Physics, we cannot ascend above the fact of Attraction (which operates according to a simple and universal law) between all masses of matter; and in Chemistry, we cannot rise beyond the fact of Affinity (limited by certain conditions which are not yet well understood) between the particles of different kinds of matter. When we say that we have explained any phenomenon, we merely imply that we have traced its origin to properties with which we were previously acquainted, and shown that it takes place in accordance with the known laws of their operation. Of the existence of the properties, and the determination of the conditions of their action, we can give no other account, than that the Creator willed them so to be; and, in looking at the vast variety of phenomena to which they give rise, we cannot avoid being struck with the general harmony that exists amongst them, and the mutual dependence and adaptation that may be traced between them, when they are considered as portions of the general economy of Nature. There is no difference in this respect between Physiology and other sciences; except that the number of these (apparently) ultimate facts is at present greater in physiology than it is in other departments, because we are not at present able to include them all under any more general expression. But, as will presently appear, a considerable degree of simplification appears practicable in our view of them; and although we may not be able to say why the structure called Muscular should possess contractility, and why the structure called Nervous should be capable of generating and conveying the force which excites that contractility to action, we may draw, from the study of the conditions under which they respectively manifest themselves, some indications of the existence of a common tie, such as that which binds together the planetary masses, at the same time that it weighs down the bodies on the surface of the earth towards its centre.

26. In the study of any branch of science, it is most desirable to commence with definite views of the nature of the phenomena with which it is concerned; and such are best gained by the examination of these phenomena under their simplest aspects. This course is most especially necessary in Physiology: since the complexity of the conditions under which its phenomena usually present themselves, often tends to mask their real character, causing that to be regarded as essential which is only accidental or contingent, and vice versa. It is extremly difficult, however, and frequently impossible, for the Physiologist to isolate these several conditions, and to study them separately, in the way that the Chemical or Physical investigator would do; and his best course is to take advantage of those "experiments ready prepared by Nature," which he finds in the variety of forms of living organized beings, with which the globe is so richly peopled. Now it is in the simplest forms of Cryptogamic Vegetation, that the phenomena of Life present themselves under their least complicated aspect; for we shall find in the operations of each of the simple cells of which such Plants are composed (all of them resembling one another in structure and actions), an epitome, as it were, of those of the highest and most complex Plant; whilst those of the higher Plants bear a close correspondence with those which are immediately concerned in the Nutrition and Reproduction of the Animal

Fig. 1.

taining reproductive mole.

body. And when we come to consider the proper Animal functions, we shall find that they are not so far removed in their essential nature from those of Plants, but that they may be ranked under the same category, and regarded as different manifestations of the same original forces. A Cell, then, in Physiological language, is a closed vesicle or minute bag, formed by a membrane in which no definite structure can be discerned, and having a cavity which may contain matters of variable consistence. Such a cell constitutes the whole. organism of such simple plants as the Protococcus nivalus (red snow'), or Palmella cruenta ('gory dew'); for although the patches of this kind of vegetation, which attract our notice, are made up of vast aggregations of such cells, yet they have no dependence upon one another, and the actions Simple isolated cells, conof each are an exact repetition of those of the rest. cules. In such a cell, every organized fabric, however complex, originates. The vast tree, almost a forest in itself, and the feeling, thinking, intelligent man, spring from a germ, that differs in no obvious particular from the permanent condition of one of these lowly beings. But whilst the powers of the latter are restricted, as we shall see, to the continual multiplication of new and distinct individuals like itself, those of the former enable it to produce new cells that remain in closer connexion with each other; and these are gradually converted, by various transformations of their own, into the diversified elements of a complex fabric. The most highly-organized being, however, will be shown to consist in great part of cells that have undergone no such transformation, amongst which the different functions performed by the individual in the case just cited, are distributed, so to speak; so that each cell has its particular object in the general economy, whilst the history of its own life is essentially the same as if it were maintaining a separate existence.

