The brain, using this term generally to include the highest centres of the nervous system, has the function of co-ordinating movements. Here the decision is made as to which impulses shall be conducted further on, when the results of stimuli arrive simultaneously from several sense-organs. In addition the brain decides into which nerve-paths these im pulses shall be switched for conduction to the effector organs. This highest connecting centre is at the same time the most important inhibition centre. It intervenes as the highest court of appeal in the course of reflex and instinctive activities. The brain, just like the lower centres, can stop or inhibit these actions. An example will make this clear. When the mouth-parts of a normal crab come into contact with a piece of food, the chemical stimuli derived from the food cause the appendages, in a reflex manner, to tear up the food and to put it into the mouth. The action stops wheh the stomach is full, when the animal's hunger is satisfied. If, now, the commissures which unite the crab's brain to its ventral nerve-cord are cut through, the mouth ap pendages continue to work as long as food is presented to them. The result is that the stomach and the gut of the animal are eventually so crammed with food that they burst. This shows clearly that the brain, which is normally kept informed of the state of repletion of the stomach through special nerves, exer cises an inhibitory influence on the reflexes concerned in feeding. Whereas it is impossible to persuade a normal frog to croak by any stimulus whatsoever, this croaking invariably follows when a frog whose forebrain has been removed is stroked down the back with the finger. Here, too, the inhibitory function of the highest centres is apparent.
Further, the brain functions as an organ of summation. It is a sort of reservoir for separate impulses, in themselves perhaps too small to result in action. The nervous energy thus stored up in the brain is from time to time liberated into motor paths in response to further incoming impulses, which can be due to in ternal causes. In this manner, so-called spontaneous actions arise, that is to say actions which take place without external cause, apparently in an arbitrary manner. Together with its functions as centres for the voluntary distribution and switching of im pulses, the brain also includes the centres concerned in the forma tion of associations. It brings about the inter-connections between nerve impulses due to different but simultaneous stimuli. Upon this the faculty of association depends. Finally, the centres are also situated here through the activity of which a certain con tinuous tension of the body muscles, the so-called muscular tone, is brought about.
The more an animal is endowed with highly developed instincts, the less does it stand in need of a large number of individually acquired experiences, memory-images, and associative faculties, to be able to behave in the most adaptive manner possible. Thus
we find that in insects in spite of the complexity of their instinc tive lives, the brain remains relatively small. Added to this, the whole bodily design of these creatures, particularly the presence of an external skeleton, prevents the volume of the body from exceeding a certain size. Thereby absolute limits are further set to the size of the brain, and hence, also, to the further develop ment of mental qualities, the beginnings of which are actually present. In vertebrates there are no such special limits set to the growth of the brain. Hence we find in the different classes of vertebrate animals a progressive development of the mental faculties. This reaches a high point in the birds, but the highest pinnacle is attained among the mammals. It is true that in the latter the lower groups are inferior to birds. It is only in the monkeys, and finally in the anthropoids, that the highest level is attained, forming an intermediate step to the intellect of man. The highest centres, particularly the association centres, lie in birds and mammals in the cortex of the fore-brain, the surface of which increases proportionately to the mental faculties.
When the hemispheres of the fore brain are removed from an animal, a number of deficiency phe nomena are evident in the animal's behaviour. These become the more apparent the higher the animal in question stands in the evolutionary series. Thus fishes or frogs without these f ore brains show no noticeable difference in their behaviour from normal individuals. Such frogs catch flies as usual. They avoid obstacles, dig themselves into the mud in winter, and betake them selves to water during the warmer months. They even possess memory, although it lasts but a short time. All this is accom plished by the centres situated in the intact parts of the brain. Reptiles and birds without fore-brains are no longer capable of spontaneous movements. A few days after the operation, when shock effects have disappeared, a pigeon thrown up into the air flies with co-ordinated movements. It alights in the ordinary way on a perch and avoids obstacles. It can still see and hear well. But all memory has vanished. It does not recognize its food and consequently does not eat or drink of its own accord. It recognizes neither friend nor enemy and if it has young it does not care for them. In mammals, the deficiency phenomena vary according to the mental capacities of the particular animals. Dogs without their fore-brains have been kept alive for years. They appear completely blind and they take no food of their own accord. For the most part they lie quietly sleeping Such a dog no longer shows any signs of joy. Rather it exhibits fear. It goes without saying that the dog is incapable of learning anything new.