The skeleton is built up of a vast number of such levers. "When- ever the organ moved must have speed but needs to overcome but little resistance, the force is supplied to the shorter arm of the lever; whenever, on the contrary, speed is not essential, but great force is required, the power is applied to the longer arm. An example of the first case is a movement of the forearm; of the second case, the raising of the body upon the toes. For all the finer movements of the more mobile parts of the body there are both flexor muscles and extensor muscles. These two are antag- onistic. The one will flex or bend the arm, for example, at the 'Hunt maintains that the sympathetic fibers also go to the muscle plasm. Kempf and also White have accepted this extremely problematical conclusion, and have tremendously overemphasized the importance of the sympathetic system.
ORGANS OF RESPONSE elbow; the other will straighten it out, extend it. Since both muscles are elastic and always under tension, the mobile organ is always delicately balanced. A slight impulse to the flexor draws the arm up smoothly, and a slight impulse to the extensor will straighten it just as smoothly. It has been shown that whenever a motor impulse goes to a flexor causing its contraction, there goes also a neural impulse to the extensor muscle causing it to lengthen or relax. Likewise, when the extensor is contracted the flexor is relaxed. The muscles around a joint are thus set in opposed groups, one group relaxing while the other contracts. The Nature of Muscular Activity. — In normal reaction the FIG. 48. — Schematic drawing of contraction of frog's gastrocnemius. Time tracing from tuning-fork giving 100 vibrations per second. A, B, latent period; B, C, phase of contraction; C, D, phase of relaxation.
muscle is contracted (shortened) by a neural impulse passing to it over its own motor nerve. The muscle itself, however, is irri- table, as is shown by the fact that when separated from its nerve supply it can be made to contract by a stimulus applied directly to it. It can be aroused by a blow, by a sudden change in tem- perature, by chemical stimuli and by electrical stimuli. The elec- trical stimuli are most convenient to use.2 Simple Contraction. — The contraction ®f a muscle to a single stimulus shows three phases; a schematic tracing is shown in 2 By irritability is meant that a tissue can be made to exhibit its own particular form of activity, whenever and however it is stimulated; e.g., the muscle cells contract no matter how stimulated, whereas the gland cells when stimulated secrete.
164 PSYCHOLOGY 164 PSYCHOLOGY Fig. 48. The muscle is stimulated electrically at a, it begins to contract at b, reaches the apex of contraction at c, begins to relax rapidly, then more slowly, becoming normally relaxed at d. The time line is indicated below the curve. The interval from a to 5 is (1) the latent period. Although stimulated, no movement can be observed. The time of the latent period is very short, possibly not over 0.005 sec. It varies greatly in the different muscles. The condition the muscle is in also causes it to vary, as do tem- perature, fatigue and the load the muscle has to raise.
The interval &-c is (2) the phase of the shortening. The muscle shortens at first slowly, then rapidly, then more slowly. This interval is about 0.04 sec.
The interval c-d is (3) the elongation or relaxation phase. Relaxation is rapid at first and then slows down. This interval is about 0.05 sec. A good many factors alter the time of these last two phases. Temperature affects both the shortening and relaxa- tion phases. At 0° C. the muscle loses its irritability; at about 9° C. the contraction phase is high. From there it falls off slightly, then begins to increase again at 18° C., becoming maximal at 30° C. The contractions then die out, heat rigor taking place at 37° C. or above. In no case does the muscle (or other protoplasmic tissue) withstand a temperature higher than 45° C. The duration of con- traction is prolonged by low temperatures. As the temperature rises the duration of the contraction as a whole decreases until at 18° C. it becomes constant. Certain drugs greatly affect the sim- ple contraction. The alkaloid veratrin has the following effect: (1) The phase of shortening is not altered, but the phase of relaxa- tion is much prolonged. After the height of the shortening has been reached there is a brief relaxation, followed by a second slower contraction.