27. We shall now examine, then, the history of the solitary cell of one of the simplest Cryptogamic Plants, from its first development to its final decay in other words, we shall note those Vital Phenomena, which are as distinct from those of any inorganic body, as is its organized structure (simple as it appears) from the mere aggregation of particles in a mineral mass. In the first place, the cell takes its origin from a germ, which may be a minute molecule, that cannot be seen without a microscope of high power.* This molecule appears, in its earliest condition, to be a simple homogeneous particle, of spherical form; but it gradually increases in size; and a distinction becomes apparent between its transparent exterior and its colored interior. Thus we have the first indication of the cell-wall, and the cavity. As the enlargement proceeds, the distinction becomes more obvious; the cell-wall is seen to be of extreme tenuity and perfectly transparent, and to be homogeneous in its texture; whilst the contents of the cavity are

The modes in which new cells may be generated are various; but the above example is purposely drawn from one of those simple Algae, whose usual mode of multiplication is by "zoospores." (See the Author's "Principles of Physiology," ¿? 139–144.)

distinguished by their color, which is very commonly either green or crimson. At first they, too, appear to be homogeneous; but a finelygranular appearance is then perceptible; and a change gradually takes place, which seems to consist in the aggregation of the minute granules into molecules of more distinguishable size and form. These molecules, which are the germs of new cells, seem to be at first attached to the wall of the parent-cell; afterwards they separate from it, and move about in its cavity; and at a later period, the parent-cell bursts and sets them free. Now, this is the termination of the life of the parentcell; but the commencement of the life of a new brood; since every one of these germs may become developed into a cell, after precisely the foregoing manner, and will then in its turn multiply its kind by a similar process.

28. By reasoning upon the foregoing history, we may arrive at certain conclusions, which will be found equally applicable to all living beings. In the first place, the cell originates in a germ or reproductive body, which has been prepared by another similar cell that previously existed. There is no sufficient reason to believe that there is any ex-. ception to this rule. So far as we at present know, every Plant and every Animal is the offspring of a parent, to which it bears a resemblance in all essential particulars; and the same may be said of the individual cells, of which the Animal and Vegetable fabrics are composed. But how does this germ, this apparently homogeneous molecule, become a cell? The answer to this is only to be found in its peculiar property, of drawing materials to itself from the elements around, and of incorporating these with its own substance. The Vegetable cell may grow wherever it can obtain a supply of water, carbonic acid, and ammonia; for these compounds supply it with oxygen, hydrogen, carbon, and nitrogen, in the state most adapted for the exercise of the combining power, by which it converts them into those new compounds, whose properties adapt them to become part of the growing organized fabric. Here, then, we have two distinct operations;-the union of these elementary substances into that composite protoplasma, which seems to be the immediate pabulum of the Vegetable tissues;-and the incorporation of that product with the substance of the germ itself.

29. The first of these changes may be, and probably is, of a purely Chemical nature; and analogous cases are not wanting, in the phenomena of Inorganic Chemistry, in which one body, a, exerts an influence upon two other bodies, B and C, so as to occasion their separation or their union, without itself undergoing any change. Thus platinum, in a finely divided state, will cause the union of oxygen and hydrogen at ordinary temperatures; and finely-powdered glass will do the same at the temperature of 572°. This kind of action is called catalysis. A closer resemblance, perhaps, is presented by the act of fermentation; in which a new arrangement of particles takes place in a certain compound, by the presence of another body which is itself undergoing change, but which does not communicate any of its elements to the new products. Thus, if a small portion of animal membrane, in a certain stage of decomposition, be placed in a solution of sugar, it will occasion a new arrangement of its elements, which will generate two new products.

alcohol and carbonic acid. If the decomposition of the membrane have proceeded further, a different product will result; for instead of alcohol, lactic acid will be generated. And in a further stage of decomposition, the ferment is the means of producing butyric acid (the fatty acid of butter). There appears no improbability, then, in the idea, that the influence exerted by the germinal molecule is of an analogous nature; and that it operates upon the elements of the surrounding water and carbonic acid, according to purely chemical laws, uniting the carbon with the elements of water, and setting free the oxygen. This result of the nutritive operations of the simple cellular plants may be easily verified experimentally, by exposing the green scum, which floats upon ponds, ditches, &c., and which consists of the cells of a minute Cryptogamic Plant, to the influence of light and warmth beneath a receiver; it is found that oxygen is then liberated, by the decomposition of the carbonic acid contained in the water. We shall presently have to return to the consideration of the Chemical phenomena of living beings; and shall pass on, therefore, to consider those to which no such explanation .applies.

