Principles of General Physiology

and deep. The last group is derived from receptors in the deeper structures and appears to have given rise to some error in previous experiments on the time of regeneration of sensory fibres. The whole of the deep structures in the hand, for example, are not supplied by the same nerve. The protopathic system is common to the whole body. It regenerates more rapidly than the epicritic system and is of a more primitive nature. The position of the point stimulated cannot be recognised and a widespread, radiating sensation arises from it. The receptors do not appear to be as sensitive as those of the other types but, when excited, the effect is a powerful one. In the skin, they respond to painful stimuli and to extremes of heat and cold. The receptors of deep sensibility answer to pressure and to movement. The fibres from the limbs run in the motor nerves. The system of Pacinian bodies is associated with deep sensibility. The epicritic system is a highly differentiated one and regenerates very slowly. It is only found in the skin and endows it with sensibility to delicate touch, with the power of localisation of stimuli, of distinguishing two points and of discriminating fine degrees of heat and cold, together with the other forms of special sensation. It is obviously the system connected with the development of specialised receptors and the intellectual powers resulting from them. Full details will be found in the paper by Head and Rivers (1908). Page May's article (1909) may also be consulted.

It is impossible to make a distinction between the differentiated chemical sense of water animals and that of smell in land animals. We are, therefore, justified in regarding the delicate appreciation of the neighbourhood of certain kinds of objects, an endowment which can be shown to be independent of sight and possessed by so many marine animals, as a form of the sense which we know as smell. The great enemy of the scallop is the starfish, so that, as soon as the near, presence of a starfish is recognised by the mollusc, its curious swimming movement is executed by this latter and it escapes beyond reach of its enemy. Although the scallop possesses numerous welldeveloped eyes around the edge of the mantle, stimulation of these by the shadow of the enemy is not sufficient to excite flight, for the reason which we shall see presently. But W. J. Dakin (see von Uexkiill, 1912, p. 329) showed that a small quantity of an extract of starfish, dropped by a pipette in the neighbourhood of a scallop, causes an immediate swimming away. This is a clear instance of the differentiated chemical sense which we may call smell.

A detailed investigation of the sense of smell in higher vertebrates has been made by Zwaardemaker (1902). It may become of a most extraordinary delicacy, thus : T^ mg. of mercaptan in. 230 cub. m. of air can be detected, and, of course, only a few cubic centimetres of this air are necessary for the production of the sensation, so that somewhere about 1 x 10~* mg. is sufficient to excite the receptors for smell. The paper by Parker (1913, 2). may be consulted with regard to the analogies

Radiant energy, from the sun or similar source, may be absorbed by the skin, converted into heat and thus excite heat receptors. But light waves have also a powerful effect in producing many chemical reactions, and these chemical changes may be such as to stimulate special end organs. Further, as we shall see in more detail in Chapter XIX., which may with advantage be read before the present section, any particular chemical reaction is produced only by a certain group of wave lengths, so that the possibility is presented of distinguishing between light of different wave length, that is, a sensibility to colour.

It is probable that appreciation of light and darkness by some photo receptor, sensitive to a photo-chemical change in a substance with which it is in contact, would be the first to appear. By this means the proximity of food or enemy would be recognised, although the aid of some other receptors, probably those of a chemical sense, such as smell, would be required to distinguish between the two. The sea anemone appears to possess photo-receptors of this simple kind (see von Uexkiill, 1909, p. 71), and according to Parker (1903 and 1905) the power is also present in the skin of fresh-water fish, such as the Ammom-tr, and in Numerous amphibia (see also the monograph by Nagel, 1896). Parker does not think that the elaborate eye of the vertebrate has been formed from this primitive sensibility of the skin to light. The receptor mechanism of the vertebrate eye is, embryologically, an outgrowth from the central nervous system, so that it seems more probable that the photo-receptors concerned may have been formed in the central nervous system itself of a transparent animal.

