Summary. — We find that the light passes through all of the transparent structures just described and falls finally upon the outer members of the rods and cones. The rods and cones are stimulated, that is, some chemical change takes place in them, and a neural impulse arises in the nerve elements with which the rods and cones are in contact, the bipolar neurones. These neurones arouse in turn impulses in the ganglion cells (neurones of the optic tract) with which they are in contact. The impulse passes! thence to the central nervous system.
Other Structures in the Retina. — There are many other ele- ments in the retina, such as the supporting fibers of Mueller, and neuroglia cells. There are certain other neural structures as well, the so-called horizontal cells of Ramon y Cajal, which apparently have as their function the tying together or association of a num- ber of retinal elements.
It should be mentioned that in addition to these sensory structures which correlate or get impulses to the central nervous system, there are endings of fibers in the retina which apparently conduct impulses from the central nervous system to the retina, that is, function as motor fibers. It may be that this centrifugal mechanism plays an important role in explaining certain visual phenomena, for example, some of those found in hallucination and in ideoretinal light. Impulses might be carried from the Central nervous system which actually arouse the retinal recep- tive elements, which in turn might cause them to function in particular cases as though light fell upon them. It has been shown that the retina is peculiarly liable to be stimulated mechan- ically by pressure as in the phosphenes, and by an electric current.
104 PSYCHOLOGY Possibly every adjustment process of the eye-ball arouses faint afferent impulses. The retina is thus probably never free from afferent impulses.
Action of Rods and Cones Under Light Stimulation.— There are some peculiarities in the action of the rods and cones when they are stimulated both by homogeneous and non-homo- geneous light. The outer member of the rod and the inner segment of the cone contract under the action of light. Again, under the influence of long continued action of light, the pigment cells which lie just outside the rods and cones (they really belong to the retina and make up its first layer) extend their processes down among the outer members of the rods and cones. In darkness these cells contract and clear the rods and cones. It has been shown further that the two changes just spoken of above (at least so far as the contraction of the cone is concerned) occur in both eyes simultaneously when only one is stimulated by light. If for example, the right eye is stimulated by light and the optic tract to the left eye is cut, there is no movement of the pigment or contraction of the cone in the eye not actually stimulated by the light. This would seem to prove that there is a centrifugal or motor path to the retina.
Probably the most interesting physiological phenomenon ob- servable in the eye is the action of the visual purple. It has been several times clearly shown that the outer members of the rods secrete a purplish-red pigment when the eye has not been exposed to light for a considerable time (two hours). Under the action of white or monochromatic light this purple gradually fades, passing through several changes in color, and disappears at the end of fifteen minutes. When first discovered it was hoped that a true physiological theory had been approached. Ebbinghaus actually formulated a theory of color vision in terms of the action of visual purple. In view of the fact that it does not appear in cones, the human fovea would thus be without any function, as would that of birds and other animals whose retime contain only cones. Nor does its assumed function as acting as a sensitizer for the rods in darkness adaptation fare much better, since many animals whose retina contain only cones or mainly cones can THE RECEPTORS AND THEIR STIMULI 105 still dark adapt, therefore the view is forced upon us that the true function, of the visual purple in the rods has not been com- pletely worked out.
Binocular Vision. — In our discussion of the eye so far, we have considered it really as a single organ. But usually the two eyes work together. In uniocular vision there is a lack of definite- ness and perfectness in reactions to objects at varying distances from the eye. If one eye is closed in a normal individual, the reactions to objects at a distance are fairly accurate. On the other hand, those to objects close at hand are very inaccurate. Try this by having some one touch your finger, having one of his eyes closed, or thread a needle or appose two sharp-pointed in- struments. The adjustments are poorly made. The moment he opens the other eye, the acts are accomplished with ease. It should be mentioned in this connection that a little practice greatly improves this type of adjustment, so that the man born with only one eye functional, or who has had one eye removed, is not at such a loss as might be supposed from these observations. When the two eyes work together, there is (a) a greater extension of the total vision field, since a part of the possible field of view of any one eye is blocked by the nose. The true binocular field,. that is, that portion of the total field from which images reflected into the eyes fall vipon corresponding points, is smaller than the sum of the fields; of the two eyes; (ft) in binocular vision, the images falling upon the retina are more complete, since each eye contributes certain elements, that is, there are two fixed points of view; (c) the response to form, size and distance of objects from the eye are more accurate; (d) defects in the one eye do not disturb the adjustments of the subject when both eyes are stim- ulated. A reference to the blind spot will simplify this. If only one eye is used, it is quite possible that the image of an object might fall upon the blind spot. No reflex tendencies would be released whereby the eye would be turned so as to receive the object upon the fovea; the object would not be reacted to. When both eyes are stimulated by an object, it is not possible for the image to fall upon the blind spot of both eyes, hence reflex tendencies will start in one eye a£ least and the other eye will be 106 PSYCHOLOGY forced to follow. The result will be a perfect focussing of the object upon the spot of clearest vision in each eye.
