General Physiology: An Outline of the Science of Life

Hence we arrive at the important fact that of all organic substances proteids alone are indispensable to the nutrition of animals, and in certain cases also they alone suffice to maintain the animal's life. Pfliiger, therefore, distinguishes proteid as the primitive food from the carbohydrates, fats, etc., which act only as substitute foods. In addition to food proper in the narrow sense, all organisms take in oxygen — a process that is termed respiration. Of course all organisms do not receive oxygen in the same form and from the same source. Terrestrial organisms take it in the form of gas from the air ; aquatic organisms use the oxygen dissolved in the water; and the tissue-cells of animals that are provided with a blood-circulation, as well as many parasitic organisms, withdraw it from chemical compounds — the tissue-cells from the haemoglobin of the blood, with which it is loosely combined, and certain parasites from relatively fixed combinations. All organisms take only a certain quantity of oxygen, even when more is offered ; their consumption of it is not essentially increased in a medium of pure

oxygen. Hence within certain limits living substance is fairly independent of the quantity of oxj-gen that is at its disposal. But all organisms without exception absolutely require for their life a certain quantity of oxygen. If separated from it they invariably die after a shorter or longer time. Without respiration no life exists. Finally, all organisms without exception take in water, and with it certain salts, which, in so far as they are not contained in the other food, are likewise essential to the maintenance of life, although wide differences prevail among the different organisms as regards the kind of salts required. Salts of sodium, potassium, magnesium, calcium, and iron, containing phosphorus, sulphur, carbon, and chlorine, appear to be essential to all organisms.

We have thus reviewed the food-stuffs of organisms ; we will now consider how the individual cell takes in this food. Food-stuffs exist partly in the gaseous, partly in the liquid, i.e., dissolved, and partly in the solid condition ; but by no means all living cells are able to take in solid food. The great majority of all cells, almost all animal tissue-cells, a great number of plantcells, and many unicellular organisms take in dissolved food only, the latter either primarily consisting exclusively of dissolved substances, or being transformed from the solid to the dissolved state by the agency of certain secretions outside the cell-body. Only relatively few kinds of cells are fitted for the ingestion of solid food.

The process of ingestion of gaseous and dissolved food-stuffs, which is termed resorption, is essentially different, according as the cells in question do or do not possess a cell-membrane. In cells that do not possess a membrane all dissolved food-substances of whatever kind pass directly into chemical relations with the materials of the living substance at the surface of the protoplasm. W'here a membrane is present, it is necessary that the food-stuffs have the power of diffusing through membranes. The substances that cannot do this must, therefore, first be transformed into diffusible substances in order to reach the interior of the cell.

Every cell, however, is capable of ingesting gaseous and dissolved food. In plants the carbonic acid and oxygen of the air come into direct contact with the cells of the leaves. A similar arrangement is found in the lungs of vertebrates. The finest branches of the bronchial tubes end in small blind sacs, the so-called pulmonary alveoli, which are formed by an extremely thin layer of epithelium-cells and are surrounded by a close network of likewise very thin-walled blood-capillaries. The oxygen of the air inspired

into the lungs passes readily through the thin walls, to be eagerly sucked up by the red blood-corpuscles and transported throughout the whole body. Dissolved substances also always bathe the surface of the cells. In the plant they ascend along with the water in fine tube-like canals and thus are brought directly to the cells. In the compound animal body some of the cells, such as those of the intestinal epithelium, are in immediate contact with the dissolved food-stuffs of the intestinal tract, while all the other tissue-cells are bathed by the blood-current, which brings to them the dissolved food in a definitely elaborated form. In such invertebrate animals also as possess no proper blood-circulatory system, the cells either stand in immediate contact with the surrounding water or are supplied with juices that bathe the cells in fine intercellular spaces. The simplest relations, finally, exist in unicellular organisms, such as Algcv, Bacteria, and others, which live constantly in a nutrient solution, either in water containing salts or in organic liquids.