However this may be, it is obvious that a mere sensibility to light and shade is of comparatively little value, until a mechanism is developed by which images of external objects are formed on a sensitive surface composed of a multitude of elements connected with separate nerve fibres. By this means a picture is, so to speak, conveyed to the brain. A fully developed eye consists, then, of some dioptric mechanism, corresponding to the lens of a photographic camera, together with a layer of a photo-chemically active substance, like the silver bromide of the plate. In the eye, we have also endings of a large number of nerve fibres, attached to complex receptors, which serve to produce nerve impulses from the photo-chemical changes. This part is called a retina. It is also necessary that stray light should be kept out by an arrangement like that of the camera bellows and the dark slide. This is done by pigment cells, which absorb the light. Fig. 160 shows a simple eye. The complex structure of the retina of the vertebrate may be seen in Fig. 161 (from the monograph by Ramon y Cajal, 1894). It is to be remembered that the peripheral receptor mechanism proper consists of the rod and cone, and perhaps of the pigment, layer ; the other layers, as will be seen from the figure, consist of neurones, interposed between the actual receptor neurones and the nerve centres. The retina thus consists in great part of nerve centres, owing to its mode of development. In the eye of the Cephalopod, which is a highly differentiated one, similar to that of the vertebrate, these intermediate . neurones form a distinct ganglionic mass, outside the eye itself. In these organisms, also, the light impinges directly on the receptors, the retina not being inverted, as in the vertebrate, where the incident light passes through the nerve layers before reaching the rods and cones.

Arrangements of the simple kind described above are found in some of the very primitive organisms, as may be seen from Fig. 160, but it is doubtful whether such simple dioptric mechanisms can do more than serve to concentrate the light on the sensitive cells. In Pecten, we have a number of eyes of an elaborate nature, shown in Fig. 162, from Dakin's monograph (1909). There are here a number of separate nerve fibres, and, in consequence, the possibility of an appreciation of something approaching an image. It is interesting to note that the arrangement noted above in the vertebrate retina, namely, the passage of the light through the nerve layer before reaching the sensitive substance, is also met with in Pecten. The fact suggests that there may be some reason for

Fn;. 160. OCKLLUS OF LIZZIA KM K i.i. IK KI; i. — Seen from the side. After treatment with dilute osmic acid. Obj. F., Oc. 2. the arrangement, in addition to that usually given, namely, the formation of the nerve layer by invagination of the front hemisphere of a spherical outgrowth. From the work of von Uexkiill (1912, p. 329) it appears that, whatever image may be formed on the retina of Pecten, no response is called forth until the object moves. Further, the movement of any object excites the same response, which is a protrusion of the long tentacles, endowed with chemical and tactile sensibility. The object of this response is obviously to obtain further information, and flight results if it turns out that the object is an enemy. Otherwise, flight would be a waste of energy.

When dioptric apparatus is present of sufficient accuracy to form a clear picture on the retina, some mechanism is obviously necessary to adjust the focus for near or distant objects. In land animals, the chief refracting surface is the curved cornea, since the refractive index of the aqueous humour on the inner side of it differs more from that of air than that of the lens does from those of the liquids in which it is immersed. This can readily be shown by observation on the eye of an albino rabbit. Owing to the absence of pigment, the image of a distant window with cross-bars can easily be seen through the outer, sclerotic, coat of the eye-ball. If a microscope slide be held in such a position as nearly to touch the cornea, and a drop of physiological saline solution be placed between the cornea and the glass, the image disappears, since the refracting surface is now a plane one. On removing the glass, the image reappears.

to do the chief work in the formation of an image. Accordingly, we find that its curvature is very much greater than in land animals, the lens being nearly spherical in shape. In land animals, the chief use of the lens is to adjust the focus of the dioptric system. The curvature of the cornea is not made to change. According to the work of Beer (1898-1901) accommodation to near or distant objects is effected in two ways. The first is that present in invertebrates, in vertebrates up to and inclusive of amphibia, together with certain snakes, and consists in the actual change of position of the lens, just as in the photographic camera. The second mechanism is found in a few snakes, in tortoises, lizards, crocodiles, birds and mammals, and consists in a change of the curvature of the lens. In its natural position in the eye, the lens, which is elastic, is focussed for distant objects, owing to the way in which it is pulled upon by the ligaments holding it in place, which cause its curvature to be a flatter one than that assumed when released from tension. But,

