General Physiology: An Outline of the Science of Life

cilia be cut into at one place, the two halves are able to act independently of one another.1 Even if a single cilium with a droplet of attached protoplasm be cut off from the cell-body, it acts rhythmically and spontaneously until it perishes. It must, therefore, be assumed that the complete dependenceof the individual cilium and the individual ciliated cell upon the one next it, is conditioned by some kind of mechanism in the basal protoplasm, which hinders all independent movement, arid mediates only impulses from that side.2 But this is only possible when in the ciliated epithelium an unbroken continuity of the basal protoplasm exists throughout the whole row of cells. It is known that protoplasmic connections between the individual cells in the cellcommunity are wide-spread in both plants and animals.

Finally, the most thorough -going despotism exists in the higher animal in the dominion of the nerve-cells over the cells of all kinds of tissues. The higher we go in the animal series, the more we see the tendency of the nerve-cells to extend their dominion to all the tissues of the body. The loss of independence thus resulting goes so far in many tissue-cells, that their vital activity sinks to a minimum so long as it is not stimulated by impulses from the nervecells. Spontaneity is apparently wholly lost. A skeletal muscle in the vertebrates never performs a contraction spontaneously ; the ganglion-cells of the central nervous system alone by their impulses are able to put it into contraction. We ought not to be misled by this lack of spontaneous contractions into believing that the metabolic processes that characterise muscular activity are at a complete standstill during rest. This is only apparently the case. As a comparison of the arterial blood streaming to the muscle with the venous blood coming from it teaches, the same metabolic processes go on in the muscle during rest as during activity, but in so slight extent and so uniformly that a contraction is not thus brought about. But if by nervous influence they undergo a sudden augmentation, the contraction appears. Wholly analogous to the dependence of the muscle-cells is the relation of many other tissue-cells, e.g., gland-cells, to the central nervous system ; and even the relation of the ganglion-cells to one another is partly of the same kind.

The general principle upon which is based the formation of the cell-community, and with it the formation of a more or less close dependence of the individual cells upon one another, is the principle that controls all development. It is the principle of utility. The fact that the cells remain together after division and thus form a community consisting of several like components, which occurs in the Protista, secures the advantage of greater protection for the individual cell. But, as has been seen, a certain dependence of the individual cells upon one another is conditioned by this simple

fact. That this dependence, the higher we ascend in the developmental series of organisms, becomes closer and more fixed, depends again solely upon utility, for the greater the unity in the government of the whole community, the surer and greater is the work of the whole, and the greater also is the advantage that the individual cell receives from the common life. Unity in the government of the cell-community is, however, determined essentially by the dependent relation of the individual cell to the other cells. Darwin's theory of selection, which contains a general explanation of adaptation in the organic world, has made it clear how such adaptative arrangements must be developed in a natural manner. Of course, the immediate mechanical causes are to be sought in each individual case.

In the evolution of mutually dependent relations between the cells in the origin of the cell-community we have become acquainted with only one result of the common life of the cells. It is the sole result, so long as the community does not surpass certain dimensions. If, however, the community becomes larger, if it develops into a compact mass, another necessary mechanical result of the association is observed, namely, the differentiation of and division of labour among the cells.

The differentiation of cells consists, as is well known, in the assumption by the cells of different characters, so that the community is no longer composed of like cells, but of cells and cell-groups of different kinds. Therewith there appear not only morphological, but also physiological differences between the individual cells, i.e., the performances of certain cells or cell-groups become different from those of others, and a division of labour between them takes place. Differentiation and division of labour are inseparable from one another.

The mechanical causes of cell-differentiation in the cell-community are fairly evident. All the properties of an organism, morphological as well as physiological, are the expression of the interaction of two factors, namely, the relations between its internal and its external vital conditions.1 If one of these two factors changes, there is a change of the properties of the organism. If, therefore, a cell divides into many like offspring, and if all these offspring remain together and form a cell-community, all the constituents of this community will remain alike, so long as the external conditions surrounding each cell are the same as those surrounding all the others. We have become acquainted with such cell-communities among the Protista. But such a community is only possible when all the cells are arranged beside one another

