Principles of General Physiology

According to this view, normal tonus is also associated with a slow consumption of protein material in the sarcoplasm under the influence of the sympathetic system. It is true that the appearance of creatinine and uric acid in the urine, referred to above (page 289), would be thus accounted for, but further evidence is required. It has recently been shown by Kure, Hiramatsu, and Naito (1914) that the diaphragm is kept in a state of tonus by impulses from the sympathetic system, conveyed by the splanchnic nerves. When these nerves are cut, the diaphragm is drawn up into the chest by the negative pressure in the pleural cavity.

Certain effects of adrenaline on skeletal muscle, which have been described, are of interest in the present connection. As we shall see in Chapter XXII., this product of the activity of the suprarenal glands has the special property of exciting the nerve endings of the sympathetic system and producing the effect due to stimulation of sympathetic nerves themselves. Has it then any effect on skeletal muscle1? Oliver and Schafer (1895, p. 263) found that the twitch of the voluntary muscles in the frog and in the dog was considerably prolonged after an injection of suprarenal extract, an effect similar to that of veratrine. But if the sympathetic endings of Boeke were stimulated by the drug, one would expect that a shortening of the muscle would take place without the application of a stimulus to the nerve. Cannon and Nice (1913) have described experiments in which injection of adrenaline enabled a muscle in situ in an animal to continue contracting longer, without fatigue, than in the absence of the drug. This effect is

regarded, no doubt correctly, as being for the most part due to increased blood supply from the rise of arterial pressure caused by the adrenaline. At the same time, the investigators are inclined to think that there is also a direct effect, since that on the muscle lasts longer than the rise of blood pressure. But might it not be the result of the previous increase of blood supply? More convincing is the experiment in which the blood pressure in the limb under experiment was prevented from rising by compression of the artery. The effect was still present. According to Panella (1907), adrenaline is an antagonist to curare, hence it must act on some receptive substance in the muscle. This is confirmed by Gruber (1914). It is clear that some of the remarkable characters of the tonic state of skeletal muscle maybe explained by the existence of the sympathetic innervation, but further evidence is required. Recently, Kuno (1915) has failed to obtain any evidence of an action of adrenaline on the voluntary muscle of the frog. Nor has he been able to confirm the production of tonus tlrrough the sympathetic rami.

The constant activity of certain nerve centres, such as the respiratory and vasomotor centres, is well known. The question has been argued as to whether the state of excitation in such cases is actually automatic, or whether it is kept up by afferent impulses from the periphery. There can be no doubt that such impulses are able to modify the state of the centres, and also that chemical substances, such as hydrogen ions, present in the blood, are able to set up a state of excitation in nerve centres. We have seen (page 457) that the heat regulating centre is accessible to the direct action of heat and cold.

On the whole, then, we must admit that afferent nerve impulses are not always necessary. It is naturally a question not easy of experimental attack to decide whether there is an automatic state of excitation apart from stimulating substances in the blood, although it does not seem impossible. There are, obviously, many centres which must not be active, except for special purposes ; such are those of the voluntary muscles. Even here, however, the question arises as to whether the inactivity of centres not in use may not be due to inhibition (see the experiments of Pavlov on conditioned reflexes, pages 503-506).

The tracing given in Fig. 126 (page 418) is of interest in the present connection. The fact shown there suggests that the respiratory centre is naturally in a state of tonic excitation ; since, when the afferent impulses from the lungs are cut off by section of the vagus nerves, the diaphragm passes into a state of partial tonic contraction. Involuntary, smooth muscle, in its various situations, is capable of remaining in a state of moderate contraction, or tonus, independent of any influence from nervous centres.

Certain muscular systems in invertebrates, such as that which closes the shell in bivalve molluscs, and many others, are able to maintain a shortened state against a heavy load for a long time without any evidence of fatigue. This state of " fixation " may occur at any length of the muscle fibres, and with any tension. A similar condition is met with in the urinary bladder of mammals, and probably in the muscular coat of the arterioles, as well as in other situations.