FIG. 161. THE RETINA OF THK DOG. — Prepared by Golgi's method. In section. e, d, Bipolar cells (inner granules) with vertical ramifications of their outer processes. In the centres of the ramifications lie the enlarged ends of rod fibres. e, Other bipolar cells with flattened ramifications abutting against ramified 'ends of /, Giant bipolar cell, with flat ramification. <j, Inner granule cell sending axone towards rod and cone fibres. A, Amacrine cell in inner molecular Ia3'er, with diffuse arborisation on ganglion cells. i, Ascending nerve fibre. j, Centrifugal fibres.

m, Nerve fibres which become lost in inner plexiform layer. »i, Ganglion cells which form synapses with the end branches of a bipolar cell belonging by the contraction of a ring of muscle, the ciliary muscle, this tension of the ligaments is released, like that of a stretched cord of india-rubber would be if the attachment of one of its ends were pulled nearer to that of the other end. In consequence of the release of tension, which admits of degrees, the lens assumes

D.c.L, Distal cell layer. R.e p.. Pseudo-rod cells. B.C., Rod cells of retina. R., Rods of retina. (Reproduced from \V. .T. Dakin's monograph on "Pecten," by permission of the Liverpool Marine Biology Committee.) more or less the form it takes when free, that is, a more spherical one ; hence it is able to focus near objects on the same plane on which it previously brought distant ones to a focus. At this point, I may stop for a moment to mention that this mechanism of accommodation was first made clear by Helmholtz, to whom we owe a very large part of our knowledge of the eye as well as of the ear. Although we have already

met with his name in connection with several other fundamental phenomena, such as the doctrine of energy, the electrical double-layer, the rate of the nerve impulse, and so on, this is perhaps the most appropriate place to call attention to his portrait, which will be found in Fig. 163 (given by the kindness of Dr J. T. Bottomley, of Glasgow). His two books on "Physiological Optics" and on The function of the dioptric system of the eye is essentially 'a question of geometrical optics. It can be satisfactorily treated by the Gauss method of reduction to certain refracting surfaces at definite distances apart. Details may be found in the textbooks; that of Parsons (1901) and the article by von Rohr (1909) may be mentioned. We must pass on to the consideration of the phenomena which have been found to occur in the retina in response to stimulation by light.

Movements of Cones. — Slow movements of the cones in the frog, brought about by contraction of the long fibres attached to their bases, were described by Van Genderen Stort (1887) (see Fig. 613, p. 522, of Schiifer's " Essentials of Histology "). They appear to result from a reflex, since light entering the other eye causes retraction of the cones in the eye which has not been exposed to light. The effect is also produced by light on the skin, by injection of strychnine and by local electrical stimulation. It is difficult to see what is the function of this movement. It has been suggested that it may be a relic of an ancestral state in which the photo receptive cells of the epidermis were connected directly to contractile cells, although, if we accept the view of the origin of the eye in nerve centres, there are obvious difficulties in t^his interpretation.

PORTRAIT OF HELMHOLTZ. — Taken in his laboratory on 7th July 1894. The Pigment Cells of the retina are also excited to movement by the action of light, and here again the use of the phenomenon remains problematical. The " Dermatoptic Function" described by Raphael Dubois (1892), is of interest, as it shows the possibility of light absorbed by pigment acting as stimulus. The siphon of the mollusc, Pholas, is sensitive to light and is retracted when light falls upon it. The response is due to the presence of pigmented cells in the epithelium, prolonged into contractile fibres. According to Dubois, the reception of the light stimulus results in contraction of the fibre, which contraction then, in some way, stimulates nerve fibres going to centres and thus setting off a reflex contraction of the siphon.

