General Physiology: An Outline of the Science of Life

Among the physical properties of living substance its specific gravity possesses an important value for the understanding of certain vital phenomena. If cells of different kinds or pieces of tissue as pure as possible be allowed to fall into distilled water, it is observed that usually they sink to the bottom. It follows, therefore, that the cell-contents, as a whole, is in general heavier than water, i.e., possesses a specific gravity greater than 1. Very recently Jensen ('93, 1) has made a careful determination of the specific gravity of the one-celled ciliate infusorian, Paramcecium aurelia, in the following manner. It is well known that the specific gravity of a liquid can be raised by the addition of soluble salts, and can be graduated very finely by increase of the concentration. Jensen placed Paramcecia in a weak solution of potassium carbonate, the strength of which he raised until they no longer sank to the bottom, but remained suspended in the solution — a sign that the solution possessed the same specific gravity as the bodies of the Paramcecia. Then the specific gravity of the solution was determined by means of an areometer. It was thus found that the cell-body of Paramcecium possesses a specific gravity of approximately 1*25. In general, the specific gravity of living substance cannot be much greater than this. So far as our knowledge at present extends, it is always a little greater than 1.

But there are certain cases in which the aggregate weight of the cell deviates from this general principle, in which the specific gravity of the cell as a whole is less than 1. These cases can be understood at once, if it is recalled that protoplasm is not a homogeneous substance. E.g., in the case of cells in which fat-droplets are stored in the ground-substance of the protoplasm it is possible that, although the ground-substance is heavier than water, the cell as a whole possesses a less specific gravity, since the accumulation of fat, which is considerably lighter than water, reaches such an

extent that it overcomes the weight of the rest of the protoplasmic body. Such cases are realised in the fat-cel]s of the subcutaneous connective tissue in man and many animals ; if such tissue be thrown into water, it floats upon the surface. For this reason fleshy men, in swimming, have to make less effort to maintain themselves above the water than thinner persons. Other substances in the cell-body can play the same rdle as fat, particularly bubbles of gas, which under certain circumstances can lower the specific gravity of the whole body of the cell enormously — a phenomenon that occurs in many shell-bearing fresh-water Rhizopoda (Arcella, Difflugia).

It follows from this fact that by the accumulation of lighter or heavier substances the cell under certain circumstances can actively diminish or increase its specific gravity, and, therefore, can actively rise or sink in water without employing locomotive organs. Under many conditions, e.g., when the vital conditions become unfavourable in the place where the organism lives, such a power is of great importance for the life of the organism. In all cases, however, where cells are found that are lighter than water, certain elements only are lighter, the whole protoplasm never. The ground-mass of the protoplasm appears always to be slightly heavier than water.

In most cases protoplasm is entirely colourless or grey ; in thin layers free from solid contents it is transparent, in thick layers opaque. It refracts light somewhat more strongly than water. As regards details, the various forms of living substance behave differently according to the condition of their constituents. Some solid elements, such as fat-droplets, drops of water, and chlorophyll grains, can be intensely coloured, so that the cells in which they are present in great quantities appear yellow, red, green, etc., as, e.g., in plant tissues. The power of refracting light also differs with the individual constituents, that of water-droplets in the vacuoles is less, that of fat-droplets greater than that of the ground-substance. It would carry us too far to examine all the individual cases, but it is of interest to consider somewhat in detail the behaviour of one form of living substance, viz., the so-called contractile substance, i.e., amoeboid protoplasm, cilia, and muscle-fibres, which execute definite changes of form, called contractions.

