Principles of General Physiology

The Eye-Media. — Of course, if the eye is exposed to ultra-violet light, severe conjunctivitis is caused. Workers in this light have to wear spectacles impermeable to the short waves and frequently also to protect all parts of the skin exposed. This is especially so in electric welding, since the arc spectrum of iron is very rich in ultra-violet lines. E. K. Martin (1912) finds that the cornea absorbs all rays shorter than 295 pp. The lens is thus protected from the most active rays, although it is capable of absorbing rays between 300 and 400 fj.fi, which might affect it and cause opacity (cataract), unless kept back by a screen outside. It was found, however, that while the mercury arc caused conjunctivitis, no change in the lens could be detected.

Hallwachs' Effect. — There is one further effect of ultra-violet light of theoretic interest in connection with the nature of photo-chemical change. Hallwachs (1888) noticed that a negatively charged insulated metal, such as a gold leaf electrometer, loses its charge when illuminated with ultra-violet light. This appears to be a case analogous to those in which light energy is stored, since the light energy is changed into the kinetic energy of moving electrons, shot from the metal.

Although ultra-violet light has so much more action on protoplasm than visible light has, it has been found by several observers that light which has no action by itself on infusoria, bacteria, or blood produces the effect of ultra-violet light when certain dyestuffs are present. Although the dyes used were, for the most part, fluorescent, this does not seem to be an essential factor. For a complete account of the work, the reader is referred to the monograph by Tappeiner and Jodlbauer (1907). The general nature of the phenomenon will be clear from the few remarks following. Hertel (1905) exposed certain bacteria to light of 448/A/tx. This had no effect on them, either with or without the presence of eosin, 1 part in 1,200. Eosin has no absorption band in this position. Ultra-violet light of 280 fj.fi. killed

them in sixty seconds, without eosin. A third experiment consisted in taking light of 518 fifji, that is, in the position of the absorption band of eosin, and making it of about the same energy as that of the ultra-violet light previously used. Alone, this light had no effect, as would be expected, since it has a longer wave length than that found ineffective in the first experiment. On the other hand, in the presence of eosin, which has no action in itself, as the first experiment showed, the bacteria were killed in seventy to ninety seconds. It appears that the action of the eosin is to be compared to that of an optical sensitiser. The above results may be put in a table as follows : —

Since chlorophyll is so active as an optical sensitiser, it has a very powerful " photo-dynamic " action. The explanation of the phenomenon seems to be of the same nature as the similar one in the case of the photographic plate. The dye is adsorbed on the surface of the organisms and the effect is probably produced by an activation of oxygen, or perhaps by a product of the oxidation of the dye, since it requires the presence of oxygen for the phenomenon to occur.

An interesting experiment by Victor Henri, etc. (1912, p. 28), with colloidal selenium throws light on the question. The solution was fluorescent, but it had no effect in the dark, on certain protozoa. Under the ultra-microscope, it was seen to be a suspension of very minute particles. A certain number of the organisms took up these particles into vacuoles, where they aggregated into small masses. On exposure to light, it was found that only those organisms which had taken up the colloid were affected, thus showing the necessity of close contact and indicating a photo-chemical reaction, although not of the nature of the formation of a product which could act independently of the light. If this had been the case, those organisms which had not taken up the colloid would have been affected by the product diffusing from the others.

The lethal effect of light on bacteria was first described by Marshall Ward (1892), who did not recognise it as an effect of the ultra-violet rays. There are other cases where light is known to have a retarding action on the growth of plants. Fungi, for example, grow more rapidly in the night than in the day time. The phenomena of heliotropic curvature, in its permanent stage, are due to diminution of the rate of growth in the places exposed to light. The first effect of light, however, as we have seen (page 126), is due to change of turgor, so that it is interesting to examine for a moment the effects of light on the permeability of the cell membrane. Trondle (1910) showed that the result depends on the intensity of the light, a weak light causing diminution ; a moderate one, increase ; a very strong one, diminution again. These effects on permeability correspond to those on heliotropic curvature, so that the conclusion seems justified that we have to deal, in the primary stage of the latter, with changes in permeability. V. H. Blackman (1914) has investigated the action of light on the permeability of the excitable structures of the sensitive plant. The method used was that of the changes in its electrical conductivity. The interesting result was obtained, that during illumination an increased permeability existed, and that the first effect of cutting off the light was a further increase, after which the normal state returned. Whether this result has any relation to the similar one in the electric response of the retina cannot as yet be stated.