Steinach (1892) states that pigmented muscle cells are to be found in the iris and that these cells can be seen, under the microscope, to contract when light falls upon them. The observations were confirmed by Guth (1901). Changes have been described in the Ganglion cells of the retina, but these clearly must be regarded as effects of prolonged stimulation on the cells of nerve centres. The Visual Purple. — There is every reason to believe that the means by which light stimulates nerve endings is through a photo-chemical reaction. There are an enormous number of chemical reactions which are affected by light, and, of these, one is known in connection with the retina, namely, the changes in the " visual purple." Whether this is the only one we cannot say with certainty, but we shall see presently that its properties are in extraordinary coincidence with certain aspects of vision. As will be shown in Chapter XIX., if a substance is sensitive to rays of a particular wave length, such as would be necessary to account for colour vision, it must absorb these rays. Since they must be in the region of the visible spectrum, the substance must have an absorption band in the region referred to, and, therefore, be itself possessed of colour. Although visual purple is the only such substance detected as yet in the retina, with the exception of certain coloured globules, which are not sensitive to light, described by Kiihne (1878), in the cones of birds, it is conceivable that others may be present in the small amount required, and even in the requisite number, to account for the number of colours to which the eye is sensitive. It is, moreover, not impossibfe. that visual purple, as obtained, may be a mixture of a number of different substances, each with an absorption for a particular group of wave lengths and giving rise to its own particular photo chemical product, to which a definite receptor only is sensitive. More probably, in the formation of the particular photo-chemical product, molecular vibrations of a certain rate are excited, possibly by resonance, with the excitation of receptors by the energy set free.

Although it had been known for some time that the retina of a frog, removed in the dark, appeared to be of a red or purple colour, when observed in the light, and that the colour disappeared more or less rapidly, the definite association of the pigment in question with visual sensation was not made until Boll's work (1876), followed by the more extensive and detailed work of Kiihne and his fellowworkers The colour of the pigment is not exactly what most people would call purple, it contains much more red. But, having a trace of violet in it, it is best described as a deep pink or rose colour.

It is bleached by light, but, in the retina, the colours return in the dark. Whether there is new pigment formed or whether the products of the action of light return to their original state in the dark, a very common phenomenon in photo-chemical reactions, is not altogether certain. It appears, however, that under some conditions, solutions of the pigment recover their colour when allowed to stand in the dark after being bleached by light. It has no isolated absorption band in the spectrum, but absorbs light almost equally in all parts, leaving a little red and violet, hence its colour. It is to be expected, then, that it would be responsive to light of all wave lengths, except the extreme red and violet. As indicated above, a series of substances with absorption bands along the course of the spectrum, when mixed together, might give a similar continuous absorption.

Visual purple is found in the rods only of the mammalian eye, in the so-called cones of birds, and in the corresponding structures of the retina of the frog, fish, and cephalopod. Since it is not present in the cones of the human eye, it is absent from the region of sharpest vision, the fovea centralis, a fact which has led some observers to doubt whether it has any real importance. Edridge-Green, however, has brought forward evidence to show that it diffuses into the fovea from the surrounding rods. The rods, themselves, he regards as being concerned only with the formation of the pig- .5 ment and not receptor organs for ? light. As regards this last point, it appears that the sensibility of the various zones of the retina in the recognition of form is directly proportional to the number of cones per unit area which they contain. Put in another way, the images of two points are recognised as distinct according to whether they fall on two cones or not, so that they must be further apart to be recognised as two in the peripheral parts of the retina, where the cones are further apart. But the microscopical appearance of the cell connections of the rods is very similar to that of the cones and does not suggest that of secretory cells only.