The ingestion of solid food occurs in only a few cell-forms. Among unicellular organisms all Eliizopoda, most ciliate Infusoria, and some flagellate Infusoria, take in solid food. In the complex cell-community this power is possessed by the leucocytes or white blood-corpuscles, which, therefore, have been termed by Metschnikoff phagocytes (eating cells), by amoeboid wandering cells, which play in the lower animals the role of leucocytes, by amoeboid egg-cells, such as occur in sponges, and by the intestinal epitheliumcells. Among these forms of cells two types may be distinguished, according to the manner of ingestion of solid food. The one type is able to take the food-masses into its living substance at any desired point upon its surface — such are all amoeboid cells, to which belong Khizopoda, leucocytes and intestinal epithelium-cells ; the other type possesses a special, constant mouth-opening — such are the ciliate and the flagellate Infusoria, which have a definitely fixed body-form with a denser cutaneous layer. All cells, however, that take in solid food are able to do it only by means of active movements of the body.

The ingestion of food by Amoeba may serve as an example of the first type. The process, which has been observed in full only relatively seldom, takes place somewhat as follows. An Amoeba, which is being observed in a drop of water under the microscope, creeps about the glass slide by letting the living substance of its formless protoplasmic body flow here and there into broad, lobate projections (Fig. 43). Suddenly it turns toward a small alga-cell lying in the vicinity, and creeps on until it touches the cell. Its protoplasm immediately begins to flow around the latter in the form of the usual lobate pseudopodia ; but the cell is shoved away by the encroaching protoplasm and the amoeba is obliged to make a new attempt to surround the cell. After several fruitless attempts it

frequently succeeds in bringing the cell into such a position and so holding it fast by a delicate viscous secretion that its pseudopodia are able to grasp the alga completely. Then, by flowing more and more about the cell, the protoplasm encloses it gradually on all sides, and the alga finds itself surrounded by a thin covering of water, forming the so-called food-vacuole, in the interior of the amoeba, which then creeps on unhindered. Amoeba, therefore, takes in solid food by causing its protoplasm simply to surround the food-mass. But the act does not always go on so smoothly. The difficulties that arise before the food-mass, which yields continually to the pressure of the encroaching protoplasm, is so fixed that the protoplasm can enclose it upon all sides, are frequently so great that not rarely the amoeba, with its pseudopodia flowing on continually in

FIG. 43. — Amoeba devouring an alga-cell. Four successive stages of the process of food-ingestion. other directions, is taken away from its victim, and must creep toward it anew in order to seize it, if it has not been taken entirely out of the sphere of influence of the food-mass. The ingestion of food by other lihizopuda takes place exactly as in the case of Amoeba, whether they have pseudopodia that are thick and broad, fine and thread-like, or branched and treelike. If the food-bodies are motile organisms, e.g., Infusoria, they usually cause the excretion of a viscous substance by stimulation resulting from their swimming against the rhizopod body ; this is increased by stimulation arising from their attempts to escape ; hence they stick firmly and can be drawn into the protoplasm. The amoeboid wandering-cells and leucocytes also, like Amoeba, ingest solid substances which exist in the blood or in the interstitial spaces between the cells. As the admirable work of Metschnikoff ('83, '84) has lately shown, they possess very great importance in the protection of the body from infectious diseases by devouring the bacteria that have entered a wound; they thus prevent the increase of the bacteria and protect the body from further infection (Fig. 44). Finally, the ingestion of microscopic fatdroplets on the part of the intestinal epithelium-cells represents the same mode of food-ingestion. In lower animals — e.g., in worms — these cells are really amoeboid cells, and by means of their pseudopodia flow around the fat-globules of

the digested food (Fig. 45, A). In the higher animals, such as man and other mammals, however, the intestinal cells are somewhat modified. They are cylindrical cells that possess upon their free surface, turned toward the lumen of the intestine, a striated border. As Thanhoffer (74) has shown, this striated border represents really nothing more or less than fine, pseudopodiumlike, protoplasmic processes, which can be extended and retracted, and with which the cells, exactly like Amoeba, flow around the fat-droplet and draw it into its body (Fig. 45, £).