Trondle holds that the effect of light on permeability is a complex one, depending on a photo-chemical change in the membrane, together with reactions on the part of the cell itself. Its use may be to facilitate the escape of assimilation products. FIG. 181. CHROMATIC ADAPTATION. — Small flat-fish (Rhomboidichthya podas), photographed on ground of natural composition, gravel and sand of various degrees of coarseness. It is to be noted that the fish is completely uncovered, although appearing to lie under

FIG. 182. — Similar views to those of the preceding figure, on backgrounds of regular black and white patterns. The imitation of the smaller pattern is better than that of the larger one. In the latter cases, although there are large patches of black and white on the fish, their shapes do not verjr closely correspond with those of the background ; there is, however, a distinct indication of square or circular areas, respectively. It may be useful to refer back for a moment to the application of photochemical facts to the retinal process. There seems no doubt that the visual purple is an optical sensitiser ; it is bleached by light as these are ; and it absorbs nearly the whole length of the visible spectrum. Whether the products of its change are themselves capable of stimulating the light-receptors, or whether they act catalytically in bringing about other changes in these receptors, is unknown. It seems clear also that the peculiar form of the rods and cones must have some significance, but the difficulty of the problem is obvious, and in the present state of knowledge, speculation is of little use.

The power which certain animals have of modifying the colour of their skins to match that of their surroundings is well known. The case of the chameleon is often quoted. The manner in which it is effected is by the contraction or expansion of pigment cells of various colours in the skin. To this is sometimes added the iridescence due to interference of waves by means of a layer of fine crystalline structure. The work of Brlicke (1851) on the chameleon was, after that of Pouchet (1848), the first experimental investigation of the question. Pouchet's work was on the frog and fish, and he introduced the name " chromatic function" to express the dependence of the adaptation on the nervous system, through the eye. Biedermann (1892) devoted much attention to the phenomena in the frog. The skin epithelium cells of this animal contain yellow granules and a deeper layer of crystalline particles, which show interference colours. In the subjacent corium, there are a set of black pigment cells. The sciatic nerve contains motor fibres for the latter cells and the chief centre appears to be in the optic lobes with subsidiary centres in the spinal cord. There is also a nerve supply to the chromatophores, by nerves accompanying the blood vessels. In the frog, the receptors for the reflex arc are in the skin, chiefly that of the discs of the toes. These are affected by the different qualities of the surfaces, which, in the habitual surroundings of the animal, are associated with definite colours, such as stone, grass, etc. When the toes are made anaesthetic, all colour adaptation disappears. The eyes play no part in the phenomenon. On the other hand, the adaptation to details, investigated by Sumner (1911) in Rhomboidichthys podas, a small flat-fish, allied to the turbot, are effected by means of eye receptors. The photographs in Figs.

181 and 182 will give an idea of the range of the adjustment. It is to be remembered that in reality the adaptation is more perfect than the monochrome reproduction indicates, since it took place to varying shades of brown, as well as to black and white. The change, when removed to a different background, was, in some cases, obvious after a few seconds. When the fish were made blind, the chromatophores went into their state of rest and no further adaptive reaction was possible.

The use of this mechanism to its possessor seems to be twofold. The animal is rendered invisible both to its enemies and also to the smaller fish which serve as its prey. Although the effects of radium and similar elements which give off charged particles, are not, strictly speaking, those of light, they may for convenience be mentioned here. It was shown by Hardy that the negatively charged /^-particles of radium produce coagulation of oppositely charged colloids, and the effect appears to be an electrical one.

The effects produced by radium on living tissues are very similar to those of intense ultra-violet light, but more powerful. They have been used in therapeutics for similar purposes. The explanation of their action is not yet clear. The advantage of radium as regards ease of application to the spot required is obvious. For further information on the extremely interesting phenomena of radio-activity, the reader is referred to the books by Rutherford (1913) and by Soddy (1911, 1914), and, as regards its action on living tissues, to that by Finzi (1914).

