THE SENSORY FACULTIES As we have seen already, very frequently the behaviour of ani mals has led to a mentality being ascribed to them similar to that of human beings. We have seen, too, that a mind, i.e. the phenomena of consciousness, cannot be experienced objectively. Nevertheless, we can attempt to form at least some idea of how the surrounding world in which animals live would appear to them if they possessed a consciousness analogous to our own. For human beings the sense-organs are the gates through which we receive the impressions of our surroundings. They constitute the only path through which what we know of the outside world reaches us. The totality of all the things and processes in our surroundings, among which our own bodies must be included, is the Urnwelt or "surrounding world." In this surrounding-world animals too are comprised. The question now arises as to how much the animals themselves can take in through their sense organs of these surroundings in which we see them. Since the type and structure of the sense-organs in animals is often quite different from what it is in ourselves, it follows that the Urnwelt of an animal must generally appear quite different from what it does to man. By documenting ourselves concerning the sensory capabilities of animals we can obtain a conception of how their Umwelt must appear, assuming that they possess a consciousness similar to our own.
A sense of time, which exists in a number of animals, is the result of complex stimuli, affecting various sense organs. Often animals which are regularly fed, brought into their stalls, or put into the open at stated times, show by their be haviour as the moment approaches that there exists in them some feeling of what is about to happen. This may have various causes. Frequently it is the preparatory actions or other behaviour of men themselves that the animals know from experience to be the pre cursors of the event. Then again, certain physiological processes go on in the animal's body with a definite rhythm, which, in conse quence of a long-standing connection with the event in question, have acquired for the animal a time-relationship with it. Such physiological processes are hunger, fatigue, etc. We find, too, in a number of the lower animals a definite rhythm due to regular repetition of recurring external stimuli. Sea anemones open and close with the rhythm of the tides and retain this same rhythm for some time in an aquarium. But a real conception of time is lack ing in all animals, even those which are mentally the most gifted, namely the anthropoids. Animals, then, live solely in the present. They know no past and no future. They are unable to survey the
span of life, youth and old age, as we do.
In the Pro tozoa the whole protoplasm of the animal body is irritable. In Metazoa the sense-organs take over the task of receiving stimuli. These sense-organs exercise a certain selection among the innumer able changes going on in the environment, all of which might act as stimuli. Each sense-organ responds only to those stimuli to which it is adapted. These are the so-called adequate stimuli of the particular sense-organ. This is very expedient, for if all the changes going on all the time in the environment were able to act upon an animal as stimuli, animals would never be at rest. Nevertheless the number of stimuli sent on as nerve impulses by the sense-organs to the nervous system is very large. But we may often observe how the behaviour of an animal does not alter at all although certainly one or several of its sense-organs are receiving stimuli. The explanation is found in the activity of the central nervous system. In general, reflex movements take place as a con sequence of stimuli acting on the organism. But we have seen above that the morphological basis of reflexes is formed by the reflex arc, which is usually composite. Association neurones inter posed in the reflex path are connected with other nerve cells, and by the massing together of such ganglion-cells centres are consti tuted. The centres of the separate senses are in communication with one another through numerous nerve fibres. This permits of the working-up of the stimuli received by the different sense organs. Other nerve centres are interposed between the principal ones. These are, in a sense, subordinate, having the function of co ordination and purposeful conduction of the impulses coming from the sensory centres. Finally a ganglionic mass, the brain (q.v.), has been developed in the course of evolution, to which the whole of the nervous system is subordinate. The brain takes over the regulation of the highest functions, those involved in purposeful behaviour. This brain development commences in flat-worms and in annelids. It attains its greatest development, on the one hand, in the supra-oesophageal or cerebral ganglion of certain inverte brates, and, on the other, in the vertebrate brain with its five divisions and far greater complexity. With the brain, that very important part of the central nervous system, the ventral nerve cord of annelids and arthropods, and the spinal cord of verte brates, is in closest connection. The brain merits our particular attention above all because in man it is rightly considered to be the organ or seat of the mind.