The phenomena are wholly different in the second type of foodingestion, where the cell has a firmer superficial layer of a fixed form, and only a small opening, the cell -mouth, which. leads directly into the liquid endoplasm. Here the movement of the cilia and flagella of the cell exclusively mediates the ingestion of solid substances. The delicate Vorticella may serve as an example, a ciliate infusorian whose bell-shaped cell-body sits upon a contractile stalk and bears at its broad end a spiral-like circlet of cilia (Fig. 46).

FIG. 44. — Leucocyte from the frog devouring a bacterium. Three successive stages in the ingestion of food. (After Metschnikoff.) FIG. 45.— A. Intestinal epithelium-cells from the liver-fluke, possessing pseudopodium-like protoplasmic processes for the ingestion of blood-corpuscles,a, &,and drops of chyle, c. (After Sommer.) B. Intestinal epithelium-cells from the vertebrate, ingesting fat. In the interior of the cells single microscopic fat -droplets are found. (After Thanhoffer.)

At the bottom of this spiral-like ciliated funnel is a cell-mouth, which is prolonged a short distance into the protoplasm as the cellpharynx, and then gradually disappears into the liquid endoplasm. The cilia of the ciliary wreath of the peristorne contract continually and rhythmically, and in this way produce in the water a whirlpool, which is so directed that it sucks small particles, such as detritus, mud, bacteria, alga3, etc., which are suspended in the water, into the cell-mouth ; from there, surrounded by a layer of water, they are shoved by contractions of the body into the cell-pharynx, and thence into the endoplasm (Fig. 46). The phenomena may be very easily observed, if, according to Ehrenberg's method ('38), granules of carmine or indigo be mixed with the water. It is seen at once how the Vorticella engulfs the red or blue granules and forms them in its protoplasm into balls which are surrounded by a covering of water and constitute food-vacuoles.

The mode of ingestion of food lay other Infusoria is entirely similar to that of Vorticella. The free-swimming forms frequently seek fixed FIG. 46. — Vorticella in four successive stages of the process of food-ingestion. An alga-cell is being engulfed into the cell-mo ath and taken through the pharynx into the endoplasm. food-masses and engulf them. Many Infusoria even, such as Goleps, a small, egg-shaped, ciliate form having a delicate latticelike surface, take in large balls of food which are broader than their mouth-opening by pressing the latter upon the ball by the force of the ciliary contraction so that the mouth-opening, as in a snake, is gradually enlarged. Thus they really suck the food-balls into their bodies (Fig. 47).

The ingestion of solid food on the part of the cell is, therefore, in every case brought about by active movements of the cellprotoplasm or its motile organoids. In the ingestion of substances by the living ceil, one phenomenon deserves special mention — namely, the fact of the selection of food. Of the various cells living in the same medium, each takes to itself different materials, and such as are necessary for the formation of its characteristic substance. This is clear in the tissuecells of highly organised animals, such as the human body. Here the blood-plasma is the common nutrient material for all tissue-

cells. But from this common nutrient liquid each kind of cell removes the substances necessary for its life ; the mucous cell takes substances different from those taken by the ganglion-cell, the muscle-cell substances different from those taken by the cartilage-cell, the liver-cell substances different from those taken by the sense-cell, and so on. The different cells choose entirely different materials, each one according to its need. This phenomenon of food-selection is, perhaps, more remarkable in certain free-living cells that take in solid food. Cienkowski