It may be compared to the putting into action of a " catch " or ratchet mechanism. To put the " catch " into action, or to remove it, requires the intervention of nerve impulses from centres ; these impulses would be excitatory in the first case, inhibitory 'in the second. They are conveyed by distinct nerve tracts. The actual mechanism may possibly consist in the prevention of the spontaneous disappearance of the products (lactic acid) caused by stimulation of the muscle system, by which its potential energy is converted into that of tension.

The "plastic tonus" of skeletal muscle, described by Sherrington in the decerebrate animal, appears to be of similar nature, although the mechanism is in the nerve centres in this case, instead of being peripheral. In this state the vasto-crureus, for example, may be maintained at different lengths with the same load, or at the same length with different loads. Thus the fibres may have a different tension with the same length. The phenomenon is a reflex from proprio-ceptors in the muscle itself.

Decerebrate tonus behaves towards inhibitory stimuli in a manner different from that shown by ordinary reflexes. There is some evidence to show that this decerebrate rigidity is not accompanied with increase of metabolism, or with comparatively little. It is affected reflexly from muscle receptors of the neck and from the labyrinth. Certain facts described in the text indicate a relationship of tonus in skeletal muscles to a sympathetic innervation of these muscles.

The question as to the automatic activity of nerve centres is discussed briefly in the text. SINCE the whole existence of living organisms on the earth depends on the receipt of radiant energy from the sun, it is unnecessary to point out the importance of the study of the way in which this energy is made use of. If it were merely converted into that form of energy in material bodies which we know as heat, it would be a very wasteful process, as our study of energetics has taught us. Much of the free energy would be thereby lost in the process of conversion to other forms. A considerable part of the sun's energy is, of course, used up in warming objects on the earth, but the study of the chemical reactions brought about by the action of light is of greater importance, although of some difficulty.

The amount of energy actually received from the sun may be somewhat realised by the following data : Suppose that the atmosphere were absent and the sun in the zenith, then each square centimetre receives per minute 1 '955 small calories, expressed in heat units. The presence of the atmosphere, which absorbs a part of the radiations, reduces the value to l~2 small calories, at the latitude of Cambridge. In other words, the energy received by 1 sq. m. (10,000 sq. cm.) in one minute would suffice to raise the temperature of a kilogram of water by 12°.

The reader will not need to be reminded that the most important of all photo-chemical reactions is that by means of which the chlorophyll system of the green plant stores up light energy and, in the process, restores to the atmosphere the oxygen which has been used up in oxidation by living beings. The stores of energy in coal and petroleum also owe their origin to chlorophyll in past ages, indirectly in the latter case, if we accept its animal origin.

In order that we may be in possession of the means of understanding, as far as is possible at present, the mechanism of the process concerned, we must first enter somewhat fully into the general theory of photo-chemical reactions. In addition to the chlorophyll system, there are other actions of light which are of physiological importance. Such are the retinal process and the action of ultra-violet light. In practical use, the various photographic methods may be mentioned, as well as those of wireless telegraphy, which makes use of waves like those of light, but of very much longer wave length.

All substances absorb radiant energy to some extent ; glass itself absorbs rays of longer wave length than those we know as light, and also those of shorter wave length. The colourless substance, anthracene, absorbs ultra-violet rays, as is shown in the photograph of Fig. 176, and many other instances might be given. In other words, a part of the energy of a beam of light which tra verses any substance is removed and held back in the substance. Something must, therefore, happen to the substance ; it may be merely warmed, or other forms of energy may make their appearance in it, chemical or electrical change, and so on.

Grotthus's Law. — It seems fairly obvious to us at the present time that no effect can be produced by light unless it is absorbed. We shall see presently also that some light energy must be used up to start any photo-chemical change, even when the reaction afterwards proceeds with evolution of energy. The law that light must be absorbed in order to produce an effect was first clearly enunciated by Grotthus (1819, p. 101) and, independently, at a later date, by Draper (1841). It is frequently known as Draper's law.