The X-rays of Riintgen, in their effect on cells, are also similar to those of ultra-violet light, but the destructive effect continues for a long time and does not appear at once in its full magnitude. The results of repeated exposure are very serious, since the rays penetrate to deep tissues and are able to produce degeneration of the seminiferous cells of the testis, for example. This fact of relatively slight absorption by tissues, other than bone or certain metallic salts, has made the use of X-rays very valuable in discovering the nature of displacement of bone, and so on. We have also seen their application to the study of the movements of the alimentary canal.

As to the nature of X-rays, the view is now generally held that they are similar to light waves, electro-magnetic, but of a very short wave length, O'l to 10 *b Further information may be obtained from the book by Kaye (1914). The use of photographic methods of recording phenomena has been described above (page 460). We may add here the photography of absorption spectra. It is plain that plates sensitive to the whole of the visible spectrum are necessary. Wratten's " Panchromatic " will be found suitable, especially in the fine-grained variety known as " M " plates. These are prepared especially for the photography of objects under the microscope. The kind of negative usually required is not identical with that of landscape photography ; for convenience of reproduction, a " hard negative " generally serves best. The developer used by Willstatter and Stoll (1913) will be found excellent for such purposes. It is made thus : Solution I., 500 c.c. distilled water, 50 g. crystallised sodium sulphite, 5 g. hydroquinone, 1 g. metol. Solution II., 500 c.c. distilled water, 50 g. potassium carbonate. For use, mix 30 c.c. of each with 60 c.c. of water (120 c.c. together) and develop for three minutes at 18° to 20° in darkness and fix in acid bath.

The methods of direct colour photography, such as Lumi^re's " autochrome " process, have been used to prepare some beautiful photographs of stained microscopic preparations. The dependence of life on the receipt of radiant energy from the sun makes it of importance to understand how this energy can be converted into forms useful to the organism without the necessity of passing through the state of heat, in which a considerable part of the free energy would be lost.

The manner in which this is done is by conversion directly into chemical energy by means of what are known as photo-chemical reactions. These reactions are also of importance and interest with relation to the receptor organs for light impressions. Light cannot act unless it is absorbed (Grotthus's law). The amount absorbed by a medium of various thicknesses is regulated by Lambert's law, which states that it is a logarithmic function of the thickness.

The " extinction coefficient " is the most useful form in which the absorption power of a particular solution is expressed. The extinction coefficient is the negative logarithm of the light which has passed unabsorbed through a thickness of 1 cm. It is different for different wave lengths. This is the form in which it is most convenient for practical use, but the original definition made it the reciprocal of the thickness of the medium which sufficed to reduce light of a particular wave length to one-tenth of its value. The manner in which it is derived from Lambert's and Beer's laws and its application to spectro-photometry are described in the text.

The phenomena of resonance play a large part in the mechanism of photochemical reactions. If the vibration period of a molecule coincides with that of any of the light waves falling upon it, the molecule will be set into resonant agitation by means of the light energy absorbed. This vibration usually leads to chemical change. Luther's theory of the mechanism of the resonance process and its consequence is given in the text. Photo-chemical reactions do not obey the law of mass action, because their rate is controlled by the amount of light energy absorbed per unit time.

In order to set a photo-chemical reaction in train, a certain amount of light energy is absorbed, so that the first stage is always associated with the taking up of energy. The later stages may be either similar to that of the chlorophyll system, in which the continuance of the reaction depends on a continual supply of light energy and results in a storage of energy, or the reaction may be one which proceeds of itself with evolution of energy, but is accelerated by means of a catalyst produced by light. The catalyst may disappear in the reaction itself and require to be formed by the continual action of light, as in the case of hydrogen and chlorine. Or it may be more or less permanent and continue to act after the light is removed. Another class of reactions is that of the coupled reactions, in which the products of the light reaction are used up at once in another reaction with loss of energy.

Optical sensitisers belong to the class of catalytic light reactions. A system, insensitive to light of a particular wave length, can be made sensitive to it if a dye be present which absorbs this wave length. Since the light is absorbed by the sensitiser, it must act by means of the changes produced in this, resulting in the formation of some substance, frequently active oxygen, or some catalyst, which acts on the system which is by itself insensitive to the particular light in question.