FIG. 47. — Four individuals of Coleps hirtus swarming about and ingesting a ball of food. ('65), who has studied in detail the life of the lowest Rhizopoda, the naked monads, gives an interesting description of how Colpodella and Vampyrella, two simple, naked rhizopod-cells, procure their food, which consists of living alga-cells. Cienkowski relates as follows : " Although the zoospore- and amoeba-conditions of the monads are only naked protoplasmic bodies, their behaviour in seeking and ingesting food is so remarkable that it seems to be the work of conscious beings. Thus, Colpodella pugnax pierces the Chlamydomonas, sucks up the chlorophyll that flows out, and

runs away. A second rare case of this kind is afforded by Vampyrella Spirogyrce. The amoeba of this species applies itself to a healthy Spirogyra, bores through the cell-wall and devours the slowly escaping primordial utricle together with the chlorophyllbands. It seems to be able to satisfy its hunger upon Spirogyra only." (Fig. 48.) But we need not search so far. In the human body there are cells that behave similarly. As Metschnikoff ('92) has shown by his researches extending over many years, the leucocytes or white blood-corpuscles, the amoeboid wandering-cells, devour and digest certain forms of bacteria present in the body, while they scorn and even directly avoid other bacteria; likewise, intestinal

FIG. 48.—Vampyrella Spirogyrce boring into and sucking out a Spirogyra-cell. A. The Spirogyra-cell is pierced and the contents are passing out into the Vampyrdla. B. The Spirogyra-cell is completely emptied. At * a cell that has been pierced and A emptied. (After Cienkowski.) epithelium-cells, as has been seen, devour only fat-droplets, while they behave wholly passively toward other small particles that are brought into the intestine, such as granules of carmine.

Finally, another very interesting phenomenon, which has to do with the ingestion, not of food, but of substances that likewise play a role in the life of the organisms in question, has also frequently been referred to, although incorrectly, as a power of selection on the part of the cell. This is the ingestion of material for shells and capsules on the part of certain shell-bearing rhizopods. The Difflugice, which are unicellular fresh-water Rhizopoda whose naked' protoplasmic bodies are fixed in a very delicate urn-shaped or flask-shaped capsule, take up the material for their tiny dwellings with their finger-like pseudopodia out of the mud of the pools and lakes at the bottom of which they live.1 The structural material of their shells is very varied, but in many

cases the shells are composed of one definite material (Fig. 49). Thus, forms occur that employ only cases of the silicious Algce or diatoms, whileothers em ploy only sand-grains of certain sizes, and still others particles of mud. It has been thought that the Difflugim select their material from substances at their command. But it can be proved, at least in some cases, that no real selection exists here in the same sense as in food-ingestion by the above-mentioned cells. The fact that forms from one and the same locality employ only a certain material depends rather upon the circum-

Fio. 49.— Various Difflugia-shells, constructed of : A, diatom-cases ; B, fine sand -grains ; C, fine and coarse sand-grains ; J), diatom cases and sand-grains ; £, coarse sand-grains ; F, the same form as E, but made of splinters of blue glass. stance that in the given locality only this one material is at hand. If, e.g., the dwelling-place of the form that constructs its shell out of mud or substance excreted from its body be examined, it is found that here other materials, perhaps diatom-cases or sandgrains, are wholly wanting. If, however, such forms be given the possibility of getting other material, by the introduction of very finely pulverised sand or, still better, very finely ground, coloured glass into the culture-vessel in which they live, it is found that the individuals arising by reproduction surround themselves with a delicate shell of sand or splinters of coloured glass.1 The circum- 1 Cf. Vervvorn ('90, 1).

stance that some shells possess small sand-grains, and others considerably larger ones is likewise to be referred in part to the character of the material at their disposal, in part, however, to other external conditions, such as the narrowness of the opening of the capsule, which does not allow the protoplasmic body to draw through large sand-grains. It accordingly appears that in most cases the construction of the capsule by Difflugice involves no real selection of material, and thus far no case has become known where such a selection has really been established with certainty. There is, therefore, no justification in drawing a parallel, as is often done, between the ingestion of structural material in the building of the Difflugia-c&psule and the act of food-selection by _the living cell.

The process of construction of living substance out of the ingested food-stuffs can be designated best by generalising, as is frequently done, a conception of the botanists and employing the word assimilation. By assimilation in the narrow sense has been understood for a long time in botany the synthetic formation in plants of the first visible organic material, starch, out of the ingested inorganic compounds. But it is advantageous to extend the conception and employ it also for the construction of higher organic compounds, especially the proteids, and, indeed, not only in plants, but also in animals. By assimilation, therefore, is understood the sum of the processes that lead to the construction of living substance to the maximum of its most complex constitution, the synthesis of proteids. Construction, or assimilation, can then be contrasted with destruction, or dissimilation.

