amœba, which bring about its peculiar "amœboid" movements, are identical in their fundamental nature with those which, occurring in a muscle, cause a contraction; a muscular contraction is essentially a regular, an amœboid movement an irregular flow of protoplasm. The body of the amœba may therefore be said to be contractile.
2. It is irritable and automatic.—When any disturbance, such as contact with a foreign body, is brought to bear n the amœba at rest, movements result. These are not passive movements, the effects of the push or pull of the disturbing body, and therefore proportionate to the force employed to cause them, but active manifestations of the contractility of the protoplasm; that is to say, the disturbing cause or stimulus sets free a certain amount of energy previously latent in the protoplasm, and the energy set free takes on the form of movement. Any living matter which, when acted on by a stimulus, thus suffers an explosion of energy, is said to be "irritable." The irritability may, as in the amœba, lead to movement; but in some cases no movement follows the application of the stimulus to irritable matter, the energy set free by the explosion taking on some other form (heat, etc.) than movement. Thus a substance may be irritable and yet not contractile, though contractility is the most common manifestation of irritability.
The amœba (except in its prolonged quiescent stage) is rarely at rest. It is almost continually in motion. The movements cannot always be referred to changes in surrounding circumstances acting as stimuli; in many cases the energy is set free in consequence of internal changes, and the movements which result are called spontaneous or automatic movements. We may, therefore, speak of the protoplasm of the amœba as being irritable and automatic.
3. It is receptive and assimilative.—Certain substances serving as food are received into the body of the amœba, and, being there in large measure dissolved, become part and parcel of the body of the amœba become, in fact, fresh protoplasm.
4. It is metabolic and secretory.—Pari passu with the reception of new material, there is going on an ejection of old material, for the increase of the amœba by the addition of food is not indefinite. In other words, the protoplasm is continually undergoing chemical change (metabolism), room being made for the new protoplasm by the breaking up of the old protoplasm into products which are cast out of the body and got rid of. These products of metabolic action have, in all probability, subsidiary uses. Some of them, for instance, we have reason to think, are of value in the solution and preliminary changes of the raw food mechanically introduced into the body of the amœba; and hence are retained within the protoplasm for some little time. Such products are generally spoken of as "secretions." Others, which pass more rapidly away, are generally called "excretions." The distinction between the two is an unimportant and frequently accidental one. The energy expended in the movements of the amœba is supplied by the chemical changes going on in the protoplasm by the breaking up of bodies possessing much latent energy into bodies possessing less. Thus the metabolic changes which the food undergoes in passing through the protoplasm of the amœba (as distinguished from the undigested stuff mechanically lodged for a while in the body) are of three classes: those preparatory to and culminating in the conversion of the food into protoplasm; those concerned in the discharge of energy; and those tending to economize the immediate products of the second class of changes by rendering them more or less useful for the first.
5. It is respiratory.—Taken as a whole, the metabolic changes are preëminently processes of oxidation. One article of food—i. e., one substance taken into the body, viz., oxygen—stands apart from all the rest; and one product of metabolism peculiarly associated with oxidation—viz., carbonic acid—stands also somewhat apart from all the rest. Hence, the assumption of oxygen and the excretion of carbonic acid, together with such of the metabolic processes as are more especially oxidative, are frequently spoken of together as constituting the respiratory processes.
6. It is reproductive.—The individual amœba represents a unit. This unit, after a longer or shorter life, having increased in size by the addition of new protoplasm in excess of that which it is continually using up, may by fission (or by other means) resolve itself into two (or more) parts, each of which is capable of living as a fresh unit or individual.
Such are the fundamental vital qualities of the protoplasm of an amœba; all the facts of the life of an amœba are manifestations of these protoplasmic qualities in varied sequence and subordination. The higher animals, we learn from morphological studies, are in reality groups of amœbæ peculiarly associated together. All the physiological phenomena of the higher animals are similarly the results of these fundamental qualities of protoplasm peculiarly associated together. The dominant principle of this association is the physiological division of labor corresponding to the morphological differentiation of structure. Were a larger or "higher" animal to consist simply of a colony of undifferentiated amœbæ, one animal differing from another merely in the number of units making up the mass of its body, without any differences between the individual units, progress of function would be an impossibility. The accumulation of units would be a hindrance to welfare rather than a help. Hence, in the evolution of living beings through past times, it has come about that in the higher animals (and plants) certain groups of the constituent amœbiform units or cells have, in company with a change in structure, been set apart for the manifestation of certain only of the fundamental properties of protoplasm, to the exclusion or at least to the complete subordination of the other properties.