Kuhne showed that the pigment is sensitive to light while in the eye, and that photographs of objects can be made on the fact. These (1878, p. 225). The difficulty of obtaining information as to the chemical nature of visual purple, as it has become the custom to call it, is obvious, on account of the very small quantity to be obtained. Its solubilities are peculiar ; according to Ktihne, it is only dissolved by bile salts with readiness. This fact suggests a colloidal suspension ; the lowering of surface tension produced so powerfully by these substances would facilitate a great dispersion of the particles. It does not, in fact, diffuse through parchment paper, so that the bile salts can be removed by dialysis. In addition to dispersion of the pigment, the bile salts appear to disintegrate the rods. The pigment is not attacked by trypsin, so that it is not of protein

nature. The method used by Kuhne to obtain his purest preparations will be found on p. 454 of his paper with Ewald (1878), and on p. 266 of his article in Hermann's " Handbuch " (1879). When light falls on the peripheral parts of the retina in man, it is found that, when diminished so as to be just visible, it is only that part of the spectrum between wave lengths 600 and 440 fj./j. (orange to blue) that is visible at all, and the sensation is one of light only, without colour, whatever the wave length

used. Now Victor Henri et Larguier des Bancels (1911, 1) have determined the amount of energy just sufficient to excite (threshold energy), the bleaching effect on the pigment and the amount of light absorbed by it, all at various wave lengths between the values named. When put into curves, these three factors are found to follow the same course (see Fig. 164). This means that to produce the same sensation, by different wave lengths, requires such an amount of radiant energy that the amount absorbed by the visual purple is the same in all cases. This is a powerful argument in favoup of the participation of the pigment in vision, at all events in that particular form of the sensation investigated. The same investigators find that the absolute quantity of energy required varies with the duration of action according to a complex law, which seems to result from a combination of that of excitation of nerve with that of a photo-chemical reaction. If the energy quantum be worked out by the formula in Nernst (1913, 255, etc.), it is 2 x 10" 1J erg for the D line, practically the same as the limit of the sensibility of the retina (page 512 above). This sensibility is then the maximum possible.

Electrical Changes. — Holmgren (1880) noticed that the incidence of light is accompanied by an electrical change in the retina, and further work was done by Dewar and M'Kendrick (1876), Kuhne and Steiner (1880), Waller (1900), Gotch (1903 and 1904), Einthoven and Jolly (1908), Piper (1905, 1910, and 1911), Frohlich (1913), and others. Although, when the interpretation of these results is better understood, they will undoubtedly help in the explanation of retinal processes, it must be confessed that, at present, they do not throw much light on the question. The main fact is that, in the uninjured eye of the vertebrate, the incidence of light causes an electrical change in such a direction that the nervous layer of the retina becomes electrically positive to the rod and cone layer. This state, with some subsidiary waves, lasts during the illumination, and disappears again, at a certain rate, when the light is removed. But, immediately after removal of the illumination, and before the effect produced by it has disappeared, a puzzling further change, in the same direction as that produced by the incidence of light, makes its appearance for a short time. Apparently darkness causes a temporary effect of the same sign as light does. When, however, the curve is carefully analysed, as can be done with that obtained by the capillary electrometer or the string galvanometer, it is found to have a complex form, which Einthoven and Jolly and Piper have resolved into a compound of three different curves of different time course, and it is important to note that, by proper construction of each of these components, it is possible to obtain a curve like the original, including the " dark " effect, from curves which have an opposite direction in light and in darkness.

This fact disposes, indeed, of some of the difficulty attending the dark effect, but seems somewhat artificial, since no explanation can yet be given of the meaning of the three hypothetical processes. It is true that the Young-Helmholtz theory supposes that there are three fundamental components in visual sensations, red, green, and violet. When in certain relative proportion, the sensation of white light results and that of various colours according to the relative proportion of the three components. But Gotch's experiments (1904) showed that the electrical response to red, green, or violet light was of the same form in each case, although differing in latent period and magnitude, while each showed a similar rise when the stimulus was removed. The three hypothetical components of the curve, assumed by Einthoven and Jolly and by Piper have, as it seems, nothing to do with the three fundamental sensations of the Young- Helmholtz theory. We shall see other reasons later why this theory cannot be regarded as a satisfactory one. Still less do the facts of the electrical change support theories such as that of Hering, where colours which are complementary to one another, that is, which make white on mixing, are supposed to cause opposite changes in the " visual substances," the one anabolic, the other catabolic. If this were so, red and violet should give electrical changes of opposite sign.

