Principles of General Physiology

Colour Vision. — This important question cannot be adequately treated here and the reader should refer to the various textbooks. There are, however, some facts, chiefly brought out by the work of Edridge-Green (1909 and 1911), to which brief reference must be made, because they are only just beginning to receive the attention they deserve. The Young-Helmholtz theory assumes that there are only three primary colour sensations, red, green, and violet. Now, while it is true that any colour may be formed by mixtures of these in appropriate proportions, it is also true that more than three primary sensations would also serve the same purpose, three is, in fact, the minimum. And it is a matter of universal experience that blue and yellow have just as much right to be considered primary as the other three. In fact, Newton's division of the spectrum into red, orange, yellow, green, blue, indigo, and violet is much nearer the truth. Indigo, however, is rarely seen as a distinct colour. Edridge-Green divides people into classes according to the number of distinct colours they distinguish and shows that there are various degrees of colour blindness according to the number of colours seen in the spectrum. From the point of view of evolution of the colour sense, he points out that it is practically certain that the distinction between different wave lengths, that is, the recognition of a difference between colours, would first show itself at the extremes of the region which is appreciated as light, the region between the

wave lengths 770 and 396 /x/z about. Red and violet would be distinguished first, next green between them would be added, finally yellow, and blue. Correspondingly, a common form of colour blindness is the tri-chromatic, where red, green, and violet are the only colours perceived. Yellow is called red-green, and blue, green-yellow. A further important point established by this investigator is that, contrary to what a casual examination of the spectrum might lead one to suppose, there is not an infinite series of gradations of colour along the spectrum, but that it can be divided up into a number of patches, each of these patches being of a uniform colour. Thus the eye is not capable of appreciating an indefinite number of spectral colours. The fact can be shown by the use of a spectrometer with adjustable shutters in the ocular. When any part of the spectrum is thus isolated, it is found that a certain breadth can be found which appears to be all of the same colour. Thus the whole spectrum is divided up, by normal sighted people, into some sixteen to twenty monochromatic areas.

Edridge-Green has brought out methods of testing colour vision on the basis of the above facts, together with other considerations. These methods are now being accepted as the only reliable ones. The existence of colour vision in the animals lower than man is obviously a difficult matter to decide. Orbeli (1909), in his work on conditioned reflexes, found the dog unable to form such reflexes to colour alone, merely to differences of luminosity. Later observers found that, with great difficulty, colour can be used in this way. The colour sense must be very rudimentary in the dog. Frohlich (1913) thinks that the difference between the electrical changes to red and to blue in the Cephalopod indicates' colour vision, but since differences in intensity of white are also associated with differences of rate of rhythm, the only evidence is the quantitative one of the rapid diminution in comparative effect as the intensity of the stimulus is increased. The apparent fondness of certain birds for brilliant colours, and, in fact, the general evolution of colour in flowers and butterflies and so on, suggests some sort of colour sense in these animals. According to Frisch (1914), a sense of colour is shown by fishes.

The numerous phenomena connected with positive and negative after-images are beyond the scope of this book. One fact should, however, be noticed, namely, that certain combinations of spectral colours give what appear to the eye to be colours as pure as the spectral colours themselves, but of a different wave length from those of which they are composed. For instance, red and green give a yellow, which is indistinguishable from spectral yellow. This fact is not easy to explain. Hartridge (1912) gives reasons for holding that the effect may be merely physical, so that the yellow-perceiving mechanism may really be excited by the mixture of red and green. (See also Edridge-Green, 1915.)

Mention should also be made of the new apparatus of A. W. Porter, which is the most perfect yet devised for the investigation of colour mixing, afterimages and other colour phenomena. This instrument has been shown at the Physiological Society's Meeting, but the description has not yet been published. Mosaic Vision. — The compound eye of the insect and crustacean is a highly developed organ and is usually considered to act as a series of tubes, with opaque walls, by which that ray only which is a continuation of the axis of the tube arrives at the receptor mechanism. In this way an image is formed. The explanation of the elaborate structures present, some of which appear to be refractile, is uncertain. The monograph by Exner (1891) may be consulted.

In order that the rhythmic vibrations of bodies, which are the material basis of what we ourselves call sound, may excite the ends of nerve fibres, it would seem that the most natural way would be to make use of the principle of resonance, which has been described on page 88 above. If we have a series of strings in regular order as regards their period of vibration, sound waves of a particular period will affect one of these strings only and set it in vibration. A structure of this nature exists in the cochlea of the internal ear of higher vertebrates and is known as the " basilar membrane.'1 It is true that it does not consist of separate strings, but as it only possesses tension in a transverse direction and not longitudinally, it is only capable of periodic vibration in the one direction. Its transverse measurement is of a regularly increasing magnitude from the base to the apex of the cochlea and the whole organ is coiled into a spiral. The nerve fibres, by means of a complex structure, the organ of Corti (see Fig. 166), are stimulated when that particular element of the basilar membrane which resonates to a given note is set into vibration by it. This is, in brief, the theory proposed by Helmholtz and further details may be found in his book " Tonempfindungen" (1863). In the sixth edition, the description of the theory will be found on p. 232.