to form a row or a surface. This is the case here. The largest cell-communities composed of like cells that are known among the Protista are those that are related to the Algce among the plants. They are either fibrous, such as the Confervas (Fig. 276), or foliaceous, such as the large Ulvacece. In the latter, cell is attached to cell to form a flat surface, so that the part of the cell-surface that is free, and the part that borders upon its neighbours are the same in every cell ; thus all cells are under like external conditions. But, if the cells proceeding from the division of one cell do not all remain under like external conditions, and if the cells do not perish, in time differences must appear. This condition is realised in the formation of every cell-community the component cells of which are not arranged in a flat surface, but are distributed in all directions as solid complexes. Here the cells that lie in the interior of the community are under wholly different external vital conditions from those at the surface. As a result of this they must form a contrast to the latter, both morphologically and physiologically; in other words, differentiation and division of labour result. The simplest examples of this are likewise met with in certain forms of the Protista, which form such an extremely interesting transit ion - stage to the cell-communities of the plants and the animals. Such an organism is the Protospongia Hceckelii (Fig. 277), a colony of flagellate Infusoria, which as regards histological structure has a certain similarity to the lowest sponges. Upon the surface of a gelatinous mass sit numerous, cup-shaped, flagellated cells, while in the interior of the mass there are many amoeboid cells without

flagella. Here, therefore, is a differentiation of the cells living in the interior as compared with those living upon the surface, which is extremely marked and the cause of which is at once evident. It is especially interesting in connection with this organism that the differentiation exists only so long as the causes exist. The amoeboid cells of the interior, for example, have the power of wandering to the surface, and in this case they likewise develop into cup-shaped flagellated cells. In these lowest forms of the differentiated cell-community, therefore, the individual cells still possess in the highest degree the capacity of changing into other forms.

Differentiation of the cells by adaptation to the external conditions afforded by different positions, which is only barely indicated in the Protista, is the fundamental principle in the construction of FIG. W^.-Spiroffyra, a multicellular, fresh-water Alga. A, Piece of a multicellular thread. B, Single cell. In every cell the chlorophyllbody winds spirally along the inside of the wall. realised most completely and in the smallest details, and finally

leads to the construction of so complex an organism as the cellmost complex animal body with all its differentiations depends solely upon the principle that the farther cell-increase proceeds upon the simple mechanical basis of the different relative positions of the cells and cell-groups arising from the continued division of the ovum, the more various must be the mutual relations and the external vital conditions of these cells and cell-groups, so that, by

adaptation to the constantly changing external conditions, the cells and cell-groups finally diverge and become gradually differentiated as regards all their characteristics. As is known from the fundaessential features in FlG- Z'n.—Prototpongia H<KckelU. (After Lang.) phylogeny. It remains for the embryology of the future to discover in detail the very manifold special relations, which are as different as the organisms themselves. While the mechanical causes of cell-differentiation in the complicated cell-community must be sought in changes of its relations with the environment, which for every cell and cell -generation are due to continued cell-division, division of labour among the cells is based upon the development of the cell-community itself1 The work of every multicellular organism is the expression of the activity of its individual cells. If the cells are different, they contribute in a different manner to the whole labour of the organism. That this combined labour must become harmonious and advantageous follows from the principle of selection, which controls all organic development, phylogenetic as well as ontogenetic. Only those cell-communities continue to live, in which the cell-generations

arising from the continued division of the egg-cell are in harmony with the special conditions under which they appear. All in which this is not the case must perish in the struggle for existence through the action of selection. But the most complete harmony is reached when the individual labours of the different cells so fit into one another that, although every cell or cell-group has developed a different labour for its own specialty, this labour is for the good of all the other cells, is, indeed, necessary to all the others. Thus, the extraordinarily far-reaching differentiation and surprisingly detailed division of labour of the individual cells and tissues in the cell-community become comprehensible.

As a result of the division of labour, every kind of cell, every tissue, every organ in the inulticellular community undertakes a special task, and since early times physiology has termed this task the " physiological function " of the cell-complex in question. All elementary vital phenomena which, in the lowest organisms, take place in the individual cell, in multicellular organisms are developed in a special degree as specific functions of definite cell-groups and become adapted very perfectly to specific purposes. Thus, in the higher animals, by the special development of contractility, movement becomes the specific function of the muscle-cells. The capacity of appreciating stimuli is developed in an especially high degree as the function of the sense-organs. The capacity of conducting stimuli is augmented to a surprising extent as the function of nerves. Secretion undergoes its greatest perfection in the function of gland-cells. Every kind of cell retains all the elementary vital phenomena, but the one becomes preeminently developed as its specialty. The more the specialties of the individual cells and cell-groups come to act for the good of all cells and assist their vital processes, the more highly evolved does the cell-community become. It represents a mechanism in which, in spite of its extraordinary extent and its excessive complication, as exhibited especially in the bodies of the higher animals, all its parts co-operate as a unit.