In the first half of the century Boeck found that certain elements of the cross-striated muscle-fibre are doubly refractive, i.e., are able to divide a ray of light into two rays, which are transmitted with different velocities. Later, Briicke, especially, investigated this property in detail. Still later, Engelmann (75) observed that not only the discs of cross-striated muscle, but in general all fibrous contractile substances, such as those of smooth and cross-striated muscle-cells, the contractile fibres or myoids of the infusorian body, and the cilia and flagella of all ciliated cells, exhibit positive uniaxial double refraction, in such a way that their optical axis coincides with the direction of the fibres. This fact indicates that the molecular structure of all these fibrous tissues must be different in the direction of the fibres from that in other directions — an inference that is important for the understanding of the phenomena of contraction in these objects. Engelmann has not been able to find double refraction in the naked contractile protoplasm of Bhizopoda, e.g., Amoeba. He, observed it only in the straight, radiating pseudopodia of Actinosphcerium Eichhornii, a delicate fresh-water rhizopod ; but here it belonged most probably, not to the contractile protoplasm, but to the stiff rays that occur as supporting-organs in the axis of the pseudopodia, and apparently have nothing to do with the contraction.

Of all the natural sciences, chemistry, in dealing with the atoms, penetrates deepest into the composition of the physical world. It must hence be employed in elucidating the composition of living substance, and thereby completing the preparation for an understanding of vital phenomena. It is well known that chemistry has arrived at the point at which it recognises the vast variety of substances in the physical world to be composed of the atoms of a small number of relatively simple substances, which thus far it has not succeeded in decomposing. But, although by means of its analytical methods the division of the sixty-eight chemical elements has so far not been accomplished, and their composition out of still simpler substances cannot yet be proved experimentally, no chemist entertains longer any doubt that in reality these elements are not final units. Accordingly, many attempts have been made to arrange them in a genetic relation to one another, and to establish the relationship that is expressed in the analogies of the chemical behaviour of individual elements and their compounds, as a natural relationship arising by the direct derivation of one from another. Especially Mendelejeff, Lothar Meyer, and, most recently, Gustav Wendt and Preyer, arguing chiefly from the relations of the atomic weights of the elements and the similarity of certain elements as regards their own behaviour towards one another and the behaviour of their compounds, have attempted this with success ; the result is shown

by the subsequent discovery of previously unknown elements, whose existence they had predicted from certain gaps existing in the genealogical table of the elements. According to Wendt ('91) and Preyer ('92), the elements have been developed in the course of the earth's history by gradual condensation from a primitive element, hydrogen, in such a way that those having a higher atomic weight have been derived from those having a lower one; finally, all have been derived from hydrogen, the element possessing the lowest atomic weight. But here scientific theory ceases and hypothesis begins. Whether hydrogen is really the ultimate unit, and in what relation its atoms stand as ponderable or mass-atoms to the imponderable universal ether, the existence of which physics finds it necessary to assume from the phenomena of light and electricity, for the present is not known.

But if we confine ourselves to ponderable matter, to which living substance, like all other bodies, belongs, chemical analysis shows that of the sixty-eight elements of which the physical world consists, twelve only are found constantly in living substance. These twelve elements which occur in every cell are :— Besides these twelve general organic elements, a small number of special elements occur which are not met with in all cells, and some of which are found only very sporadically. These are : —

Among these, silicon is wide-spread and fluorine is infrequent, while the others, which likewise have a very limited occurrence, and certain metals, such as copper, which are occasionally found in traces in living substance, perhaps possess no importance at all for the vital processes of the organisms in which they have been observed. But no one of all these organic elements is limited exclusively to organic nature. Carbon occurs in the air, combined with oxygen, as carbonic acid, and in large masses in the calcium carbonate of sedimentary rocks.

Hydrogen, likewise combined with oxygen, as water, covers the greater part of the earth's surface. Oxygen occurs both free as a gas in the atmospheric air, of which it constitutes about 21 per cent., and also combined with a large number of other elements. Nitrogen occurs likewise both in the free state in the air, comprising about 79 per cent., and also combined with hydrogen and oxygen in the compounds of ammonia, both ammonium nitrate and nitric acid.

Sulphur is wide-spread in combination with oxygen in sulphates. Phosphorus behaves similarly, and is to be found everywhere in the phosphates of the alkalies and the alkaline earths. Chlorine is very widely distributed, combined with sodium, as common salt. Potassium occurs in combination with chlorine as potassium chloride, and with acids as nitrates, sulphates, and phosphates. Soditim, chiefly in the form of sodium chloride or common salt, is found everywhere on the surface of the earth ; it is in solution in the sea, in the earth, and forms large solid masses in salt strata.