We discussed on page 119 the relation of strength of stimulus to response in nerve tissue. We found that in the nerve the "all- or-none law" seemed to hold and we mentioned the fact there that it held in the muscle as well. If a single muscle fiber is con- tracted it contracts to its full capacity; increase of stimulus merely brings more muscle fibers into operation. As soon as all are stimulated the response of the muscle is maximal. Gradation of ORGANS OF RESPONSE 165 muscular activity is thus brought about by the number of fibers at work and not by the intensity of the stimulation.
Effect of Repeated Contraction. — If instead of stimulating the muscle with a single induction shock we give it repeated stim- ulations all equal in intensity and equally spaced in time, we find that the first contraction (shortening phase) is highest, then there follows a decrease for about four contractions. The muscle de- creases slightly in irritability; irritability then increases and the shortening increases again for some time. The effect of activity is thus beneficial to the muscle. After a time under repeated stimulation, the muscle begins again steadily to lose its irritabil- ity. The height of contraction becomes less and less. Finally the muscle ceases to shorten. This condition is known as "fatigue."
Tetanic Contraction. — Most muscular responses brought about by the action of motor neurones are unlike the simple twitch we have thus far studied. The impulses come so rapidly that the muscles cannot react to them separately. There is no time for relaxation. The muscle then contracts and stays per- manently in that phase as long as the stimulus endures. This is called compound contraction or tetanus. It can be produced in the laboratory by sending in separate electrical shocks very rap- idly to the muscle. The number of stimuli per second required to produce tetanus varies' with the muscle and with the species of animal from which the muscle is taken. Twenty to thirty stimuli are required for mammalian muscles. Of course, all of the con- ditions mentioned above which tend to slow the relaxation phase tend to produce tetanus with less frequency of stimulus.
Summation of Stimuli. — The most interesting phenomenon to be observed in compound contraction or tetanus is summation. If conditions are so arranged that a second impulse strikes the muscle just at the instant that it is most shortened, it will still further shorten. If three instead of two stimuli are thus properly timed, a still further shortening will occur. It thus becomes pos- sible in a muscle completely tetanized to obtain two to three times the extent of shortening that can be obtained in the simple twitch.
Muscular Contraction in Habit Responses. — In habitual ac- tivity, boxing, swimming, tapping, for example, is there a com- PSYCHOLOGY pound contraction, tetanus, or a simple twitch? Most movements of this character are so long continued that they must be of the tetanus type. It has been recently shown that even the most rapid trills that can be made upon a musical instrument are of this type. How do sufficient impulses reach the muscles to pro- duce tetanus? Investigation shows that the motor nerve has its own rhythm of discharge. When a muscle is in tetanic contrac- FIG 49.— Mosso's ergograph: c, carriage moving to and fro on runners by means ?h -VJi « P*88*8, from *he carriage to a holder attached to the last two phalanges f th*™r?i * finSeV(the adjoining fingers are held in place by clamps); p, writing point sfr«a?fl^ie^ tion the separate impulses do not observably change its form, but under such conditions the muscle (1) emits a musical note which has a frequency that within certain limits corresponds to the fre- quency of the impulses reaching it. The same phenomenon can be detected by the string galvanometer. The number of impulses reaching the striped muscles in habit contraction varies with the muscle— from 47 in the flexor of the arm to 100 in the masseter (jaw). The various motor cells of the central nervous system bus have very different rates of discharge. If the frequency of the impulses reaching the muscles is greater than the above " the muscle cannot keep in step. It, however, maintains its intrin- sic rhythm.
Muscular Work.-Fig. 49 shows the ergograph, an instru- ORGANS OF RESPONSE 167 ment by means of which we can study graphically the curve of work of certain muscular organs.