If the last point, namely, the development of a unity in the cooperation of the cells and tissues of the cell-community be developed more in detail, it is found that in addition to the principles of dependence and cell-differentiation, a third principle comes into consideration, namely, that of centralisation of administration. This principle is connected very closely with the two others ; considered from the point of view of natural selection, it is in a certain sense a necessary result of those, and it is developed pari passu with them.

The farther the differentiation of the cells goes and the closer becomes the dependence of the cells upon one another, the stronger is the necessity of bringing into relation with one another the more outlying cells, tissues and organs of the cell-community, in order that unified co-operation may take place ; selection must make this relation constantly more intimate, the more complex the structure of the cell-community becomes. Along with this there arises in the community a tendency toward centralisation.

The first step in the direction of centralisation is really taken by means of the division of labour, when certain cellgroups or organs undertake a definite function for the whole community. Thus the function in question becomes centralised for the whole body in one place, and as many centres arise as there are organs differentiated for definite functions. This first step toward centralisation of administration is met with in the cell-community of the plant. Here the synthesis of starch, upon which the nutrition of the whole plant depends, is centralised in the green cells of the leaf. Further, the function of taking up water, without which life cannot continue to exist, is localised in the roots alone. Corresponding localisations are present in the animal cellcomm unity. Thus, in the higher animals the nutrition and respiration of the individual tissue-cells are centralised in the heart, which drives the blood, rich in food and oxygen, to all the cells of the various tissues and organs (Fig. 278).

In the animal cell-community the second important step toward centralisation is taken, namely, the union of all individual centres or organs of function with one another by the appearance of a central nervous system with its paths of conduction. This principle, in greater and greater perfection, leads finally in the animal series to a far-reaching centralisation, such as is met with in the complex cell-community of the vertebrates, and especially of man. We have in the central nervous system a central organ which alone has the function of uniting cells, tissues, and organs with one another, so that an advantageous co-operation of them becomes possible ; and the farther we ascend in the animal series,

FIG. 278.— Scheme of the circulation of blood in man. Centralisation of the nutrition of all cells in the blood -current. The darker half is the venous, the lighter the arterial system. The two are united by the capillary network of the lungs (above) and of the other tissues (below). In the capillaries the blood - current bathes all the tissues, the cells of which take food from it and give off to it their useless substances. (FromRanke.) the more we find the tendency of the central nervous system to extend its authority toward a unified control of all cells and cellcomplexes of the animal body. In order to make graphic the principle upon which the mechanics of the central nervous system is based, it will be advantageous to consider the simplest form in which the function of the latter is expressed, namely, the reflex action.

The essence of the reflex action consists in the fact that an element that appreciates stimuli and an element that reacts to stimuli are so put into relation with one another by a central bond, that every stimulus acting upon the appreciating element is conducted first to the centre, and thence, as an impulse to a reaction, to the reacting element. Such a mechanism, in which every stimulus acting upon the sensory end calls out with machine-like certainty a reaction at the other end, is a reflex arc. The most primitive

Fia. 279. — Primitive reflex arc in a single cell. /, Poteriodendron , a flagellated cell fixed in a cup-shaped sheath upon a myoid-fibre. //, Neuro-muscular cells from an actinian. (//, after Hertwig.) form of a reflex arc exists in unicellular organisms, the cell-body of which possesses both the sensory and the motor elements, and even functions also as the central bond for the two. A single Poteriodendron represents a reflex arc of the simplest kind (Fig. 279, /). The cell-body, fixed upon a myoid-fibre at the bottom of a delicate, cup-shaped sheath, bears a flagellum which is extremely sensitive. The slightest stimulus which acts upon the latter is conducted centripetally to the cell-body, and from there centrifugally to the myoid-fibre, and the action of the stimulus upon the flagellum is followed at once by the contraction of the fibre. Wholly analogous to this is the behaviour of Vorticella, except that in the latter the sensory elements are present chiefly in the form of the cilia of the peristome. The same relations, further, exist in the so-called neuro-muscular cells of the Ccelen-