These groups of cells, thus distinguished from each other, at once by the differentiation of structure and by the more or less marked exclusiveness of structure, receive the name of "tissues." Thus, the units of one class are characterized by the exaltation of the contractility of their protoplasm, their automatism, metabolism, and reproduction, being kept in marked abeyance. These units constitute the so-called muscular tissue. Of another tissue—viz., the nervous—the marked features are irritability and automatism, with an almost complete absence of contractility and a great restriction of the other qualities. In a third group of units, the activity of the protoplasm is largely confined to the chemical changes of secretion, contractility and automatism (as manifested by movement) being either absent or existing to a very slight degree. Such a secreting tissue, consisting of epithelium-cells, forms the basis of the mucous membrane of the alimentary canal. In the kidney, the substances secreted by the cells, being of no farther use, are at once ejected from the body. Hence the renal tissue may be spoken of as excretory. In the epithelium cells of the lungs, the protoplasm plays an altogether subordinate part in the assumption of oxygen and the excretion of carbonic acid. Still, we may, perhaps, be permitted to speak of the pulmonary epithelium as a respiratory tissue.
In addition to these distinctly secretory or excretory tissues, there exist groups of cells specially reserved for the carrying on of chemical changes, the products of which are neither cast out of the body nor collected in cavities for digestive or other uses. The work of these cells seems to be of an intermediate character: they are engaged either in elaborating the material of food that it may be the more easily assimilated, or in preparing used-up material for final excretion. They receive their material from the blood, and return their products back to the blood. They may be called the metabolic tissues par excellence. Such are the fat-cells of adipose tissue, the hepatic cells (as far as the work of the liver other than the secretion of bile is concerned), and, in general, the blood.
Each of the various units retains, to a greater or less degree, the power of reproducing itself, and the tissues generally are capable of regeneration in kind. But neither units nor tissues can reproduce other parts of the organism than themselves, much less the entire organism. For the reproduction of the complex individual, certain units are set apart in the form of ovary and testis. In these, all the properties of protoplasm are distinctly subordinated to the work of growth.
Lastly, there are certain groups of units—certain tissues which are of use in the body of which they form a part, not by reason of their manifesting any of the fundamental qualities of protoplasm, but on account of the physical and mechanical properties of certain substances which their protoplasm has been able, by virtue of its metabolism, to manufacture and to deposit. Such tissues are bone, cartilage, connective tissue in large part, and the greater portion of the skin.
We may, therefore, consider the complex body of a higher animal as a compound of so many tissues, each tissue corresponding to one of the fundamental qualities of protoplasm, to the development of which it is specially devoted by the division of labor. It must, however, be remembered, that there is a distinct limit to the division of labor. In each and every tissue, in addition to its leading quality, there are more or less pronounced remnants of all the other protoplasmic qualities. Thus, though we may call one tissue par excellence metabolic, all the tissues are, to a greater or less extent, metabolic. The energy of each, whatever be its particular mode, has its source in the breaking up of the protoplasm. Chemical changes, including the assumption of oxygen and the production, complete or partial, of carbonic acid, and therefore also entailing a certain amount of secretion and excretion, must take place in each and every tissue. And so with all the other fundamental properties of protoplasm; even contractility, which, for obvious mechanical reasons, is soonest reduced where not wanted, is present in many other tissues besides muscle. And it need hardly be said that each tissue retains the power of assimilation. However thoroughly the material of food be prepared by digestion and subsequent metabolic action, the last stages of its conversion into living protoplasm are effected directly and alone by the tissue of which it is about to form a part.
Bearing this qualification in mind, we may draw up a physiological classification of the body into the following fundamental tissues:
1. The eminently contractile: the muscles.
All these separate tissues, with their individual characters, are, however, but parts of one body; and in order that they may be true members working harmoniously for the good of the whole, and not isolated masses, each serving its own ends only, they need to be bound together by coordinating bonds. Some means of communication must necessarily exist between them. In the mobile, homogeneous body of the amœba, no special means of communication are required. Simple diffusion is sufficient to make the material gained by one part common to the whole mass, and the native protoplasm is physiologically continuous, so that an explosion set up at any one point is immediately propagated throughout the whole irritable substance. In the higher animals the several tissues are separated by distances far too great for the slow process of diffusion to serve as a sufficient means of communication, and their primary physiological continuity is broken by their being imbedded in masses of formed material, the product of the indifferent tissues, which, being devoid of irritability, present an effectual barrier to the propagation of molecular explosions. It thus becomes necessary that, in the increasing complexity of animal forms, the process of differentiation should be accompanied by a corresponding integration, that the isolated tissues should be made a whole by bonds uniting them together. These bonds, moreover, must be of two kinds.