Fig. 50 shows the curve of work of the flexors of the middle finger of the right hand, lifting a load of three kilograms at in- tervals of two seconds. This curve is usually called a fatigue curve. It will be noticed that the height to which the load is raised is great at first, that it decreases rapidly for a time, then FIG. 50. — Normal fatigue curve of the flexors of the middle finger of right hand. Weight 3 kilograms, contractions at intervals of 2 seconds. (Howell's Text-Book of Physiology.) W. B. Saunders Co.
much more slowly, decreasing very rapidly thereafter until fatigue results. If now the load is lightened the muscle again is capable of doing work. The ergograph thus does not afford a means of completely determining the amount of work a muscle can do. Certain interesting facts have been obtained with the ergograph: (1) If a sufficient rest period is allowed after each contraction, no fatigue occurs; (2) after complete fatigue with a given load, a rest of two hours is required for the muscle 's com- plete recovery; (3) if after complete fatigue abortive contractions 168 PSYCHOLOGY 168 PSYCHOLOGY are continued for some time, the period of rest must be much longer than two hours. This shows the deleterious effect of at- tempting to continue much muscular activity in a fatigued con- dition; (4) loss of sleep, hunger, and ansemia of the muscles lower the amount of work which can be done; (5) improved cir- culation produced by massage, better foods and sugar solutions increase the muscular work; (6) the total amount of work done is greater with small loads than with large; (7) fatigue of one set of muscles, for example, those in the leg, will decrease the amount of work which can be obtained from another muscle due to the circulation of fatigue products in the blood.
Muscle Tone. — We have several times spoken of muscle tone. This is poorly understood in physiology. That a muscle is never fully relaxed in the normal state appears from the fact that when the muscle is cut the two ends draw away from each other. Tone in both flexors and extensors is generally supposed to be due to a reflex through the central nervous system (without assistance of sympathetic neurones). It is supposed that when the muscle relaxes to a certain point, afferent endings in the muscle are stim- ulated. These in turn arouse motor impulses in the central nervous system which pass directly to the motor end plates and are distributed to each motor fiber. Certain authors have recently suggested that the sympathetic system contributes the tone: that the answering impulses to the afferent stream from the muscle goes out from the cord over the pre-ganglionic fibers, then to the post-ganglionic (sympathetic) and to the muscle. Sectioning of the white rami is said to destroy muscle tone. The whole matter is still a research problem.
Fatigue Products.— When an individual exercises, CO0 is given out by the muscle. It is absorbed into the blood stream, carried to the lungs and then given off in the expired air. Twice as much C02 is given off on a working day as on a resting day. The further products as a result of activity are lactic acid (de- rived probably from sugar, the sugar in turn being obtained from the glycogen), and possibly acid potassium phosphate. If ex- tracts are made from the muscles of a fatigued animal and admin- istered to a rested animal, the latter shows signs of fatigue.
ORGANS OF RESPONSE 169 Functioning of Reflex Arcs: The "Final Common Path".— We have on page 117 already touched upon reflex action. Now that we have studied all of the elements comprising a reflex arc, it would be well to consider a little more closely some of the phenomena which appear when reflex arcs function. From our survey of the sense organ structures, it becomes clear that there are many more points where afferent impulses can be aroused than there are separate motor outlets. Indeed, a count of axones shows that there are about five afferent neurones to one efferent (cerebro-spinal axis as a whole). Hence we must look upon each motor neurone as the possible outlet for impulses aroused at many different sensory points on the bodily surfaces. It is the sole outlet, the final common path for impulses which are on their way to the muscle in which the given motor neurone ends. Cer- tain consequences result from this relationship. Two sensory impulses aroused at points a and b upon a sensory surface might pass out over a common path to a given muscle, but the impulse aroused at a if acting alone might excite one type of motor re- sponse, say, a contraction of the flexors, whereas that aroused at b if acting alone might excite the muscle to act in a wholly different way, for example, produce the contraction of the ex- tensors. Obviously both flexion and extension of the leg do not and cannot occur at the same time. What usually occurs is that either the one or the other reflex appears; or if both flexor and extensor muscles do contract trembling will occur. In observing the behavior of others, we see their separate acts taking place in an orderly way, although we know it is not usual for one stimulus to begin to act at the moment another ceases to act. A host of stimuli act concurrently, but the organism reacts now to one, now to another, depending upon which group of stimuli becomes prepotent. To give the reasons for now the one, now the other group of stimuli becoming prepotent is to give an outline of the whole of physiology and psychology. Although we can- not profitably discuss this question, it is possible for us to examine certain close relations existing among different reflex arcs.