terata (Fig. 279, II). Here, likewise, a cell possesses, upon the one side, a sensory element, and, upon the other, a contractile fibre, which contracts as soon as the sensory end-organoid is stimulated. What in all these cases is differentiated within a single cell, is in the nervous system of animals distributed to several cells. In the simplest case of the latter, three different cells are concerned. One cell, the sensory cell, receives the stimulus ; from this a centripetal nerve-path conducts to a central cell, the ganglioncell, and from here a centrifugal nerve-path conducts to a cell that performs the reaction, the motor end-cell (Fig. 280, A). But this form of reflex arc is realised perhaps only in the invertebrates. In vertebrates, so far as the conditions are known, a fourth cell at least is interpolated in the arc, since in place of one ganglion-cell at least two are present, one of which receives the stimulus from the sensory-cell and conducts it to the other, while the other transfers the impulse to the motor end-cell (Fig. 280, B). In a given case the end-cell of the centrifugal path may be either motor or secretory, or may produce light or electricity. Thus reflexly by the ganglion-cells parts of the cellcommunity, wholly different and far removed from one another, are put into union and activity by impulses from the central nervous system.

If we start from the scheme of the reflex arc, the further factors that come into consideration in the mechanism of the central nervous system are very simple. They consist only in the facts that, upon the one hand, between the sensory and the motor endorgan more than two ganglion-cells possessing different functions are interpolated, and, upon the other hand, certain ganglioncells are innervated not simply from one side, by a single other ganglion-cell, but by several, and under certain circumstances by many others. Thus, by means of their nerve-fibres very complex arid intricate connections are formed between the ganglion-cells and the individual systems of ganglion-cells, which latter are the centres of definite vital processes and hence the seat of definite impulses. A network of ganglion-cells and uniting nerve-fibres results, which is apparently inextricable, but in reality insures a very definite and unified co-operation of the various parts of the organism that it binds together. By the proper innervation of all

FIG. 280.— Schemes of the reflex arc. A, Simple scheme of reflex arc. At the left, below, a sensory cell ; in the middle, above, a central ganglion-cell ; at the right, below, a muscle-cell. B, Scheme of a reflex arc in vertebrates. At the left, below, a sensory cell, at the left, above, a sensory ganglioncell. At the right, above, a motor ganglion-cell, at the right, below, a muscle-cell. (After Gegenbaur.) kinds of cells, tissues and organs of the cell-community by the central nervous system, the cells of which form in vertebrates the brain and the spinal cord together with the sympathetic nervous system, a central system of administration for the whole cellcommunity is inaugurated, which from the brain and spinal cord by means of their long paths of conduction brings even most distant parts of the community under a unified control (Fig. 281). Hence the nervous system has been compared very graphically to a telegraphic network, the wires of which put the most distant regions of a country into connection with a central place of government. The comparison of a central nervous system to a great telegraph station and the nerve-fibres to the telegraph wires is very fitting with respect to the principle of centralisation upon which the two are based. But, as has sometimes happened, such a comparison ought not to be carried too far ; for example, the nerves should not be regarded simply as conducting-wires for electricity. In reality, nerves are extensions of ganglion-cells, and, like these, consist of living substance, i.e., they have a metabolism with which their life and, therefore, their function are inseparably connected. This follows directly from the fact that the nerve invariably perishes,

Fm.281.-Ner.vous system of man. The nerve-trunks, ^ any non-nucleated prowhich contain centrifugal and centripetal paths of tOplasmiC maSS, alter being conduction, pass from the brain and spinal cord to ~, - , ^l," all parts of the body, and thus unite the latter Cllt Ott irom tne ganglion- The manner in which the elements of the nervous system are united with one another anatomically and functionally deserves special attention, since the later researches upon the finer structure of the central nervous system, which have been made possible by the extraordinary development of the microscopic technique, especially by Golgi, Weigert, Ehrlich, and others, have led to the discovery of very peculiar but fixed relations. The element of the central nervous system is the ganglion-cell, but the ganglioncell with its characteristic differentiations. From the body of the cell there extend processes, more or less numerous according to the function of the cell, among which two kinds may be distinguished sharply from one another. Some form a more or less richly branched structure, and are, therefore, appropriately termed dendrites. The older histologists termed these protoplasmic processes. The others are the nerve-processes. So far as we now know, as regards the number of the latter there are only two varieties of ganglion-cells: unipolar (previously called multipolar on account of the numerous dendrites), provided with only one nerve-process, and bipolar, with two nerve-processes. These nerveprocesses are simply the beginning of the nerve-fibres, which not rarely reach a length of one metre and more. The conducting nerve puts even the most distant cells of the animal body into physical connection with the ganglion-cells, and transmits the impulses that go out from the bodies of the ganglion-cells to the tissue-cells, or in specific cases to other ganglion-cells. In its course from the body of the ganglion-cell to the cell that it innervates, the nerve-process appears different at different points. It sends off here and there collateral branches, and a little beyond its origin is surrounded by a sheath consisting of my elm, the medullary sheath.

