A further stage in evolution is that the muscle-cells lose their connexion with the epithelium and come to lie entirely beneath it, forming a sub-epithelial contractile layer, developed chiefly in the tentacles of the polyp. The of the evolution of the ganglioncells is probably similar; an epithelial cell develops processes of nervous nature from the base, which come into connexion with the bases of the sensory cells, with the muscular cells, and with the similar processes of other nerve-cells; next the nerve-cell loses its connexion with the outer epithelium and becomes a sub-epithelial ganglion-cell which is closely connected with the muscular layer, conveying stimuli from the sensory cells to the contractile elements.
By means of vibrations or shocks transmitted through the - Sub water, or by displacements in the balance or position of the animal, the otoliths are caused to impinge against the bristles of the sensory cells, now on one side, now on the other, causing shocks or stimuli which are transmitted by the basal nerve-fibre to the central nervous system.
They are the power Of receiving impressions or stimuli from the exterior, and of communicating with each other, with the view of co-ordinating a suitable response.
There is, however, very great delicacy of perception or appreciation on the part of the sense organ, stimuli being responded to which are quite incapable of impressing themselves upon the most highly differentiated animal.
It is easy to realize how such a rhythm can be modified by the reception of stimuli, and can consequently serve as the basis for the movement of the stimulated organ.
The now well-known fact that small doses of poisonous substances may act as stimuli to living protoplasm, and that respiratory activity and growth may be accelerated by chloroform, ether and even powerful mineral poisons, such as mercuric chloride, in minimal doses, offers some explanation of these phenomena of hypertrophy, wound fever, and other responses to the presence of irritating agents.
Special Cell-Modifications for the Reception of Stimuli.In studying the physiology of movement in plants certain modifications of cell-structure have been observed which appear to have been developed for the reception of the stimuli by which the response to light, gravity and contact are brought about.
We are now in a position to give an expanded definition of instinctive behaviour as comprising those complex groups of co-ordinated acts which, though they contribute to experience, are, on their first occurrence, not determined by individual experience; which are adaptive and tend to the well-being of the individual and the preservation of the race; which are due to the co-operation of external and internal stimuli; which are similarly performed by all members of the same more or less restricted group of animals; but which are subject to variation, and to subsequent modification under the guidance of individual experience.
The impulse to migrate, that is to say, the calling forth of specific activities by climatal or other presentations, appears to be instin tive; whether the direction of migration is in like manner instinctive is a matter of uncertainty; and, if it be instinctive, the nature of the stimuli and the manner in which they are hereditarily linked with responsive acts is unexplained.
Thus it is found that the action of the heart is accelerated by pleasant, and retarded by unpleasant, stimuli; again, changes of weight and volume are found to accompany modifications of affection - and so on.
The second is Fechner's method; it consists of recording the changes in feeling-tone produced in a subject by bringing him in contact with a series of conditions, objects or stimuli graduated according to a scientific plan and presented singly in pairs or in groups.
The loss of an eye will be followed by atrophy of the optic nerve; the tissues in a stump of an amputated limb show atrophic changes; a paralysed limb from long disuse shows much wasting; and one finds at great depths of the sea fishes and marine animals, which have almost completely lost the organs of sight, having been cut off for long ages from the stimuli (light) essential for these organs, and so brought into an atrophic condition from disuse.
At a later date in the life of the individual, by some unknown stimuli, they resume their active power of proliferation and so give rise to new growths.
It may merely act locally in some way, and so render that part susceptible to unknown tissue stimuli which impart to the cells that extraordinary power of proliferation characteristic of new growth.
The capability of reacting to stimuli, the result is stimulation (Verworn).
Stimuli comprise chemical, mechanical, thermal, photic and electrical changes in the environment of the organism.
Stimuli applied generally, not unilaterally, in most cases induce increased divisibility of the cells of the part.
Among stimuli acting unilaterally, perhaps none has proved more interesting, in late times, than what is known as Chemiotaxis.