Allied Reflex Arcs. — Certain reflexes combine harmoniously, being mutually reinforcing reactions. This can be shown most 170 PSYCHOLOGY easily in the scratch reflex of the dog (Sherrington). If at the moment the scratch reflex is being elicited by stimulation of a skin point on the shoulder, another point 10 cm. distant is stim- ulated, the result is favorable to the action already in progress. If the two stimuli are each made just subliminal so that the reflex cannot be obtained by stimulating either the one or the other of the skin points, the reflex can be obtained by simulta- neously stimulating the two points. The greater the similarity in the type of action each stimulus separately applied would call out, the greater the reinforcement when both stimuli are applied simultaneously. We may look upon the whole skin area (or skin and kinsesthetic area) from which variations of the same reflex can be obtained as the receptive field of the reflex. These recep- tive fields are quite extended. In the scratch reflex, the different points in the receptive field when touched all produce variations in the scratching reaction. Such reflexes are called type reflexes. There is thus a completely harmonious relationship existing among all of the separate reflexes comprising a given type.
Antagonistic Reflexes. — Many of the reflexes which can be aroused by the use of the same common path are thus allied. But many arcs are antagonistic when functioning. If while the scratch reflex is in progress with the left foot, due to a stimula- tion of a skin point of the left shoulder, the right foot is stimu- lated, the scratch movement is halted. Depending upon the time relations of the stimuli, the contact applied to the right foot can interrupt the scratch reflex, cut it short or delay its progress. The stimulus to the right foot does not have to be very intense in order to produce this result. We have here an example of interference between two reflexes. The final common path used by the scratch reflex is used also by the reflex elicitable from the right foot. The latter reflex has as a result extension at the left knee. In other words, stimulation of the right foot arouses the extensors at the left knee and simultaneously inhibits the flexors. The scratch reflex involves a rhythmical use of the flexor neurones. There is thus a definite conflict in use of the flexors. The scratch reflex involves them in action four times per second, whereas the stimulus from the right foot would block action in them altogether.
ORGANS OF RESPONSE 171 The Knee Jerk in Man. — Entirely aside from the subject of allied and antagonistic reflexes the fact is worth considering that apparently any form of reflex action taking place under the im- pulse of a given stimulus may and probably is altered by the presence of another stimulus if the application of the latter is timed with respect to that of the first. This can be illustrated by the reflex knee jerk, which occurs when the tendon is struck. If the extent of the jerk of the leg is being measured, it will be found that blowing upon the eye, squeezing a dynamometer with the hand at the instant that the blow is struck on the tendon augments the extent of the jerk. If the reinforcing action (or stimulus) precedes the blow on the tendon by too great an inter- val, the extent of the jerk is decreased: there is inhibition. Inhibition begins to appear when the reinforcing act (squeezing the dynamometer with the hand, for example) precedes the blow on the tendon by.22 sec. to.6 sec.; the maximum inhibiting effect is obtained at from.66 to.9 sec. If the interval is greater, inhibition becomes less noticeable. At 1.7 sec. to 2.5 sec., the reinforcing stimulus has no effect.
Latent Time in Reflex Arcs. — After a reflex arc has con- ducted an impulse, it shows a resting or unstimulable phase for a short time. Stimuli impinging upon the afferent nerve endings will no longer excite the muscle. This is known as latent reflex time. It seems to be never longer than one second or thereabouts. It is sometimes much shorter; in the knee jerk, lOo-; in the reflex eyelid closure, 45<r.
General Considerations. — This completes our survey of the efferent motor control of the system of skeletal muscles. We have entered somewhat into the details of reflex action in order that we may have before us some of the factors which may later on throw light upon the phenomena we meet with in instinct and in habit. We have gained in the various parts of the text some little insight into such physiological factors as (1) fatigue in cell body and possibly in axones; (2) the nature of simple and tetanic contractions in muscles (latent period, shortening and relaxation phases, and summation of stimuli); (3) the nature of work, fatigue and fatigue products in the muscles, and finally 172 PSYCHOLOGY 172 PSYCHOLOGY (4) the nature of simple allied and antagonistic reflex action. If there were space at our disposal, we should find it helpful to consider the compounding of reflexes, coordination of reflexes, and the so-called laws of reflex action. The final explanation of many of these topics in sensory physiology is still in question, but most of the phenomena actually appear in the behavior of organisms, whatever their final explanation in terms of neural action may be. We cannot apply in detail all of the data we have gained from this study. Man's acts from day to day are too complicated. We see him running to catch his train, tossing pennies to his newsboy, speaking fluently, meeting his family, playing, painting, building and running mechanical contrivances. It does help us not a little in observing these acts to have some insight into the factors we have studied, even though we cannot put our finger from moment to moment upon a reinforcement, an inhibition, or action in a partially fatigued group of muscles.