" Corpora non agunt nisi soluta" This old dictum plays in the life of the cell a very great role. In order that the ingested foodstuffs may work chemically and be of use for the construction of living substance, they must be in a dissolved condition ; since, however, the food taken in by the organism is in part solid food, it must first be transformed into soluble form, and this process is termed digestion. It has been seen that only a few cells have the power of taking in solid food; in these there occurs so-called intracellular digestion, the transformation of the solid food into soluble compounds taking place in the interior of the cell. The great majority of cells, however, cannot take in solid food ; in them, therefore, the transformation of the solid into the soluble form must take place outside of the cell, in order that ingestion may be possible ;

this transformation is, therefore, termed extracellular digestion, and the ingestion of the dissolved food, resorption. The change of solid food, such as coagulated proteids, starches and fats, into soluble compounds takes place through the action of definite secretions which the cell-body gives off to the outside. These characteristic secretions are called enzymes or unorganised ferments. The result of their action can be demonstrated outside FIG. 5Q.—lieberMhnia, a fresh-water rhizopod, from the egg-shaped shell of which branched pseudopodial filaments protrude.

the organism by allowing an enzyme, e.g., pepsin, which is produced by the cells of the gastric glands, to act upon a bit of coagulated proteid. If, e.g., there be placed in a beaker a solution of pepsin in water to which has been added an equal volume of 0*4 per cent, hydrochloric acid, there is obtained an artificial gastric juice. If there be put into this digestive solution a flake of fibrin, i.e., the proteid the spontaneous coagulation of which causes the clotting of

the blood outside the blood-vessels, and the beaker be warmed in a digestion-chamber to the body-temperature, it is found after some time that the solid flake of fibrin begins to swell, to become transparent upon the outside, and gradually to become dissolved in the liquid. Finally, the whole flake, as such, disappears, and in its place there is found dissolved in the liquid peptone, that modification of proteid which, as has already been seen, arises by a hydrolytic cleavage of the polymeric proteid molecule, is soluble in water, and diffuses through organic membranes. Besides the peptone there are found also certain transition-stages between the native albumin and the peptone, which are likewise soluble in water and are termed albumoses. We shall presently discuss more in detail the peculiar manner of working of the ferments.

That which happens, in extracellular digestion, outside the cellbody, and which can be imitated even in the test-tube, takes place in intracellular digestion within the protoplasm. Likewise here the process can be followed best in the naked protoplasmic body of Rhizopoda. Lieberkuknia is a large fresh-water rhizopod, from the egg-shaped, membranous shell of which thick, branching pseudopodial filaments protrude through an opening at the pointed pole (Fig. 50). When the Lieberkuhma seizes and digests1 an infusorian that carelessly swims against its pseudopodia, it can be seen with the microscope that the prey first becomes attached to the pseudopodia, entangles itself more and more firmly by its strong efforts to escape, and gradually becomes surrounded either wholly or partially by the pseudopodial protoplasm (Fig. 51). For some time the movements of the infusorian continue ; then they become feebler, and at the same time its body-form begins to change. It decreases in size constantly, while the liquid and granular parts of its protoplasmic body pass over into the pseudopodial protoplasm, mix with it, and are no longer seen to stream to the central body of the Lieberkuhnia. Thus, gradually, the whole body of the infusorian becomes dissolved and its liquefied contents mix with the protoplasm of its captor, until none of it is longer distinguishable. In other cases of intracellular digestion the food-body, e.g., in Amoeba and Infusoria, becomes surrounded by a food-vacuole within the endoplasm, and is dissolved in the same manner as in the exoplasm of the Lieberkuhnia. Further, the observations that have been made upon the Infusoria by Greenwood ('94) are very interesting. She followed the fate of the ingested food-masses in the Vorticellinw, especially in Carchesium (Fig.