Muscles of the Eye and Their Action.— The movements of the two eyes are controlled by six large striped muscles attached to each eye-ball. Their points of attachment are shown in Fig. 18.
Levator palpebrse superioris Sclera Tendon of superior oblique Lacrimal gland -Lateral rectus rnea • Inferior rectus Orbitala dipose Medial rectus Inferior oblique FIG. 18. — Dissection of the left orbit from in front. (Morris' Anatomy, Courtesy P. Blakiston's Son & Co.)
The joint action of the medial rectus and of the lateral rectus rotates the eye inward and outward. The superior rectus and the inferior rectus rotate the eye upward and downward and somewhat inward. The inferior oblique and the superior oblique rotate the eye outward and upward or outward and downward. These are the simple eye movements which result from the action of pairs of muscles. In many cases three or more muscles are involved and the relationships become very compli- THE RECEPTORS AND THEIR STIMULI 107 cated indeed. Attention is called to the fact that the muscular system of the eye is at the same time a part of the kinaasthetic system and indeed an extremely important part.
Condition for Single Vision. — Owing to the coordination and balance existing in the eye movements, the two eyes become really a binocular instrument. In order to have single vision with both eyes functioning, it is necessary that the images of the objects fall upon certain points in the two retinae which are called ' * iden-. tical or corresponding points. ' ' Whenever this happens the sub- ject reacts to a single object. In all cases where the images fall upon disparate points, drglopia, or double images, results, that is, the subject reacts to two objects. Occasionally one of these images is so blurred and indistinct that its presence does not very much disturb reaction. One can learn, of course, to react to either image. This appears quite clearly in the use of a microscope. After a little practice the individual becomes monocular in his use of the microscope, and the image from the unused eye is "neg- lected. ' ' Corresponding points on the retina may be made clear by making a paper image of the right retina and one of the left, first dividing each into quadrants, and sliding the left model into the right without rotating or otherwise disorienting them: the corresponding quadrants in the two retinae will be super- imposed. The upper and lower nasal quadrants of the left eye correspond respectively to the upper and lower temporal quad- rants of the right eye; the upper and lower temporal quadrants of the left eye correspond respectively to the upper and lower nasal quadrants of the right eye. The most important corre- sponding points are the two foveae.
Horopter. — In each fixed position of the eye there are a num- ber of points in the field of view, images from which will fall upon the corresponding points of the two retinas. It we plot a figure embracing all such points, the figure is called the horopter for that position. If a distant point is fixated upon the horizon, the two images of the point fall, of course, upon corresponding points, namely, upon the fovese. But it is found that even if this fixa- tion is held, the images of many other objects above, below, right and left, while not falling upon the foveae, fall nevertheless upon 108 PSYCHOLOGY corresponding points in the retinae. They -are reacted to singly; the subject can enumerate them or name them correctly.