II. THE NATURE AND FUNCTION OF SMOOTH MUSCLE.
The most important area of unstriped muscle is to be found in connection with the alimentary canal and the other visceral Fio. 51.— Showing smooth muscle cells arranged to form Anatomy.)
organs. The gross divisions of the alimentary canal are the mouth, pharynx, oesophagus, stomach, the small and the large intestine. The unstriped muscular tissue is to be found mainly in the lower portion of the oesophagus and throughout the stom- ach, the large and the small intestines. These latter1 structures have (1) an inner lining of epithelial cells supported by a fibrous tunic and a thin layer of smooth muscle, (2) a muscular coat com- d of two layers of smooth muscle, one circular and the other ntiidinal; this muscular layer propels the contents along the al, and (3) an outer fibrous coat (several subsidiary coats are ORGANS OF RESPONSE ORGANS OF RESPONSE not given). Smooth muscle is found almost exclusively in the veins and arteries throughout the body, in the bronchi, in the genital and urinary organs, and to some extent in the skin (muscles attached to hairs, ducts of sweat glands, for examples). The structure of the smooth muscle differs markedly from the striated. It consists of minute spindle-shaped cells (Fig. 51) with a single nucleus. These cells generally unite to form mem- branes as in the intestines. Fig. 52 shows the details of the construction of the nucleus.
The Nerve Supply of Smooth Mus- cles.— In general the smooth muscles are under the control of the sympathetic sys- tem, that is, of the post-ganglionic neurones (page 154). They can also, as will be shown later, be stimulated to action by secretion from the ductless glands, and by other chemical agents. They are thus under two forms of control: under a nervous control, and a secretion control. The endings of the post-ganglionic neu- rone are like those shown in Fig. 46, but the afferent endings have not been surely differentiated from the sympathetic. The endings at times are quite complicated. Attention is called to the fact that several of the efferent neu- rones of the brain and cord are (apparently) distributed to the visceral organs controlled by the smooth muscles. It is generally supposed that these pre-ganglionic neurones of the central sys- tem are never distributed directly to smooth muscles fibers or glands (the adrenal gland being possibly an exception). They end in the various plexuses around cell bodies of post-ganglionic neurones (sympathetic ganglia). Emerging from these plexuses are the post-ganglionic neurones which control the bladder, sex organs, and the contraction and dilatation of the intestines.
Importance of the Action of Smooth Muscles. — A little ob- servation shows that the vegetative life of the organism is con- FIG. 52. — Showing details of nucleus of smooth muscle cells. (From Piersol's Anatomy.)
174 PSYCHOLOGY trolled by smooth muscle. Its integrity *and smooth working condition all of the functions of the organism as a whole. We have emphasized certain of these factors on page 66.
Contraction in Smooth Muscles. — Speaking broadly, the general phenomena of contraction already discussed in connec- tion with the skeletal muscles appear here. The most important difference is that of sluggishness in the changes. The latent period in smooth muscle is long, sometimes 100 to 500 times longer than in striped muscle. The phases of shortening and relaxation are also prolonged, summation effects appear as well as tetanic contractions. It maintains any tone given to it for a much longer period of time than striped muscle. Intestinal muscles show rapid increase in tone upon little stimulation, par- ticularly when evoked by chemical stimuli, and maintain it against pronounced resistance with slight production of heat. They show, under proper conditions of stimulation, rhythmic activity. This is seen in the hunger contractions, in the ureters, and in the bladder.
