The manifestation of so-called vital phenomena in man forms the subject-matter of the following chapters, and some explanatory definition of the vital characters of the simpler organisms will be useful in preparing the beginner's mind for the more intricate questions in human physiology. This, with the foregoing short account of the chemical and structural peculiarities of animals, will complete a rough outline of the general character of organisms.
Protoplasm has already been referred to as the material capable of showing vital phenomena, the most obvious and striking of which are its movements.
Besides the common molecular or Brownian movement of the granules in protoplasm - which may be seen in most cases where fine granules are suspended in a less dense medium - protoplasm can perform motions of different kinds which must be regarded as distinctly vital in character. This movement may be said to be of three different kinds, according to the results produced, viz.: (i) The production of internal currents. (2) Changes in form. (3) Locomotion. In reality, the two latter are dependent on the first.
The existence of currents moving from one part of the protoplasm to another can be well seen in vegetable cells, when the cell wall restricts the more obvious change in form or place. Thus in the cells forming the hair on the stamens of Tradescantia Virginica the various currents can be seen in the layers of protoplasm which line the cell wall.
The granular particles course along in varying but definite directions, passing one another like foot passengers in a crowded street. The first and most obvious result of this is, that the different parts of the substance are frequently brought into contact with one another, and thus the products of any chemical changes taking place at a given part of the cell body are rapidly distributed over the entire mass of the protoplasm.
The change inform occurs if there be no definite cell wall - as in naked vegetable spores and amoeboid forms of animal life - to restrict or direct the current of protoplasm: it flows unto various directions in bud-like processes, which appear at various parts of the protoplasmic mass, so as to cause a constant change in the form of the cell. These outstretched processes sometimes flow together and become fused, often enclosing some of the medium in which the creature is suspended, or catching some foreign particle floating near them.
The flowing out of these pseudopodia usually takes place for some time persistently from one side of the cell; and the body of the cell has to follow, as it were, the protrusion of the processes in such a manner that in a short time definite change in position or movement in a certain direction occurs: thus the protoplasmic unit may be said to perform definite progression or locomotion. All these movements may be seen in the white blood corpuscle of a cold-blooded animal, such as a frog, and still more easily in the unicellular amoeba.
Various influences may be seen to affect the rate of movements and probably influence at the same time the other activities of the protoplasm. Foremost among these must be named: (i) Temperature. If a protoplasmic unit, which is observed to be motile, be gently warmed, the movements become more and more active as the temperature is raised, up to a certain point, about 35°-42° C, when a spasm occurs, resulting in the withdrawal of the pseudopodia; soon after this the cell assumes a spherical shape. If the heat be carefully abstracted before it has attained too great a height, the protoplasm may recover and again commence its movements. If, on the other hand, cold be applied to moving protoplasm, the motions become less and less active, and commonly cease at a temperature about or a little above o° C. (2) Mechanical irritation also produces a marked effect on the movements of protoplasm. This may be well seen in the behavior of a living white cell of frog's blood under the microscope. It is spherical when first mounted, owing to the rough treatment it goes through while being placed on the glass slide and covered; shortly its movements become obvious by its change in form, which may again be checked by a sudden motion of the cover glass. (3) Electric shocks given by means of a rapidly-broken induced current cause spasm of the protoplasm, the cell becoming spherical. (4) Chemical stimuli also have a marked effect; carbonic acid causing the movements to cease, and a supply of oxygen making it active. The movements and other activities of protoplasm are, during life, frequently modified and controlled by nerve influence, as will appear in the following pages. This may readily be seen in the stellate pigment cells of the frog's skin, which can be made to contract into spheres by the stimulation of the nerves leading to the part.
The motions of protoplasm are thus seen to be affected by external influences, but the most careful observer cannot find physical explanations of the various movements which have been described. It is necessary, therefore, to ascribe this power of motion to some property inherent in the protoplasm, and hence the movements are called automatic. We are unable to follow the chemical processes upon which the activities of the protoplasm depend, and we therefore call them vital actions; but we must assume that these so-called vital properties depend on certain decompositions in the chemical constitution of the protoplasm. We know that some chemical changes take place, as we can find and estimate products which indicate a kind of combustion; but we know little or nothing of the details of the chemical process.