Visual Reactions Involve Habit Systems. — Attention must be directed to the fact that in our visual reactions to objects there are many complex non-visual factors. In the first place, visual reactions are really habit systems visually set off or initiated. We have in the past not only reacted visually to such objects; we have touched them in the dark, handled them, walked toward them, away from them, in intense illumination, in shadow and in fog, now with the eye in one position, now in another, now with one set or accommodation of the lens, now with another, with competing objects in the field of view, and again with few or none. Various habit systems thus grow up, so that if even a part of the original visual component is actually present, the absent motor elements are redintegrated or associatively aroused (there is undoubtedly present a vast system of conditioned reflexes). This can be seen very plainly in watching any complex motor act. With every change of movement in the performer there goes along a more or less complete change of tensions in the mus- cular system of the observer, so that he is thrown into the proper "attitude" to make the next adjustment. For example, in watch- ing a fight, we tend to ward off a blow or to strike a blow. The inter-relations of these factors and the dependence of our im- mediate visual reactions upon past habits can best be seen, pos- sibly, in the way an experienced hunter sets his rifle sights to bring down his prey. The novice, going into a clear mountain- ous country for the firsit time, sets his sights for a too close range and the bullet falls short. The experienced hunter has learned by trial and error that he must increase his range over that which he would use on the plains. In the same way habits grow up based upon the size of the retinal image and upon the presence or absence of a clear image upon the retina. The presence or absence, too, of intermediate objects in the field of view to which we have previously made adjustments markedly affect our visual reac- tions to any distant abject. An individual unused to the sea may start out to row his boat to a distant object. The, chances are that it will take him two or three times as long as he had cal- THE RECEPTORS AND THEIR STIMULI 109 culated. The point to be emphasized in all of our visual adjust- ments is. that we are dealing with integrated habit systems and conditioned reflex systems. Each time the visual receptor is stim- ulated, the kingesthetic receptors (eye muscles, the ciliary muscles controlling adaptation and other factors) are simulta- neously stimulated and the complex of impulses resulting redinte- grates the established habit systems. The reactions which we make, whether a mere verbal reaction as to the form, size or distance of an object, or the starting out to walk to a distant point, are dependent upon such complex factors. It must be noted, too, that we can give no verbal report about the various roles played by the different factors. A deer springs up in the fog, in shad- ows, in dim light, in the red light of early morning, in the clear light of noon, on a mountain or on a plain: immediately the rifle is raised, pointed at a varying distance ahead of the animal, and fired. The animal falls. No hunter (not even a psychological one) can give a verbal report- that contributes one iota to the event. We have made a visual motor adjustment and the com- plex factors involved in it all work harmoniously together.
The Stereoscope. — Since visual stimuli really touch off habit systems, it follows that we can, by a very simple arrangement of visual conditions, initiate reactions which really belong to another general environmental setting. For example, if we obtain two photographs of a landscape taken simultaneously with a twin camera with the lenses fixed, at the same distance apart as the two eyes, or slightly wider, and place these photographs in a stereoscope which allows the two images to fall upon correspond- ing points without fatigue of accommodation and without the intrusion of lateral images, we have a close approximation to the landscape situation as it stimulates the eye in nature — we obtain - the "stereoscopic" effect even though the stimulus in each of the eyes is presented in one plane. The reaction obtained from the subject is like that obtained in ordinary binocular vision. The subject states that the various objects appear in different planes and at different distances from the eye. On the other hand, the visual conditions can be so arranged that the image which usually falls upon the right eye has to fall upon the left, and vice versa.
HO PSYCHOLOGY HO PSYCHOLOGY When this is done, an inverted relief is reported. The pseudo- scope is an instrument admirably arranged to make the image ordinarily falling upon the right eye to fall upon the left and vice versa. Hollow objects are thus reacted to as solid objects, and solid objects as hollow.
Untrustworthiness of Vision.— The various "illusions" or errors in visual reactions to which the eye is subject probably ,Fio. 19.— The Muller-Lyer figure.
depend to some extent at least upon a certain lack of balance and coordination in the functioning of the motor system of the eye. For example, the eye under-reacts to acute angles and, over-reacts to obtuse angles. If the movements of the eyes are photographed while traversing the two forms above (Fig. 19), it will be seen that the movements are more extended on the right-hand figure (obtuse FIG. 20.— The Franklin figure.