Magnesium is a constant accompaniment of' potassium and sodium, and is similarly combined, occurring as magnesium chloride, carbonate, sulphate, and phosphate. Calcium, in the form of calcium carbonate, silicate, sulphate, and phosphate, occurs in the vast limestone strata of the sedimentary rocks. Iron is very wide-spread over the earth's surface in the form of sulphur compounds, oxides and their salts. Silicon appears almost exclusively combined with oxygen in the form of silicic acid and its salts in igneous rocks.

Fluorine occurs chiefly in combination with calcium as fluor spar. Bromine and iodine are present in many salt strata, as well as in sea-water, as sodium bromide (iodide), and potassium bromide <(iodide). Aluminium is spread over the whole earth in combination with oxygen as clay, and in the latter form, combined with silicic acid, as feldspar. Manganese and all the other'metals that are observed occasionally here and there in the living organism are met with everywhere in rocks in the form of their oxides, sulphur compounds, and various salts.

This survey shows that all organic elements help at the same time to constitute the inorganic portion of the earth's surface. Since, moreover, chemical analysis of living substance has shown that no constituents but these organic elements are to be found in the organism, the important fact follows that an elementary vital substance exists no more than a specific vital force. The conceptions of a " vital ether," a " stpiritus animalis" a " vital matter," etc., with which the earlier physiology so freely dealt, have, therefore, in harmony with the advanced development which analytical chemistry has undergone at the present time, completely disappeared from the present theory of life; living substance is composed of no different chemical materials from those occurring within lifeless bodies.

Nevertheless, one fact deserves mention, viz., that the few general organic elements are not scattered irregularly here and there through the natural system of elements, but they occupy a definite position, being remarkable as elements having very low atomic weights. Hence the conclusion may with great probability be drawn that in the evolution of the elements the organic elements arose by condensation very early, and therefore existed in the very early stages of the development of our planetary system, at a time when other elements, such as the heavy metals, had not yet been formed.

Living substance must be killed before its chemical composition can be learned. Paradoxical as this may sound, at present it is the only way by which a knowledge of the chemistry of living substance can be obtained. The biting sarcasm that Mephistopheles pours out before the scholar upon this practice of physiological chemistry must be quietly endured. It is not possible to apply the methods of chemistry to living substance without killing it. Every chemical reagent that comes in contact with it disturbs it and changes it, and what is left for investigation is no longer living substance, but a corpse — a substance that has wholly different properties. Hence ideas upon the chemistry of the living object can be obtained only by deductions from chemical discoveries in the dead object, deductions the correctness of which can be proved experimentally in the living object only in rare cases. This alone is responsible for the excessively slow advance of the knowledge of the chemistry of the vital process. It is evident that the greatest foresight is necessary in applying results obtained upon the dead object to conditions in the living, and it must con-

stantly be borne in mind that the chemical relations of the latter are to be distinguished sharply from those of the former. Although there is no fundamental difference between the elements composing living and those composing lifeless substance, in other words, although no special vital element exists in the organic world, some of the elements in living substance form unique compounds which characterise it only, and are never found in lifeless substance. Thus, there exist in the former, besides chemical compounds that occur also in the latter, specific organic complexes of atoms.

Many of these organic compounds, especially those that are of special importance to living substance, possess so complicated a constitution that thus far chemistry has not succeeded in obtaining an insight into the spatial relations of the atoms in their molecules, although the percentage composition of the molecules is known to a greater extent. There are especially three chief groups of chemical bodies and their transformation-products, by the presence of which living substance is distinguished from lifeless substance ; these are proteids, fats, and carbohydrates. Of these only the proteids and their derivatives have been demonstrated with certainty as common to all cells ; hence they must be set apart among the organic constituents of living matter as the essential or general substances, in contrast to all special substances.