In the first place, there must be a ready and rapid distribution and interchange of material. The contractile tissues must be abundantly supplied with material best adapted by previous elaboration for direct assimilation, and the waste products arising from their activity must be at once carried away to the metabolic or excretory tissues. And so with all the other tissues. There must be a free and speedy intercourse of material between each and all. This is at once and most easily effected by the regular circulation of a common fluid, the blood, into which all the elaborated food is discharged, from which each tissue seeks what it needs, and to which each returns that for which it has no longer any use. Such a circulation of fluid being in large measure a mechanical matter, needs a machinery, and calls forth an expenditure of energy. The machinery is supplied by a special construction of the primary tissues, and the energy is arranged for by the presence among these of contractile and irritable matter. Thus, to the fundamental tissues there is added, in the higher animals, a vascular bond in the shape of a mechanism of circulation.
In the second place, no less important than the interchange of material is the interchange of energy. In the amœba, the irritable surface is physiologically continuous with the more internal protoplasm, while each and every part of the body has automatic powers. In the higher animals portions only of the skin remain as eminently irritable or sensitive structures, while automatic actions are chiefly confined to a central mass of irritable or nervous matter. Both forms of irritable matter are separated by long tracts of indifferent material from those contractile tissues through which they chiefly manifest the changes going on in themselves. Hence the necessity for long strands of eminently irritable tissue to connect the skin and contractile tissues, as well with each other as with the automatic centres. Similar strands are also needed, though perhaps less urgently, to connect the other tissues with these and with each other. To the vascular bond there must be added an irritable bond, along the strands of which impulses, set up by changes in one or another part, may travel in determinate courses for the regulation of the energy of distant spots. In other words, part of the irritable tissues must be specially arranged to form a coördinating nervous system.
Still further complications have yet to be considered. In the life of a minute homogeneous amœba, possessing no special form or structure, there is little scope for purely mechanical operations. As, however, we trace out the gradual development of the more complex animal forms, we see coming forward into greater and greater prominence the arrangement of the tissues in definite ways to secure mechanical ends. Thus the entire body acquires particular shapes, and parts of the body are built up into mechanisms, the actions of which are to the advantage of the individual. Into the composition of these mechanisms, or "organs," the active, fundamental tissues, as well as the passive or indifferent tissues, enter; and the working of each mechanism, the function of each organ is dependent partly on the mechanical conditions offered by the passive elements, partly on the activity of the active elements. The vascular mechanism, of which we have just spoken, is such a mechanism. Similarly, the urgent necessity for the access of oxygen to all parts of the body has given rise to a complicated respiratory mechanism; and the needs of copious alimentation, to an alimentary or digestive mechanism.
Further, inasmuch as muscular movement is one of the chief ends, or the most important means to the chief ends of animal life, we find the animal body abounding in motor mechanisms, in which the prime mover is muscular contraction, while the machinery is supplied by complicated arrangements of muscles with such indifferent tissues as bone, cartilage, and tendon. In fact, the greater part of the animal body is a collection of muscular machines, some serving for locomotion, others for special manœuvres of particular members and parts, others as an assistance to the senses, and yet others for the production of voice, and, in man, of speech.
Lastly, the simple automatism of the amœba, with its simple responses to external stimuli, is replaced in the higher animals by an exceedingly complex volition, affected in multitudinous ways by influences from the world without; and there is a correspondingly complex central nervous system. And here we meet with a new form of differentiation unknown elsewhere. While the contractility of the amœbal protoplasm differs at the most but slightly from the contractility of the vertebrate striated muscle, there is an enormous difference between the simple irritability of the amœba and the complex action of the vertebrate nervous system. Excepting the nervous or irritable tissues, the fundamental tissues have in all animals exactly the same properties, being, it is true, more acute and perfect in one than in another, but remaining fundamentally the same. The elementary muscular fibre of a mammal is at most a mass of but slightly differentiated protoplasm, forming a whole physiologically continuous, and in no way constituting a mechanism. Each fibre is a counterpart of all others; and the muscle of one animal differs from that of another in such particulars only as are wholly subordinate. In the nervous tissues of the higher animal, on the contrary, we find properties unknown to those of the lower ones; and, in proportion as we ascend the scale, we observe an increasing differentiation of the nervous system into unlike parts. Thus we have what does not exist in any other tissue—a mechanism of nervous tissue itself, a central nervous mechanism of complex structure and complex function, the complexity of which is due not primarily to any mechanical arrangement of its parts, but to the further differentiation of that fundamental quality of irritability and spontaneity which belongs to all irritable tissues and to all native protoplasm.
In the following pages I propose to consider the facts of physiology very much according to the views which have been just sketched out. The fundamental properties of most of the elementary tissues will first be reviewed, and then the various special mechanisms. It will be found convenient to introduce early the account of the vascular mechanism, and of its nervous, coördinating mechanism, while the mechanisms of respiration and alimentation will be best considered in connection with the respiratory and secretory tissues. The description of the purely motor mechanisms will be brief, and, save in a few instances, confined to a statement of general principles. The special functions of the central nervous system, including the senses, must of necessity be considered by themselves. The tissues and mechanism of reproduction naturally form the subject of the closing chapter.