angles) than on the left where they are acute angles. In the latter case the eye movement is checked. The two horizontal lines are of course equal. Again, if a subject is asked to draw equal horizontal and vertical lines (in the form of a cross) with- out the aid of a rule, it will be found that he draws the vertical line too short. This is supposed to be due to the fact that hori- zontal eye movements are made with less effort than vertical ones. Occasionally the eye is downright untrustworthy. Make up a drawing like that shown in Fig. 20;.have some one hold the drawing flat at the level of the chin six or eight inches from the face, and tell him to fixate on the point of intersection of any two lines. He will tell you that there are three lines, two hori- THE RECEPTORS AND THEIR STIMULI 111 zontal and a third line standing vertical to the drawing. This is really a very special case of diplopia or double vision. In the various text-books of psychology a great deal of attention is paid to these illusions, but little can be said about them at present which will be contributory.
Visual Hallucinations. — From "illusions" to "hallucina- tions ' ' there is only a step. We have already said that the eye is always under stimulation. This is shown by the presence of action currents, the ideoretinal lights, and the hypnogogic images (long-continued after-effect of stimulation). Possibly also slight stimulation is afforded by each lens adjustment and by each change in convergence and divergence of the eye-ball. In addition, we know that there are centrifugal neural impulses reaching the eye from the central nervous system. The retina is peculiarly a sense organ that is never at rest and its activity is not wholly dependent upon outward stimulation of light. At- tention, too, has been called to the fact that visual impulses of a very simple character sometimes touch off complicated visual- motor habits. It is small wonder, then, that, in pathological cases where the retina is probably over-active so far as these internal changes are concerned, as in fevers, delirium tremens and the like, we see the subject reacting apparently to a visual object to which other persons present do not react. In under- standing this, attention is called to the fact that in normal per- sons, when there is a readiness of certain action systems to function the moment a certain visual stimulus appears, the action is often touched off by any visual stimulus; for example, a con- cealed enemy is ready to fire at the head of a soldier. The soldier, instead of exposing himself, holds up his hat or a dummy. The enemy's fire is drawn and a momentary respite is obtained with the chance to make the next objective. The higher the emotional tension, the easier it is to set off such premature reactions. This may account for the fact that a man in the throes of delirium tremens covers his head to shut out the vision of the snakes that squirm around the wall. His reading and his conversation with other imbibers have taught him that after drinking too much he will ' ' see snakes. ' ' Long indulgence has made such habit systems ready to function. Any wavering of fixation, the presence of 112 PSYCHOLOGY sinuous shadows on the wall, the presenci of any entoptic phe- nomenon may touch off both explicit and implicit types of reac- tion. We shall return to the fact again and again that visual reactions may become short-circuited into word reactions. The patient blind from birth can describe in appropriate words all of the beauties of the setting sun. Provided such; a person were in the throes of delirium tremens and there were conditional emotional reflexes of a proper kind present, there is some reason to think that he might exhibit many of the characteristics of one suffering from visual hallucination. We have no such factual case at hand, be it said. This explanation would hold perfectly well for the asserted cases of hallucinations appearing in patients long after both eye-balls have been removed.
General Summary. — In considering, as has been do"ne in this chapter, the various factors which must be taken into account in providing stimuli for the control of human action, it may be argued that we have not offered to the subjects experimented upon the same kind of situations they will meet in daily life. Rarely are they stimulated with pure monochromatic lights, pure tones, with two olfactory substances which balance or cancel one another, and only rarely does the environment offer stimuli to which they make the simple types of reaction that they make in the laboratory. The laboratory obviously selects its problems and investigates one phase of them at a time. For this it has been criticised. The criticism would be justified if the laboratory made no effort at other times to make good this defect. That this science does attempt with some success to deal with larger human problems, total situations and total reactions will appear from some of the material we later present. Even granting the nar- rowness of our conclusions in sensory physiology, it is safe to say that most of the facts we have presented here have been of service and will continue to be of service to one or another group of scientists outside of psychology. Findings in sensory physiology are used by the physiologists themselves, by the neurologists, by specialists in ear, nose and throat, by the surgeon, by the psychi- atrist, and in the Army and Navy, as well as in the arts and trades. To trace their use in these fields is beyond our present aim.