The proteids play the most important rdle in the composition of living substance, since they are absolutely indispensable to all life that exists at present upon the surface of the earth, and quantitatively they constitute the chief constituent of all the organic compounds of the cell. Without exception they consist of the elements carbon, hydrogen, sulphur, nitrogen, and oxygen ; of these, nitrogen especially distinguishes proteids from the two other chief groups of organic bodies, carbohydrates and fats, so that the former, as nitrogenous bodies, are to be contrasted with the latter two as non-nitrogenous. The stereo-chemical composition of the proteid molecule is not yet known, but from analyses, in which the molecule is split up into a large number of still very complex molecules, it is known that it must have an excessively complex constitution ; although it contains only the five elements C, H, N, S, and O, the number of its atoms often reaches far beyond a thousand. In the year 1866 Preyer made the first analysis of haemoglobin, the proteid that gives the characteristic colour to the blood, more exactly to the red corpuscles, and, as a carrier of oxygen from the lungs through the blood to the cells of the tissues,

plays an extremely important role in the animal body. found the composition of haemoglobin to be — Although at first this formula caused surprise, a number of later analyses have since given quite similar results.1 Thus, according to Griibler's investigations ('81), the composition of the crystallised proteid which occurs in the squash-seed may be estimated as — ZinofFsky ('85) found the formula of hsemoglobin from horse blood even still larger than Preyer —

Similarly complex formulas have been derived for the proteid that constitutes the white of the hen's egg. From all these analyses it follows that because of the mass of its constituent atoms the proteid molecule must be enormous. The great size of the molecule explains an important characteristic of proteids, viz., that, in contrast to other bodies, they do not diffuse from solutions through animal membranes or artificial parchment. If an aqueous solution of common salt or any other soluble salt be placed in a wide glass tube, the lower end of which is closed by a membrane, preferably artificial parchment (Fig. 37), and the tube be suspended in a vessel of pure water, it is found after a short time that the concentration of the salt solution in the inner tube has decreased considerably, while the water in the outer vessel has come to have an equal percentage quantity of salt. Hence salt has diffused from the tube through the membrane into the outer water until its percentage composition has become equal in the two liquids. But if instead of the solution of salt a solution of egg albumin be employed, which can be obtained by rubbing up thoroughly the white of a hen's egg with about 100 cubic centimetres of water and filtering, the solution can be allowed to stand in the dialyzer (as the apparatus is called) for hours and days without a trace of albumin diffusing from the inner tube into the outer water. This phenomenon may be explained very simply from

the size of the albumin molecule ; the latter is too large to pass through the excessively fine pores of the membrane, while no obstruction stands in the way of the small molecules of salt. This property is of practical importance in the chemical investigation of proteids, for by dialysis the proteids can always easily be separated from the salts that may be present with them in solution. The fact that proteids and a host of other substances which behave similarly do not diffuse through membranes, has led to the idea that these bodies, in contrast to diffusible substances, dissolve in water only apparently, and form no real solutions ; their apparent solubility may be only a largely developed power of swelling. Proteids in a dry state are, in fact, capable of taking up very large quantities of water, and thereby gradually swelling. In 1861 Graham contrasted these bodies as colloid substances from crystalloid substances ; and this distinction has been handed down and been generally accepted. The colloids are said to be capable of swelling only, not of crystallising ; the crystalloids, on

the other hand, to be really soluble and capable of crystallisation. But such a sharp distinction is scarcely admissible ; for in the first place, proteids are known that can form genuine crystals, like the above-mentioned proteids in squash-seeds, which occur widespread in plant seeds as aleurone-grains, and like the haemoglobin of the red blood-corpuscles. If, e.g., whipped blood from the guineapig be shaken for a time with ether, by which the haemoglobin is extracted from the substance of the red corpuscles and driven out into the blood-serum, and a drop of this liquid be allowed slowly to evaporate upon a glass slide, very delicate tetrahedral crystals gradually separate (Fig. 38, //), which consist of pure haemoglobin. In the second place, under the influence of certain reagents, proteids can pass over into modifications that diffuse through membranes, without losing in the process the chemical characteristics of proteids. These modifications, which, e.g., proteids undergo in the body under the influence of the digestive juices of the stomach and the pancreas, are termed peptones : it is known