Grotthus found, for example, that ferric thiocyanate, which is red, is decolorised by A, Series of photographs of ultra-violet absorption spectra of anthracene. Below each one there is a normal spectrum. The relative intensities of the two spectra are varied in known ratio by means of rotating adjustable sectors. At a certain wave length in each pair, marked with a white spot, the intensities of the light are equal. Ordinates— amount of absorption corresponding to the frequency of vibration of light indicated by the abscissae.

green light, yellow gold chloride by blue light, blue starch iodide by yellow light. Each is attacked by light of the colour complementary to its own colour, that is, by the light which it absorbs. The Laws of Lambert and of Beer. — In order to be able to compare the amount of light absorbed by one substance or solution with that absorbed by another, it is necessary to take some standard of nirasuivmrnt. Bunscn and Roscoe (1855-1859) introduced the extinction coefficient for this purpose. Their definition of it will be found on p. 6 of the reprint in Ostwald's " Klassiker," No. 38.

When light of a particular wave length is absorbed by any substance, it is clear that the intensity of the light issuing from it is less than that which enters it, and that there must be some particular thickness of it which reduces the intensity of the light to one-tenth of the value it had on entering. In order that the numbers, characteristic of different substances, should rise or fall in the same direction as the absorbing power of the substance or solution, Bunsen and Roscoe defined the extinction coefficient as being the reciprocal of the depth of the solution required to reduce the intensity of light of a given wave length to one-tenth of that which it had on entering. It is plain that the greater the absorbing power, the less the depth required ; hence the advantage of the inverse value, the extinction coefficient being directly proportional to the absorbing power.

The symbol e is generally used for the extinction coefficient, so that if d is the depth of solution required to reduce light of a given wave length to one-tenth of its value, then the extinction coefficient for this wave length is Now in practice it is the intensity of the issuing light that is measured, and it is more convenient to use a constant thickness of solution, and to measure the intensity of the light that has passed through this, than to vary the thickness of the absorbing layer. It is therefore necessary to know the laws which express the relation of the one to the other.

We have already seen (page 35) that the relation is a logarithmic one, and it is known as Lambert's law when applied to the case of a pure substance, solid or liquid. Beer showed that the same law applies to solutions. Let us call the original intensity of the light I, and that after passing through a layer d, I'. Then, by the definition of the extinction coefficient, I' = I x y~ After passing through a second layer, I' = (Ix— )x— = Ix — -- and after x such layers I' = — .

In general, if light of unit intensity is reduced to -th by a certain thickness of solution, after passing through x times this thickness, its value will be — • In order to get rid of the exponent we take logarithms, thus : — Hence, e = - — ^ — and, knowing the ratio of the light transmitted to that entering a solution of depth x, we can calculate the extinction coefficient. For convenience, x is taken of one centimetre depth, so that c = — log I', that is, the negative logarithm of the unabsorbed light.

For example, suppose the intensity of light of the wave length of the D line is reduced to two-thirds of its intensity by passing through a stratum of a particular solution of one centimetre thickness. Then, By Beer's law, the absorption of light by solutions is directly proportional to the concentration ; so that, if we know the extinction coefficient for a known concentration, we can estimate an unknown concentration by measuring its extinction coefficient. Hence the practical value of the form given to the extinction coefficient. Thus : —

c and c being the respective concentrations, and e and e' the respective extinction coefficients, Such measurements have played a large part in the investigation of the blood pigments. Resonance.— Light consists of a series of periodic electro-magnetic disturbances of various periods of vibration, or wave length. We have seen (page 88) that when a system, such as a pendulum, has the same period of vibration as that of a series of minute impulses delivered to it, the system is set into vigorous movement by the heaping up of the effect of a number of small impulses. It is, in fact, a means of accumulating energy. Consider now the effect of a set of various wave lengths, such as we find in the sun's light, on a chemical molecule, which has itself a definite rate of vibration. Some of the rates of vibration of the different light waves will almost certainly coincide with that of the molecules of the absorbing substance and will therefore set these into resonant vibration, which may reach an amplitude great enough to bring about chemical change. At the same time, those rays of the vibration period in question will be absorbed and, if situated in the visible part of the spectrum, there will be an absorption band seen by the eye. If in the ultra-violet, as in the case of many colourless organic compounds, the band, although invisible, may be photographed.