Other theories have been suggested, such as the " sound pattern" theory, in which the basilar membrane is supposed to vibrate as a whole, but with " nodes," or lines of rest, in different places according to the pitch of the note. None, however, seems to agree with the general facts of the structure of the organ of Corti as well as that of Helmholtz does. This theory has, moreover, recently received a striking confirmation in the experiments of Yoshii (1909). Guinea pigs were exposed to the sound of a particular note on an organ pipe or siren for thirty to forty days in succession. Local degenerations were then found to have been produced in the organ of Corti. These degenerations were in different places according to the note made use of, and were transverse, not longitudinal. It is true that the degeneration extended somewhat on both sides of the actual region corresponding to the tone itself, but Helmholtz's theory of resonance would not exclude the possibility of neighbouring portions of the membrane being, to some extent, also set in vibration and it is clear that the nature of the experiments of Yoshii could scarcely afford evidence as to the minimal intensity of sound necessary to cause resonance of a very limited element of the membrane. Secondary changes were also found, in the experiments quoted, in the nerve fibres and ganglion cells belonging to the particular region of the organ of Corti affected by the sound ; none in the tympanic membrane nor in the transmitting structures of the middle ear.

The hair cells are supposed to act as transmitters of the vibrations of the membrane, possibly after these vibrations have been magnified by the structures forming the pillars of the arch of Corti, which rest on the membrane. The tympanic membrane is interesting physically. It is so formed, by its shape, tension, and attachments, as to be what is known as " aperiodic " ; that is, it has no definite period of vibration of its own, so that it can transmit any rate of vibration indifferently.

The perception of sound seems to have arisen somewhat late in the course of evolution. There is no satisfactory evidence that invertebrates or even fishes possess it. Of course, the periodic vibrations of a sounding body, if sufficiently strong, can affect the touch receptors of the skin, but, as we know from our own experience, the periodic impulses in the nerves from these organs do not give rise to the sensation of sound ; the cerebral " analysers " necessary for the purpose are not brought into play. This fact serves to confirm the view of the indifference of the actual nerve impulses themselves.

In birds and mammals the auditory organs, as valuable distance receptors, are highly developed, as we saw in discussing the conditioned reflexes of the dog. Their importance when speech, even in its most rudimentary forms, makes its appearance will be sufficiently obvious. In fact, the more or less musical notes made by certain insects, such as the cricket, by the aid of special apparatus, seems to imply the_ presence of an auditory organ of some kind.

Certain organs, present in most animals, even in the Medusae, were supposed at one time to be connected with the sense of hearing and were called " otocysts." These organs consist essentially of sacs, lined with cells, and containing a liquid in which a loose "otolith," or several of them, is freely movable. Nerve fibres terminate in the cells of the sac, and the " otoliths " may be sand particles or any similar substance, insoluble in the liquid.

Although Farre (1843) showed that these organs in the Crustacea act as " delicate antennae " and have no auditory functions, it was not until comparatively recently that it has been generally recognised that their function is to serve as receptors for the sense of position with regard to the direction of gravity. Verworn proposed that they should be called "statocysts" and the solid bodies within them, "statoliths." Beer (1898) showed definitely that Crustacea have no receptors for sound as such.

Fig. 167 shows the structure of a typical "statocyst" from Pterotrachea, and it is plain that the weight of the statolith will rest on different receptor cells according to the position of the animal and thus afford information of its position with regard to the vertical. An ingenious experiment of Kreidl (1893) neatly demonstrated the fact in Crustacea. As is well known, these organisms periodically shed their outer Ot, Statolith in the interior of the sac, which is filled with liquid. Wz, Hair cells on the inner surface of the wall. Hz and Cz, Cells with short bristles, supposed to be the sensitive cells.

(From Claus's ' ' Elementary Text-Book of Zoology. " Translated by Adam Sedgwick. London : Swan Sonnenschein, 1884, p. 86.) chitinous covering as they grow too large for it. Along with it, in sonic specie-, the inner lining of the statocvsts comes awav, naturallv taking the statoliths also. These latter must therefore be replaced by new grains of sand or similar bodies. Kreidl placed the animals under such circumstances that the only grains available were iron filings, which were duly taken into the statocvsts. On bringing a magnet into various positions with relation to the animals, the iron filings were attracted and pressed against various points of the lining of the statocyst, and the animals showed by their movements that the effect on them was the same as if, under normal conditions, they had been turned into such a position that the weight of the grains would have excited the cells in question.

