Comparative Psychology

We have seen already, from the example of the tadpoles and other animals in small vessels, that there is no necessity for a tropism to be present in an animal equally under all circumstances. Rather, it is called forth by certain definite stimuli. Conversely, the tropisms which are present are frequently reversible. When a stimulus becomes too strong, a positive may be converted into a negative tropism. Thus, many lower crustaceans such as Daphnia, become positively phototactic when the temperature is lowered, negatively when it is raised. Daphnia which has become adapted to a light of medium intensity swims towards the source of light when the intensity of the light is diminished and away from the light when it is increased. The previous milieu is an optimum which the animals seek to re-attain after it has been altered.

Reflexes.

We have seen that animal tropisms are automatic in appearance only. Reflexes, on the other hand, are stimulation effects which are really purely mechanical. Reflexes are invariably dependent upon the presence and interaction of a nervous system. The morphological basis of every reflex consists, in the first place, of a receptor, or sensory neurone. This is a ganglion cell with defi nite processes which take up the stimulus and conduct the excita tion to the cell-body and to other processes which transmit the ex citation further on. The second component of the reflex path is another ganglion cell, an effector, or motor neurone. One out growth of this is in connection with the last-mentioned process of the sensory neurone, which conducts the excitation received to the cell-body of the motor neurone. The excitation is then sent on by other processes of the motor neurone and conducted to a muscle or gland, which is thereby activated. The sum total of these nerve conduction paths is called a simple reflex arc. The connection between the sensory and the motor neurone constitutes a simple nerve centre, or reflex centre. But in the animal kingdom composite reflex arcs are most usually found. This comes about by further ganglion cells, called association or internuncial neu rones, being intercalated between the sensory and the effector neurones of the arc. These association neurones are in communica tion with yet other centres. Further, the reception of a stimulus usually takes place through particular sense-cells, which are nor mally found as parts of sense-organs having complicated struc tures. Yet the essential physiological elements of a reflex are invariably reception of stimulus, conduction of stimulus, and final result. Reflexes were so named by Descartes (Lat. reflexus, reflection). The stimulus coming from the sensory neurone is conceived as being reflected on to the motor neurone. Such reflexes are widespread in the animal kingdom wherever a nervous system has been developed. It can truly be said that by far the greater number of movements of animals, or of their organs, depend upon reflexes. These can be observed in numberless cases from the coelenterates upwards, to the highest mammals, and in man himself. Quite apart from the purely reflex movements of

inner organs, a number of reflexes can be observed in the external actions of human beings. The contraction of skin in the cold and the reddening or turning pale of the face are examples.

Among animals sea-urchins show most clearly to what an extent the whole behaviour can be dominated by such reflexes. Von Uexkiill rightly says of these creatures that they are purely reflex animals. All their reactions can be explained by reflexes. If a mechanical stimulus affects a spot on the skin of a sea urchin, the neighbouring spines incline towards the spot in ques tion. This occurs because the excitation is conducted by nerve paths towards those muscles surrounding the bases of the spines, which lie nearest to the stimulated spot. Von Uexkiill was able to show that the movement of a sea-urchin along the ground, following such stimulation, depends upon these reflex movements of the spines. The same is true of the so-called "turning-over reaction" of the sea-urchin, in which an animal that has acci dentally fallen on to its back rights itself again by purposeful movements of the spines. Between the spines of the sea-urchin are found so-called pedicellariae. These are small three-clawed pincers, each on a long stalk. At rest, the slender stems lie on the body of the animal. There are four kinds of these pincers, each of which is erected and brought into action only by quite specific stimuli. Long, thin, motile snap pedicellariae shut to the light touch of any small animal which may serve as prey and happens to be swimming by. The short strong biting pedi cellariae rise to a stronger mechanical stimulus and then open, closing again on further touch. They will bite, for example, slowly but strongly into the leg of a small crustacean. Quite weak mechanical stimuli, such as those produced by small foreign bodies falling on to the surface of the sea-urchin, excite the small cleansing pedicellariae which serve to clean the surface. The dis position of the reflex arcs is such that when a stronger stimulus occurs, suitable pincers are held ready while the type which is too weak stands back. Thus a chemical stimulus repels the biting pincers and causes the spines to bend away from one another, so that room is made for the fourth set, the poison pedicellariae. This type, which possesses poison glands, now enters into activity. For if a mechanical stimulation of a region of the body-surface now occurs, the stalks of these pedicellariae, together with the spines, all bend towards the stimulus. The jaws of the pincers close together as soon as they come into contact with the object. The erection of the stalks as a result of the chemical stimulation is an essential preliminary to the functioning of the snap-reflex. The weapon may be said thereby to be loaded and put into a state of tension. Thus for the poison-pincers to act a coupling of two small stimuli is essential.