Spectrophotometry. — Observations by the spectroscope give us information of the position of absorption bands, but we often require measurements of the degree of absorption by different substances in various regions of the spectrum. This is done by the method of Spectrophotometry, based on the laws of Lambert and Beer. In practice it consists in the comparison of the intensity of the light of a particular wave length, which has passed through a known thickness of the solution investigated, with light of the same wave length which can be diminished in intensity in a known degree.

The practical methods of doing this and of calculating the extinction coefficients will be found in Gamgee's article (1898, pp. 213-225). The price lists issued by Messrs Adam Hilger are instructive, especially with regard to the beautiful instruments made by them for the registration, photographic or otherwise, of spectrophotometric measurements. Fig. 176 (page 549) is a copy of the curve of the ultra-violet absorption of anthracene as obtained by one of these instruments. The paper by Eckert and Pummerer (1914) may also be consulted with respect to photographic registration.

We may take it then that light of some particular wave length is absorbed, and that it sets into resonant vibration the molecules of the absorbing substance, if any of the vibration periods of the light waves coincide with those of the latter. What is the further course of events ? We know that, in many cases, chemical reaction follows. Let us see first what are the general phenomena with which we have to deal. The article by Luther (1908) may be referred to for more details of the general theory than can be given here, and the monographs by Weigert (1911) and Sheppard (1914) for the whole subject.

In the first place, we find that the rate of the reaction does not follow the simple law of mass action. This is due to the fact that it is controlled by the amount of light energy absorbed per unit time and not by the actual number of molecules present. An instructive case is that of the oxidation of quinine by chromic acid in light, as investigated by Luther and Forbes (1909). The order of this reaction depends on the colour of the light; violet light is only slightly absorbed and the reaction is unimolecular, ultra-violet is strongly absorbed and the order is very much lower ; since this light is totally absorbed, the rate of the reaction is independent of the concentration of the reacting substances.

A curious result of this fact is that the order of the reaction depends on the thickness of the layer of solution through which the light passes. In a thick layer the relative amount of violet light absorbed increases, so that the order of the reaction is higher than in a thin layer, where the violet light is scarcely absorbed at all. We have then a reaction, whose order depends on the shape of the vessel in which it takes place. In practice, it is found that the majority of reactions brought about by light are of the nature of oxidations or reductions, that is, reactions in which changes of valency take place, as we shall see in more detail in the next chapter. But all kinds of reactions are also to be met with.

As the phenomena of resonance imply an increase of free energy in the system concerned, it is not unexpected to find that when chemical change takes place it is in the direction such that the resonance is diminished or ceases, according to the second law of energetics. The mechanism of this resonance process is, according to Luther (1908), essentially as follows, on the basis of the electro-magnetic theory of light, the electronic (atomic) nature of electricity, and the electrical nature of chemical combination. Compounds consist of molecules or atoms smaller than themselves and between these constituent elements, that is, between their electrons, there is an electric field. The stronger the field the firmer the combination, or the more inactive the compound, and the shorter the period of vibration of the (negative) electron ; that is, the further in the ultra-violet the absorption bands lie, the more stable the compound. Conversely, the further the absorption band lies towards the red end, the more sensitive is the compound to light. Thus anthracene, with its bands in the ultra-violet, is less sensitive than chlorophyll, with its band in the red. Researches by Luther and Nikolopoulos (1913) on a series of organic compounds confirm this view.