A detailed account of the properties of the statocysts of Pecten will lie found in an article by von Buddehbroek (1911). In addition to organs of this kind, the vertebrate pos-e— e- a remarkable organ, the labyrinth or semicircular canals. The statocysts are known in the vertebrate as utricle and saccule. There are three semicircular canals on each side, and from Fig. 168 it will be seen that these loops are arranged in the three dimensions of space. This fact, in itself, suggests that they are concerned with the sense of position, but Crimi Brown (1874) and Cyon (1873) were the first to draw attention to the relation. It is, howe\er, to the experiments of Mach (IS?.")) and of Breuer (1891) that we owe the clear present ment of their modiof action. Flourens (1828) had obtained, on section of the semicircular canals, peculiar movements of the head, dill'ering according to the canal injured. It appearthat their structure is such as to enable rapid changeof position of the head in

space to be appreciated and their particular direction to be known. The receptor cells lining a part of the tube of which each of these loops is composed, and especially those of the dilated ampulla at the end of each, have long delicate hairs projecting into the liquid filling the tube. Since there is free circulation all round the circle, any movement of the head of the animal, to which the base of the hair cell is attached, will move these cells through the liquid, owing to the inertia of the latter not allowing it to share the movement at once. The result of this relative movement is to drag the sensitive hairs through the more or less stationary liquid, bending them and thus exciting the nerve endings at their attached ends.

The investigations of Ewald (1892), chiefly on the semicircular canals of the pigeon, will l>e found of much interest. They show how the sense of the position of the head in space is disturbed by injury or stimulation of various component parts of the labyrinth mechanism. The recent work of Wilson and Pike (1912) is a valuable contribution to the knowledge of the effects of stimulation and of extirpation of the labyrinth in various mammals. The reflex tonic contraction, especially of the muscles concerned in the maintenance of posture, is controlled by the labyrinth, and will be described in the next chapter.

FHJ. 168. SEMICIRCULAR CANALS OF MAX.— Photograph of an enlarged model made by Tramond, Paris. The three arcs are seen to lie in the three dimensions of space. The membranous canals are shown inside the bony canals, which are partly cut away. The cochlea is at the bottom. It is not to be supposed that the only information obtained as to the position of the head in space is derived from the labyrinth. The eyes, as well as the proprio-ceptors of muscles, play a large part in the process, these various receptor organs mutually correcting each other. The reader will probably have noticed how the eye is liable to be deceived by passive movements of the body, when these are unnoticed. A railway train rounding a curve deviates from the vertical owing to the " banking " of the track and, if the line is well laid, it is hard for a passenger to convince himself that the buildings which he sees through the windows are not leaning, although he may know that it is the train itself which is out of the perpendicular. The reader should, perhaps, be reminded that this observation is not possible on such lines as those where it is the custom of the guard to inform passengers in the restaurant car when the train is coming to a curve, so that there may be no " slip between the cup and the lip."

The astonishing sense of direction possessed by some animals, such as the carrier pigeon, is difficult to explain except as a wonderful memory for labyrinthine sensations received as it is taken from place to place in a basket. It appears that gradual change of position is appreciated rather by the statocyst organs and does not readily excite the receptors of the semicirpular canals, which respond to rapid changes to which the relatively inert particles of the former organs would not react sufficiently quickly.

It will have been noticed how sensations from different kinds of receptors are combined together to give more accurate and detailed information of external objects. This is particularly the case with the proprio-ceptors of those muscles which move receptor organs in a definite way, such as those of the eye and the hand, when combined with the sensation derived from these sense organs themselves. In this way the notions of space and so on are formed. But here we pass over to the province of psychology, and it is difficult to avoid the use of such words as "sensation," which imply consciousness, in the description of receptors. The reader must understand that nothing further is to be assumed here than the existence of certain nerve impulses passing to particular regions of the brain becoming connected up with other neurones, according to states present in other parts of the nervous system, and finally giving rise to the activation of some effector.

The discussion of binocular vision and similar aspects of the photo-receptor mechanism is beyond the space permissible here. Plants, like animals, are in relation with changes in their environment, and have also developed means of intensifying and determining the direction of the action of external forces. The former mechanism is especially well marked in the so-called "excitable organs," in the narrow sense, where rapid movement exists. The bristles of the leaf of Dionaea are quite entitled to be called receptors ; they make the leaf very sensitive to the contact of insects. A similar phenomenon is to be seen in the leaf stalk of Mimosa pudica, also in the stamens of various species of Centaurea (the blue corn-flower) and in other situations.