30. The second stage in the nutritive process consists in the appropriation of the new products thus generated to the enlargement of the living cell-structure; a phenomenon obviously distinct from the preceding. It is well to observe, that this process, which constitutes the act of Organization, may be clearly distinguished in the higher Plants and Animals, as consisting of two stages;-the first of these being that of assimilation, which consists in the further preparation or elaboration of the fluid matter, by certain alterations whose nature is not yet clear, so as to render it plastic or organizable;-the second being the act of formation, or the conversion of the organizable matter into the solid texture, in which process the properties that distinguish that texture come to manifest themselves. Thus, for example, we do not find that a solution of dextrin (of starch-gum) is capable of being at once applied to the development of Vegetable tissue, although it is identical in composition with cellulose; for it must first pass through a stage, in which it possesses a peculiar glutinous character, and exhibits a tendency to spontaneous coagulation, that seems like an attempt at the production of organic forms. And in like manner, the albumen of Animals is evidently not capable of being applied to the nutrition of the fabric, until it has been first converted into fibrin; which also is distinguished by its tenacious character, by its spontaneous coagulability, and by the fibrous structure of its clot. Now, in both these cases, there is probably some slight modification in chemical composition, that is, in the proportions of the ultimate elements; but the principal alteration is evidently that which is effected by the rearrangement of the constituent particles; so that, without any considerable change in their proportions, a compound of a very different nature is generated. Of the possibility of such changes we have abundant illustrations in ordinary Chemical phenomena; for there is a large class of substances, termed isomeric, which, with an identical composition, possess chemical and physical properties of a most diverse character.

31. But we cannot attribute the production of Fibrin from Albumen,

the organizable from the unorganizable material, to the simple operation. of the same agencies as those which determine the production of the different isomeric compounds; for the properties of Fibrin are much more vitally distinct from those of Albumen, than they are either chemically or physically; that is, we find in the one an incipient manifes tation of Life, of which the other shows no indications. The spontaneous coagulation of fibrin, which takes place very soon after it has been withdrawn from the vessels of the living body, is a phenomenon to which nothing analogous can be found elsewhere; for it has been clearly shown not to be occasioned by any mere physical or chemical change in its constitution; and it takes place in a manner which indicates that a new arrangement of particles has been effected in it, preparatory to its being converted into a living solid. For this coagulation is not the mere homogeneous setting, which takes place in a solution of gelatine in cooling; nor is it the aggregation of particles in a mere granular state (closely resembling that of a chemical precipitate), which takes place in the coagulation of albumen: it is the actual production of a simple fibrous tissue, by the union of the particles of fibrin in a determinate manner, bearing a close resemblance to the similar process in the living body (§ 188). We say, then, that the coagulation of Fibrin, and the production of a fibrous tissue, are the manifestation of its vital properties, rather than the direct result of chemical or physical agencies; because no substance is known to perform any such actions, without having been subjected to the influence of a living body; and because the actions themselves are altogether different from any which we witness elsewhere.

32. The act of Formation seems to consist of a continuation of the same kind of change,-that is, a new arrangement of the particles, producing substances which differ both as to structure and properties from the materials employed, but which may be so closely allied to them in chemical composition, that the difference cannot be detected. Thus, from the "protoplasma "* of Plants are generated, in the process of celldevelopment, the membranes which constitute the walls of the cells: chemically speaking, there seems to be no essential distinction between. these substances; and yet between the living, growing, reproducing cell, and the gelatinous, semifluid matter in which they are imbedded, how wide the difference! So in the Animal body, we find that the composition of the proper muscular tissue scarcely differs, in regard to the proportion of its elements, from the fibrin, or even from the albumen, of the blood; and yet what an entire rearrangement must take place in the particles of either, before a tissue so complex in structure, and so peculiar in properties, as muscular fibre, can be generated!

33. Both in the Plant and the Animal, we find that tissues presenting great diversities both in structure and properties, may take their origin in the same organizable material; but in every case (at least in the ordinary processes of growth and reparation) the new tissue of each kind is formed in continuity with that previously existing. Thus in

This term is now commonly employed to designate that combination of starchy and albuminous matters, in which all newly-forming cells appear to originate. See & 28.