CHAPTER IV THE ELEMENTARY FACTS ABOUT THE NEURO-PHYSI- OLOGICAL BASIS OF ACTION Introduction. — Having- studied the receptors and found that their activity involves the initiation of neural impulses, our next problem is to learn something about neural conduction and the arrangement of pathways over which such impulses must pass in order to reach the effectors — the muscles and glands. We should say in the beginning that all neural impulses initiated in a sense organ have to pass either through the spinal cord or the brain or both of these organs before reaching the muscles and glands. Hence it is necessary for us to take up the elementary facts about their structure and functions. If we attempted to make a study of the whole nervous system, even in outline, we should find that our task could not be accomplished without going into a laboratory and actually working with neurological material. We can, though, apart from such laboratories, obtain a fairly good working notion of (1) many of the things which the nervous system as a whole has to do, (2) of the elementary neural structures and (3) the way the latter are chained to- gether to form the reflex arcs that make possible our acts in daily life.
The Unit of the Nervous System. — The unit of the nervous system is the neurone. A complete neurone is shown in Fig. 21. It consists (1) of a cell body with (2) its axone and (3) its den- drites. The cell body is a somewhat complicated and not thor- oughly understood structure. It contains a nucleus which does not differ greatly from the nucleus of any other cell. The most char- acteristic part of the cell is its cytoplasm, which is made up of neurofibrils, fine fibrils' that are continuous throughout the axone, cell body and dendrites. The perifibrillar substance is a fluid- like substance that surrounds the neurofibrils. The cell contains in addition chromophilic substance, flake-like masses scattered PSYCHOLOGY throughout the cell body and larger dendrites, but never in the axone or the axone hillock. These subdivisions are shown in Fig. 22. In the embryonic or developing nervous system the cell body which is called a neuroblast, first appears. If one were to watch the neuroblast develop into a partially complete neurone (as has actually- been done), he would see the axone hillock first form and from it would develop the slender axone process, and later the dendrites.
The axone, ax, Fig. 21, is a slender outgrowth from the cell body varying in length from a fraction of a mm. to more than a meter. Under the high power microscope it is seen to be made up of elementary neuro- fibrils. There is usually only one axone to a cell body. It is to be distinguished from the dendrites by its straighter course, its uniform diameter and smooth outline, Fig. 24. Shortly after leaving the cell body the axone may become myeli- nated, i.e., covered by a fatty sheath whose function may be insulating or nutritive. It may possibly also be concerned with conduction. In addition to the medullary sheath, many of the axones are covered also with the more primitive sheath of Schwann or neurilemma. The sheath of Schwann is probably not found in the central nervous system. Cer- tain axones, for example, those of the sympathetic neurones and of the FIG. 21.— Scheme of peripheral motor neurone. The cell body, dendrites, axone, collaterals, and terminal arborizations in the muscle are seen to be parts of a single cell, the neurone, c, cytoplasm of cell body containing chromophilic bodies, neurofibrils and perifibrillar sub- stance; n, nucleus; n', nucleolus; a, dendrites; ah, axone hillock, free from chromophilic bodies; ax, axone; «/, collateral; m, medullary sheath; nn, node of Ranvier where branch U given off; «/, neurilemma (not present in central nervous system); m', striated muscle fiber; tel, motor end plate. (From Bailey's Text-Book of Histology.) Courtesy Wm. Wood NEURO-PHYSIOLOGICAL BASIS OF ACTION 115 olfactory, may have a sheath of Schwann with- out a medullary sheath. We have then two main divisions of axones: my- elinated axones, with or without a neurilemma; and non-myelinated ax- ones, with or without a neurilemma. The axone on ending usually splits up into a terminal brush. These brushes always end (1) around the dendrite of another neurone or (2) in a muscle or gland or (3) in some sense organ structure (if the peri- pheral process of an affer- ent neurone is classed as an axone) (page 116). On their course inside the central nervous system axones give off collaterals Or Side branches Which FIG. 22.— Diagram of a cell body from the J j j j 'j. £ ventral horn of the gray matter of the human spinal end around dendriteS OI cord showing arrangement of ^ neurofibrils. ax, axone; neurOnCS WllOSe Cell DOdieS stances; n, nucleus; x, neurofibril passing from one ,..,,...dendrite to another; y, similar fibril passing through lie in the brain and COrd. the body of the cell. (Herrick's Introduction to m,.. Neurology.) W. B. Saunders Co.
