PHYSIOLOGY 13 Con structive and de structive meta bolism. the midst of this protoplasmic body there is seen a peculiar body of a somewhat different and yet allied nature, the so-called nucleus ; but this we have reason to think is specially concerned with processes of division or reproduction, and may be absent, for a time at all events, without any injury to the general properties of the proto plasmic body. Now such a body, such a mass of simple protoplasm, homogeneous save for the admixtures spoken of above, is a living body, and all the phenomena which we sketched out at the very beginning of this article as characteristic of the living being may be recognized in it. There is the same continued chemical transformation, the same rise and fall in chemical dignity, the same rise of the dead food into the more complex living substance, the same fall of the living substance into simpler waste-products. There is the same power of active movement, a move ment of one part of the body upon another giving rise to a change of form, and a series of changes of form resulting eventually in a change of place. In what may be called the condition of rest the body assumes a more or less spherical shape. By the active transference of part of the mass in this or that direction the sphere flattens itself into a disk, or takes on the shape of a pear, or of a rounded triangle, or assumes a wholly irregular, often star-shaped or branched form. Each of these transformations is simply a rearrangement of the mass, without change of bulk. When a bulging of one part of the body takes place there is an equivalent retraction of some other part or parts ; and it not unfrequently happens that one part of the body is repeatedly thrust forward, bulging succeeding bulging, and each bulging accompanied by a corresponding retrac tion of the opposite side, so that, by a series of movements, the whole body is shifted along the line of the protuber ances. The tiny mass of simple living matter moves on ward, and that with some rapidity, by what appears to be a repeated flux of its semi-liquid substance. The internal changes leading to these movements may begin, and the movements themselves be executed, by any part of the uniform body ; and they may take place with out any obvious cause. So far from being always the mere passive results of the action of extrinsic forces, they may occur spontaneously, that is, without the coincidence of any recognizable disturbance whatever in the external conditions to which the body is exposed. They appear to be analogous to what in higher animals we speak of as acts of volition. They may, however, be provoked by changes in the external conditions. A quiescent amoeba may be excited to activity by the touch of some strange body, or by some other event, by what in the ordinary language of physiology is spoken of as a stimulus. The protoplasmic mass is not only mobile but sensitive. When a stimulus is applied to one part of the surface a movement may com mence in another and quite distant part of the body ; that is to say, molecular disturbances appear to be propagated along its substance without visible change, after the fashion of the nervous impulses we spoke of in the beginning of this article. The uniform protoplasmic mass of the amoeba exhibits the rudiments of those attributes or powers which in the initial sketch we described as being the fundamental characteristics of the muscular and nervous structures of the higher animals. These facts, and other considerations which might be brought forward, lead to the tentative conception of proto plasm as being a substance (if we may use that word in a somewhat loose sense) not only unstable in nature but subject to incessant change, existing indeed as the ex pression of incessant molecular, that is, chemical and phy sical change, very much as a fountain is the expression of an incessant replacement of water. We may picture to ourselves this total change which we denote by the term "metabolism" as consisting on the one hand of a down ward series of changes (katabolic changes), a stair of many steps, in which more complex bodies are broken down with the setting free of energy into simpler and simpler waste bodies, and on the other hand of an upward series of changes (anabolic changes), also a stair of many steps, by which the dead food, of varying simplicity or complexity, is, with the further assumption of energy, built up into more and more complex bodies. The summit of this double stair we call " protoplasm." Whether we have a right to speak of it as a single body, in the chemical sense of that word, or as a mixture in some way of several bodies, whether we should regard it as the very summit of the double stair, or as embracing as well the topmost steps on cither side, we cannot at present tell. Even if there be a single substance forming the summit, its existence is abso lutely temporary: at one instant it is made, at the next it is unmade. Matter which is passing through the phase of life rolls up the ascending steps to the top, and forthwith rolls down on the other side. But to this point we shall return later on. Further, the dead food, itself fairly but far from wholly stable in character, becomes more and more unstable as it rises into the more complex living material. It becomes more and more explosive, and when it reaches the summit its equilibrium is overthrown and it actually explodes. The whole downward stair of events seems in fact to be a series of explosions, by means of which the energy latent in the dead food and augmented by the touches through which the dead food becomes living pro toplasm, is set free. Some of this freed energy is used up again within the material itself, in order to carry on this same vivification of dead food ; the rest leaves the body as heat or motion. Sometimes the explosions are, so to speak, scattered, going off as it were irregularly throughout the material, like a quantity of gunpowder sprinkled over a surface, giving rise to innumerable minute puff s, but pro ducing no massive visible effects. Sometimes they take place in unison, many occurring together, or in such rapid .sequence that a summation of their effects is possible, as in gunpowder rammed into a charge, and we are then able to recognize their result as visible movement, or as appre ciable rise of temperature. These various phenomena of protoplasm may be conven- iently spoken of under the designation of so many properties, or attributes, or powers of protoplasm, it being understood that these words are used in a general and not in any definite scholastic sense. Thus we may speak of proto plasm as having the power of assimilation, i.e., of building up the dead food into its living self; of movement, or of contractility as it is called, i.e., of changing its form through internal explosive changes; and of irritability or sensitiveness, i.e., of responding to external changes, by less massive in ternal explosions which, spreading through its mass, are not in themselves recognizable through visible changes, though they may initiate the larger visible changes of movement. These and other fundamental characters, all associated with the double upward and downward series of chemical changes, of constructive and destructive metabolism, are present in protoplasm wherever found ; but a very brief p i asm> survey soon teaches us that specimens of protoplasm existing in different beings or in different parts of the same being differ widely in the relative prominence of one or another of these fundamental characters. On the one hand, in one specimen of protoplasm the energy which is set free by the series of explosions constituting the down ward changes of destructive metabolism may be so directed as to leave the mass almost wholly in the form of heat, thus producing very little visible massive change of form. Such a protoplasm consequently, however irritable and Proper- ties of Diffev- entia
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