It is different, too, for different senses with the same observer, and different even for the same sense when the external stimuli differ in intensity.
That they are in some cases produced by physical or sensory stimuli does not constitute them irrational, and it is purely arbitrary to confine the word pleasure to those cases in which such stimuli are the proximate causes.
One end of the body, through contact, during locomotion, with fresh tracts of medium and other forms of stimuli, has become more specialized than the rest, and here the nervous system and sense-organs are more densely aggregated than elsewhere, forming a means of controlling locomotion and of correlating the activities of the inner organs with the varying stimuli that impinge upon the body.
From antiquity men had applied themselves to determine the relations between the physical stimuli and the socalled " quality " of sensations.
But what was new was the application of this doctrine to the relations between the stimuli and the socalled " intensity " of sensations.
There are then, at least within the limits of moderate sensations, concomitant variations between stimuli and sensations, not only in " quality," as in the intervals of sounds, which were understood long ago, but also in " intensity "; and the discovery of the latter is the importance of Weber's and Fechner's law.
By the rules of induction from concomitant variations, we are logically bound to infer the realistic conclusion that outer physical stimuli cause inner sensations of sensible effects.
In point of fact, many stimuli are beneath the " threshold " of a man's consciousness.
But he had also to endure countless objections to his mathematical statement of Weber's law, to his unnecessary assumption of units of sensation, and to his unjustifiable transfer of the law from physical to physiological stimuli of sensations, involving in his opinion his parallelistic view of body and mind.
He agrees with Fechner that physical process of nerve and psychical process of mind are really the same psychophysical process as appearing on the one hand to an observer and on the other hand to one's own consciousness; and that physical phenomena only produce physical phenomena, so that those materialists and realists are wrong who say that physical stimuli produce sensations.
He accepts Fechner's extension of Weber's law of the external stimuli of sense, while judiciously remarking that " the physiological interpretation is entirely hypothetical."
The increments of contraction become, however, less and less, until the succeeding stimuli serve merely to maintain, not to augment, the existing degree of contraction.
When muscle that has remained inactive for some time is excited by a series of single and equal stimuli succeeding at intervals too prolonged to cause summation the succeeding contractions exhibit progressive increase up to a certain degree.
Of the response of the nerve, it is found that unmistakable signs of fatigue appear even very soon after commencement of the excitation of the nerve, and the muscle ceases to give any contraction in response to stimuli applied indirectly to it through its nerve.
By direct application of electric currents, contract vigorously after all response on its part to the stimuli (nerve impulses) applied to it indirectly through its nerve has failed.
In them areas are found whence stimuli excite movements of this or that finger alone, of the upper lip without the lower, of the tip only of the tongue, or of one upper eyelid by itself.
In deep sleep the threshold-value of the stimuli for the various senses is very greatly raised, rising rapidly during the first hour and a half of sleep, and then declining with gradually decreasing decrements.
Certain it is that in the course of the waking day a great number of stimuli play on the sense organs, and through these produce disintegration of the living molecules of the central nervous system.
The nerve cell just prior to sleep is still well capable of response to stimuli, although perhaps the threshold-value of the stimulus has become rather high, whereas after entrance upon sleep and continuance of sleep for several hours, and more, when all spur to the dissimilation process has been long withheld, the threshold-value of the sensory stimulus becomes enormously higher than before.
The long-continued incitement to catabolism of the waking day thus of itself predisposes the nerve cells towards rebound into the opposite phase; the increased catabolism due to the day's stimuli induces increase of anabolism, and though recuperation goes on to a large extent during the day itself, the recuperative process is slower than, and lags behind, the disintegrative.
The threshold-value of the stimuli adequate for the various senses may be extraordinarily lowered.
This theory of complementary colours as due to the polarity in the qualitative action of the retina is followed by some criticism of Newton and the seven colours, by an attempt to explain some facts noted by Goethe, and by some reference to the external stimuli which cause colour.