The remaining parts of the neurone are the dendrites, d, Fig. 21. They have a structure similar to that of the cell body. They may be exceed- ingly numerous or lacking altogether. They exhaust them- selves by subdivision and end near the cell body. To this general statement there is one exception: the dendrite of an afferent spinal neurone (Fig. 23) consists of a single process which is exactly like the axone — it runs a straight course, it is smooth and myelinated. However, it ends in a sense organ and PSYCHOLOGY PSYCHOLOGY conducts toward the cell body. We have»learned that axones conduct the impulse centrifugally, that is, away from the cell body, and that the dendrites conduct centripetally. Hence the outgrowth from such a cell body as we are considering is an axone so far as structure is concerned but a dendrite in its func- tion. Dendrites of all other types of neurones, since they are so intimately a part of the cell body, are probably important devices for securing nutrition for the whole neurone. They are beautifully "j contrived for this purpose since they offer many points of contact with their nutrient environment. The dendrites must also take part in conduction since the end of axones often comes in contact only with the dendrites.
The neurone as a whole is thus the an- atomical, embryological, functional and Fia. 23.— Schema of peripheral afferent neurone. 1, spinal ganglion cell; 2, dendrite (or peripheral axone process); 3, free nerve ending in epithelium; 4, line to show entrance of axone or central process into the central nervous system; 5, short caudal branch ending in the gray matter of dorsal column; 6, collateral of the cephalic branch likewise ending in the gray matter of the dorsal column; 7, end arborization of the axone; the axone may end around cell bodies in the medulla or turn in at any point to end around cells in the .dorsal column.
trophic unit. There are no other structures in the nervous sys- tem, so far as we know, which take part in neural activity. Sev^ eral types of neurones are shown in Fig. 24.
Those to "be emphasized here are as follows: A, the peripheral afferent neurone; B, the peripheral motor neurone, and E, the NEURO-PHYSIOLOG1CAL BASIS OF ACTION 117 interconnecting central neurone (Golgi Type II). There are very many types of neurones not shown in this figure.
The Reflex Arc. — Although the neurone is a unit of the ner- vous system it cannot function alone. It becomes functional from a conduction standpoint only when its connections are estab- lished. The functional unit of conduction is called a reflex arc.
FIG. 24. — Showing some of the varieties of the cell bodies of the neurones of the human nervous system including the dendrites and small portions of the axones, axone sheaths not included. A, from spinal ganglion; B, from ventral horn of spinal cord; C, pyramidal cell from the cerebral cortex; D, Purkinje cell from cerebellar cortex; E, Golgi cell type II from spinal cord; F, fusiform cell from cerebral cortex; G, sympathetic, a, axone; d, dendrites; c, collateral branches; ad, apical dendrites; bd, basal dendrites. In the cell marked A p is the peripheral process ending in a sense organ, c is the central process ending within the central nervous system. (Morris* Human Anatomy.)
A schematic diagram of a reflex arc involving a segment of the spinal cord is shown in Fig. 25. It consists of a neurone of type A in Fig. 24. In Fig. 25 the neurone is marked 1. One of its processes (the dendrite) ends in a sense organ structure, SS, in the periphery, while the other process (the axone) ends around the dendrites of a neurone of type E above, numbered 2, the com- PSYCHOLOGY plete neurone lying usually in the gray matter of the cord; and of a third neurone (spinal motor neurone) whose cell body lies in the gray matter (anterior horn) of the cord, but the axone of which ends in a motor structure, MS. This neurone is numbered 3. Occasionally possibly it may happen that a reflex arc is com- pleted without the intervention of the intermediate central neurone. In such a case the peripheral sensory neurone, 1, may end, or one of its collaterals may end, directly around the den- drites of the motor neurone, 3. These are only schematic pic- tures, however. Usually many neurones are involved in any reflex arc.
The Synapse.— There has been a great amount of discussion