that they arise by the hydrolytic cleavage of the original proteid molecule, so that the peptones represent the hydrates of the original proteids. Important conclusions follow from this fact. Since the proteid molecule, which was originally not diffusible on account of its enormous size, is split up in the peptonising process into the peptone molecules, which are much smaller and therefore diffusible, but which have the chemical characteristics of proteids, it follows that the proteid molecule is not simple but polymeric, i.e., it consists of a chain-like combination of many similar groups of atoms. In the transition to the peptone condition the proteid molecule is broken up with hydration into these single, similar atomic groups, all of which, however, have the chemical characteristics of proteids, but represent much smaller molecules. The inability of proteids to diffuse through membranes depends, therefore, solely upon their polymerism. Wholly analogous cases occur in inorganic nature ; e.g., certain forms of silicic acid are unable to diffuse through membranes because of their polymerism. Hence it is evident that no fundamental difference exists between solutions of simple molecules, as found in peptones, and those of polymeric molecules, as in ordinary albumin.

A further physical property, which is perhaps connected with the polymerism of the ordinary proteid molecule, and which belongs to almost all proteids with the exception of their hydrates, the peptones, is their capacity of clotting, or coagulating. Coagulation consists in the passing of the substance from the dissolved to the solid condition within the solvent medium. Boiling is a method that causes coagulation in almost all proteids. In the fresh hen's egg the proteid is present in a thick clear viscous solution. In the boiled egg it has become a solid white opaque mass ; it is coagulated. By boiling, proteid can be separated out of thin solutions in the form of fine curdled flakes. Other methods, such as the use of inorganic acids and alcohol, also cause proteid in solution to be coagulated and precipitated, the result being indicated by a clouding of the liquid. That the power of coagulation is in some way connected with polymerism is indicated by the fact that inorganic polymeric molecules, such as the above-mentioned silicic acid, in aqueous solution can likewise be coagulated into a jelly. If, e.g., hydrochloric acid be added to a solution of sodium silicate, free silicic acid and sodium chloride are produced ; the silicic acid may then be separated from the salt by dialysis, since, in contrast to the salt, the former, like a polymeric body possessing very large molecules, does not diffuse through membranes. By the addition of a few bubbles of carbonic acid this solution of silicic acid may be changed at once into a coagulated jelly-like mass. Since our knowledge of the chemical composition of proteids is at present very incomplete, it is not easy to produce definite chemical reactions with them. Nevertheless, a number of tests have been empirically

determined, which are characteristic for proteids and in doubtful cases make known their presence. What chemical transformation the proteid molecule undergoes in these tests is of course practically unknown. The best-known tests are the following, any one of which alone is riot sufficient to prove with absolute certainty the presence of albumin : — 1. The xanthoproteic test : a solution of proteid is coloured yellow by boiling with nitric acid ; by the addition of ammonia the colour changes to orange.

2. The biuret test : if a solution of proteid be made alkaline by causic potash or soda, it takes on in the cold, by the addition of a drop of cupric sulphate solution, a clear violet colour. 3. Milton's test : coagulated proteid, boiled for a time with a solution of mercuric nitrate, and a little nitrous acid, becomes rose-red. 4 The hydrochloric acid test : boiling with concentrated hydrochloric acid dissolves coagulated proteids and colours the clear liquid violet.

5. The potassium ferrocyanide test : a solution of proteid to which acetic acid has been added shows by the addition of a solution of potassium ferrocyanide a white cloudiness. 6. The iodine test : the addition of tincture of iodine, or a solution of iodine in potassium iodide, serves as a good microscopic method for recognising proteids ; the clot becomes yellowish-brown. Besides these tests, a great number of others have been suggested by different investigators, but they fail in individual cases.

According to their different solubilities in water, three groups may be distinguished among the simple proteids — viz., the albumins, the globulins, and the vitellins.1 The albumins are directly soluble in pure water. To them belong egg-albumin, which forms the great mass of the white of eggs ; serum-albumin, an albuminous body contained in blood-serum ; muscle-albumin, the proteid of muscle-cells, soluble in water ; and plant-albumin, which is dissolved in the sap of plant-cells.