In their sensibility to gravity, whose direction they are able to appreciate plants have an actual separation in space of the receptor and effector as there is in animals. It is the point of the growing rootlet that is sensitive, while tlie response occurs in a region at some distance from this. It may be mentioned here that the proof that the roots of plants are sensitive to gravity was first afforded by Knight (1806), who used centrifugal force to replace gravity and thus obtained a more powerful stimulus. In such cases, it would be quite justifiable to speak of a "reflex action."

As to the mechanism of the gravity receptors, a similar view was arrived at independently by Haberlandt (1900) and by Nemec (1900). It assumes that each cell of the sensitive tissue corresponds to a statocyst of the animal. In the plant cell, the statoliths are usually starch grains, which fall and form a little heap on the lowest part of the cell. The precise part of the cell thus affected depends on the position of the root or stem as regards the vertical line. In plants in which the statoliths consist of starch, exposure to cold causes them to be used up and the reaction to gravity is abolished until more are formed in warmth and light.

The direction of light is appreciated by the leaves of plants, as shown by their setting themselves at right angles to it. Haberlandt (1909, p. 557) points out that, in many cases, the outer ends of the epidermis cells are of a vaulted shape, so that parallel rays of light would be brought to a focus somewhere near the inner ends of the cells. If the axis of the cell is directed towards the light, the middle of the base of the cell is most brightly illuminated, and it is to be presumed that when the brightest part moves to one side or the other a reaction takes place in the stem, the result of which is to bring the bright spot to the centre again. In other cases the cuticle is formed into a lenticular shape. Haberlandt has shown photographically that the light is actually brought to a focus on the inner ends of the cells by these arrangements.

The articles on sense organs in plants by Haberlandt (1904 and 1909) will be found of interest. The finer the differences between external forces which an organism is able to appreciate, the better equipped is it to make use of or to defend itself against these forces. Nerve fibres themselves are not sufficiently easily stimulated by these forces, except in cases where the latter are actually injurious and damage the structures of the organism. There are, in fact, free nerve endings in the skin for the appreciation of such nocuous stimuli.

A mechanism of some sort is therefore necessary to magnify the various minute forces acting on the organism, so as to produce a force of sufficient magnitude to set up a propagated disturbance in nerve fibres. Such mechanisms may be of different kinds, since nerve fibres are excitable by electrical, mechanical, chemical, and other stimuli. These mechanisms are the " receptors." A primitive kind of chemical sense, allied to taste and smell, seems to be one of the first developed. Touch receptors, to appreciate delicate contact, would also be of early formation.

Events occurring in the organism itself, as well as those of the external world, require to make their existence known to the nerve centres. Hence we have intero- and extero-ceptors. Amongst the former are the proprio-ceptoro, l>v which an organ under the influence of excitation from the centres gives information of its state of activity to the centres themselves. The distance receptors, such as the eye, ear, and, to a certain extent, those for smell, are the most important in the development of the highest intellectual qualities.

Since nerve disturbances are all of identical nature, whatever be the kind of external energy which acts on the receptor organ, it is clear that the ditVerence between sensations derived say from the eye and the ear, must lx> due to the arrangements of the nerve centres, the "analysers." This is Miiller's "law of specific senserenergies." A nerve fibre of special sense, ho\\e\er excited, alwavs gives rise to the same sensation. A receptor organ differentiated for a particular kind of stimulus, differs from other receptors, in that it is sensitive to very small stimuli of the appropriate kind, which would be far below the limit of appreciation by a receptor adjusted for another kind of stimulus. The amount of light energy required to excite the

retinal receptors is very small indeed, compared to that of the same form of energy required to excite the heat receptors of the skin, for example. In the skin, there are receptors for heat, cold, touch, and pain. These are again grouped by the first relay of central analysers into the two groups of protopathic and epicritic sensibility. These two groups also apply to other regions of the body, some regions, however, being possessed of receptors for the protopathic group only. Their more precise definition will be found in the text.

Receptors for light are, in all probability, arranged to make use of a photochemically sensitive substance. The products of this reaction, or possibly the changes of energy involved in the course of the reaction, are such as to excite the nerve terminations. Thus we may have a primitive sensibility to light situated in the skin generally. But, to be of value as a distance receptor, an organ for light stimuli requires to be able to form images of external objects. So that we find a dioptric mechanism present to produce an image on a sensitive surface composed of a number of elements each connected with a separate nerve fibre.