The Bunsen-Boscoe law states that the product of the intensity of the light and its time of action produces a constant effect, so that intensity and time of action can mutually make up for each other. This law is replaced by an exponential ratio when a photographic plate is developed after exposure. This fact seems to depend on the intervention of an adsorption phenomenon in the process of development. The phenomena of fluorescence and phosphorescence are shown to be, probably, cases of photo-chemical reactions with storage of light energy, which is given off again afterwards.

Chemi-luminescence is the name applied to the emission of light in a reaction at a temperature much below that corresponding to the wave length of the light given off, if the system had merely been raised in temperature by the application of heat. It consists in the direct conversion of chemical energy into light, without passing through heat, and is the converse of those photo-chemical reactions in which light energy is converted directly into chemical energy, as in the chlorophyll system of the green leaf.

The reactions brought about by the chloroplasts of the green leaf result in the storage of a large amount of energy derived from the sun. Carbon dioxide and water are changed into starch and oxygen. Although the presence of the pigment chlorophyll is indispensable for the occurrence of the reaction, owing to its absorption of the light energy, there is no satisfactory evidence to show that carbon dioxide can be reduced by the pigment alone. Chlorophyll is the ester of a complex acid, consisting of pyrrol derivatives linked to magnesium. This acid is combined with a monatomic hydrocarbon alcohol, phytol, with twenty carbon atoms.

There are two forms of chlorophyll in the leaf, one a product of oxidation of the other. These are accompanied by two yellow pigments, carotin and xanthophyll, unsaturated and autoxidisable hydrocarbons. These latter are not related to cholesterol. The relation between chlorophyll and haemoglobin is described in the text. The absorption of light by chlorophyll is chiefly in the red and is in the position of the maximum energy of the solar spectrum during the greater part of the day.

The maximum action of chlorophyll is in light of a wave length corresponding to that of its absorption bands. There is reason to suppose that formaldehyde is the first product of photosynthesis. This is subsequently, perhaps also under the action of light, polymerised to higher carbohydrates. Although a substance giving aldehyde reactions is split off from chlorophyll by the action of light in the presence of oxygen, there is no evidence that this substance is other than a decomposition product of the pigment itself, perhaps of the phytol constituent. Its production takes place in the absence of carbon dioxide.

Along with the production of an aldehyde by light and oxygen, a peroxide of some kind is formed, just as in the case of ordinary optical sensitisation by dyestuffs. It is doubtful whether this is the source of the oxygen evolved in the normal course of the photo-synthetic process. At the present time, therefore, we have 110 evidence that chlorophyll acts otherwise than as an optical sensitiser for the reactions going on in the complex system of the chloroplast. We have no information as to these complex reactions, except that possibly iron plays a part in them. It must be remembered, however, that the chemical structure of chlorophyll is a rather remarkable one, so that conclusions as to its behaviour must not be made too hastily.

Certain electrical changes have been noticed to occur in green leaves under the action of light. They appear to be connected with the photo-synthetic action, since they are absent unless carbon dioxide is present. When the light available is deficient in the rays absorbed by chlorophyll, other optical sensitisers are found to be present and these absorb the rays which actually reach the cell. A brief discussion of the factors affecting the rate of photo-synthesis is givm in the text.

The efficiency of the photo-synthetic process is, at its optimum, a high one. Many of the constituents of living cells absorb ultra-violet light to a considerable degree. The reaction of certain small animal organisms to ultra-violet light obeys definite laws. A minimum duration is necessary; there is a limit of intensity below which no reaction occurs, and the effect is independent of temperature. These facts point to the occurrence of a photo-chemical reaction, whose products excite skin receptors

The direct action of ultra-violet light on micro-organisms and on tissue cells is a lethal or destructive one. The shorter the wave length, the more powerful the effect with equal energy of the radiation, probably owing to the greater absorption of the short waves by protoplasm. Similar action can be produced by visible light in the presence of a dye which can absorb this light (photo-dynamic seiisitisation). Application of the facts of photo-chemical reactions to the retinal process is suggested in the text.