As examples of actual experiments, Fig. 165 (according to Piper. 1910) may be of interest. The electrical change in the mammal is of a simpler nature than that in the bird. Piper also shows (1911, Figs. 38 and 39) that the response i- a simple one in the retina of the Cephalopod, a fact which indicates that a part of the complexity of the vertebrate effect may be due to the nervous elements of the retina. If we regard it as probable that the electrical response is connected with a photo - chemical reaction, we may consider for a moment such a reaction as that of the decomposition of silver chloride by light in the simplest conditions, so that the products are not removed from the sphere of action by other reactions. In the dark, we know that silver and chlorine reunite to form the chloride again, and at a rate controlled, in the main, by mass action. As soon as light begins to act on the chloride, a portion is decomposed and the products begin to recombine again owing to their own affinity and independently of the action of light. It will be clear, therefore, that, according to the intensity of the illumination, an equilibrium will be established at such a position that the rate of recombination is equal to that FIG. 165. ELECTRICAL CHANGES PRODUCED BY LIGHT IN THE of decomposition RETINA. — String galvanometer. The white areas are

As soon as illumination ceases, recombination begins at its own proper rate, since the opposing reaction no longer exists. Further particulars of such photo-chemical changes will be found in Chapter a, Eye of pigeon. Brief illumination (0'23 second). Negative and positive b, Eye of pigeon. Longer period of action of light (072 second). <j, Effect of brief periods of darkness (O'l second each) on the pigeon's eye previously exposed to continuous illumination.

d, Eye of rabbit. Illumination for 0'5 second. Positive variation with light and secondary slow rise. Small, slow negative variation, after short latent period, with subsequent darkness. e, Eye of Cephalopod (Eledone). Simple positive variation with light, after latent period of 0'023 second ; remaining practically constant during illumination. Return to original value with darkness, also after a latent period. see that there are other reactions which would illustrate the point more accurately, since the decomposition of silver salts by light is not quite so simple as assumed above. It serves, however, to illustrate the nature of the apparent equilibrium

attained under the action of light, which is only kept up by the continuous supply of light energy. (See Bauer, 1911, on the regeneration of visual purple, which continues during the action of light.) It may be mentioned that Brossa and Kohlrausch (1913-1914) have found that the form of the electrical response varies with the wave length. The work of Frohlich (1913), also, on the eye of the Cephalopod requires consideration. This eye presents certain advantages as regards the question before us. As already pointed out, the nervous elements are situated in a ganglion outside the eye itself. Although the retina is very highly developed, the visual elements are of one kind only, similar to the rods of the vertebrate retina. It has visual purple and the receptors are exposed directly to the light rays. Frohlich confirms the result of Piper that the electrical response is less complex than that of the vertebrate. But the chief interest of his work consists in the demonstration of the fact that the retinal electrical response is not a steady one, but consists in a series of rhythmic waves, from 20 to 100 per second, being more rapid the more intense the illumination. These waves are also to be seen on the return of the curve after cessation of the stimulus, but of a lower rate. There is no indication of a " dark " response in the same direction as that on illumination. The effects of red and of blue light were also compared with that of white and the interesting fact found that red light required to be increased enormously more than white or blue to give the same increase in electrical effect, as shown in the table : —

With the same intensity, the rate of the rhythmical changes is less with red than with blue. Whatever may be the significance of the vibratory nature of the electrical change, it is clear that it does not represent the actual rate of vibration of the light itself, but it does not appear to me that the author's conclusion that the red end of the spectrum is exciting, the blue end inhibitory, on account of the rapid rate of the waves produced, is a necessary one. The view of the Verworn school that rapid rates of nerve impulses produce inhibition has been discussed previously (page 426).