The latter is divided into segments by the so-called nodes of Eanvier, and disappears shortly before the cell which the nerve supplies is reached. The medullary sheath, in which the nerve-fibre runs as the axis-cylinder, is itself usually surrounded by a membranous sheath, the neurilemma. The end of the nerve shows very characteristic differentiations according to the kind of cell which it innervates. Such a complete cell, i.e., a ganglion-cell with all its appendages, represents the elementary constituent of the nervous system, and can fittingly be termed with Waldeyer a neuron (Fig. 282). The combination of the innumerable neurons with one another constitutes the nervous system of the animal. According to the later researches of Golgi, Kb'lliker, His, Ramon y Cajal, and others, the connection of the neurons with one another appears to be everywhere of such a kind that the dendrites of the ganglion-cells receive the stimulating impulses, while the nerve-process transmits them from one ganglion-cell to the dendrites of another. The bipolar ganglion-cells, which are contained chiefly in the spinal ganglia lying at the two sides of the spinal

Fio. 282. — Scheme of a neuron ; a, free axis-cylinder ; 6, axis-cylinder surrounded by iieurilemma alone ; c, axis-cylinder surrounded by the medullary sheath alone ; d, axis-cylinder surrounded by the medullary sheath and the neurilemma, and divided intokegments by the nodes of Ranvier. (From Stohr.) cord, alone possess in their one nerve-process a sensory path, which receives impulses from the periphery in the form of external stimuli and transmits them to the cell-bodies ; thence the impulses are continued through the other nerve-process to other neurons. Hence, as regards the body of the ganglion-cell to which they belong, the dendrites conduct always centripetally, the nerveprocesses in the unipolar ganglion-cells always centrifugally. The greater or smaller number of the dendrites of a ganglion-cell

FIG. 283.— Cell of Purkiiije from the grey cortical layer of the brain. (From Stohr.) appears to depend upon the question, with how many other neurons the ganglion-cell is in connection. Thus, the cells of Purkinje in the grey cortical layer of the brain, in which the most complex psychic processes are believed to be localised, have an extraordinarily richly developed system of dendrites (Figs. 283 and 284). The nerve-fibres of one ganglion-cell pass to the dendrites of another ganglion-cell. It is here a noteworthy fact, that, according to the later investigations, the connection between the two takes place not by direct continuity of their substance, or, as

is said "per continuitatem" but through simple contact, "per contiguitatem" The end of a nerve-fibre and the end of a dendrite join at their tips, but a piece that does not consist of nerve-substance is intercalated between them. It must be assumed that this intercalated piece, which is to be seen only with very strong magnifying powers, consists also of living substance, else it would be difficult to understand how it is able to conduct the excitation from the nerveprocess to the dendrites.

While there is great unanimity in the mode of union of the neurons with one another, the kind of transition of the nervefibres into the end-cells, which they innervate, or from which they FIG. 284. — Section through the cortex of the cerebellum of a calf. The large, branched cells are Purkinje's cells. (After Schiefferdecker.) spring, is very various. The nerves (sensory) that conduct centripetally from the periphery of the body, as well as those (motor, secretory, electric, etc.) that conduct centrifugally to the periphery vary according to the organ in which they end. Among the former there are some that end free in the skin in the form of an end-bulb, without being in connection with a sense-cell (Fig. 285, 12). The others appear to go out directly from a sense-cell, which is specially developed for the reception of the stimulus, as, e.g., the rods and cones of the eye, the hair-cells of the ear, the olfactory cells of the nose (Fig. 285, 7), etc. Among the endings