III. GLANDS AND THEIR ACTIONS.
In addition to the skeletal muscles and the smooth muscles, there is another group of effectors or expressive organs — the glands. These organs are fundamental to animal existence, since they play the principal role in the digestion of food and in the control and the regulation of growth and metabolism (secretion). and in the elimination of certain waste products of the body (excretion). While each cell in the body must abstract from the blood and lymph its own nutrient material and give off its own waste products, still in all complex animal forms certain cells are grouped in the glands which either secrete certain substances for the use of other organs, or else eliminate waste products from the bodily fluid. There are many scattering groups of cells which perform these functions for the body as a whole that are not grouped into glandular structures, but we shall have to neglect any lengthy discussion of them and confine our survey to the most important and largest of the gland structures. We may divide glands into (a) duct-glands, which have a well- ORGANS OF RESPONSE 175 marked opening or outlet through which their products are deliv- ered, and (6) ductless-glands, which have no outlets. Their secretion is absorbed directly into the blood stream and is dis- tributed by it to other bodily tissues.
A. THE DUCT-GLAND (EXTERNAL SECRETION).
The duct-glands were studied earlier than the ductless-glands, and their action was thought to be better understood. Recent investigation has thrown a great deal of light upon the action of these glands and new points of view have been obtained. It is now generally recognized that their activity is very complex indeed. There are so many problems connected with the action of duct-glands that we can survey only certain features which are at the present moment of most interest to objective psychol- ogy. Due largely to the work on animals carried out by Pavlow and his students in Russia, and to that of Lashley on man in this country (page 30), it has been found that the action of certain duct-glands can be profoundly modified by habit influences.
The Salivary Glands. — The principal glands which at pres- ent have been found to show this influence (conditioned reflex) are the glands of the stomach and the three pairs of glands in the mouth cavity — the parotid, the sublingual and the submax- illary. The latter three pairs of glands are shown in Fig. 53. They manufacture and secrete jointly the fluid called saliva, which they pour directly through ducts into the mouth cavity. The saliva is thus the first digestive fluid with which the food comes into contact. These glands are made up of several types of secreting cells (epithelial). Contained in the glands besides the secreting cells are blood-vessels, connective tissue, smooth muscle tissue and nerve endings. The nerve supply is complex. The motor neurones actually ending in the glands are post-ganglionic, but the pre-ganglionic fibers belong to both autonomic systems (thoracico-lumbar autonomic and cranio-sacral autonomic). There are, too, medullated afferent endings present (cerebro- spinal). The glands are excited usually reflexly (normal reflex) by food substances coming into contact with the lining of the mouth. They seem to possess in addition to their function PSYCHOLOGY PSYCHOLOGY as digestive organs a certain protective function. Small pieces of rock fail to excite the glands, but if the rock is powdered, copious fluid appears which apparently makes the spitting out of the indigestible substance more feasible. Strongly irritating substances, acids, salts, etc., produce a copious flow of saliva which reduces their irritating action. As has been brought out Duct of accessory parotid Duct of parotid Bristle inserted into duct Frenulum linguae ~~ Major sub- lingualduct Sublingual gland Duct of submaxillary gland ^ Mylo-hyoid muscle Parotid gland Masseter •muscle Sterno- lastoid muscle Posterior .belly of digastric muscle Lingual "nerve Submaxil- .lary gland, drawn backward \Loop of fascia •Hyoid bone Anterior belly of digastric muscle / Deep portion of submaxillary gland, x Fio. 53.— Salivary glands. (Morris's Human Anatomy.) P. Blakiston's Son & Co on page 32, conditioned reflexes can be aroused through the nose, the eyes, the ears, etc. This proves that definite reflex arcs control the action of the glands. This can be shown even more clearly by cutting the cranial pre-ganglionic fibers (lingual nerve or the chorda tympani) and electrically stimulating the peripheral stump. A copious, thin, watery secretion results after an appreciable latent period and at the same time there occurs an increased flow of blood through the gland. Excitation of the post-ganglionic fibers belonging to the sympathetic (thoracico- ORGANS OF RESPONSE 177