Book - Vertebrate Zoology (1928) 30

From Embryology

Vertebrate Zoology G. R. De Beer (1928)

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Chapter XXX The Brain, And Comparative Behaviour

The brain is the anterior region of the spinal cord, modified, specialised, and enlarged in connexion with the development of special sense-organs in the anterior region of the body. That these sense-organs should be accumulated here rather than elsewhere is due to the fact that chordate animals are bilaterally symmetrical and move along a definite axis with one end constantly leading. This end is the first to come into contact with new surroundings, information concerning which is of the highest value to the animal.

In order to understand the evolution of the spinal cord and brain, it is necessary to consider what is known as a reflex arc. An afferent fibre brings an impulse from a receptor, and if this afferent fibre were to connect with only one efferent fibre going to a particular muscle, whenever the receptor was stimulated the response would be the contraction of this muscle. Nothing else in the way of response would be possible. But actually the afferent fibre when it has run into the brain or spinal cord makes a large number of connexions with other neurons. Some of these may be efferent neurons and connected with various effectors ; others may be neurons which carry the impulse to other parts of the spinal cord or brain : the so- called association-neurons. By this means a receptor can be connected up with several effectors, or one effector may be stimulated by impulses coming from several different receptors. This possibility of one efferent neuron being used by impulses coming from several afferent neurons, as a " final common path " for their reflex circuits, is of the greatest importance. The efficiency and economy of using what may be called interchangeable standard units (the neurons), capable of an infinite variety of combinations is one of the main factors of the success of the higher vertebrates. An animal possessing this type of nervous system can make many kinds of response, and indeed by suitable connexions and adjustments there is no limit to the number of combinations which may be formed between receptors and effectors. These adjustments are made in the central nervous system, and they are its function, just as that of a telephone exchange is to make adjustments between calling and answering subscribers. The key to the whole system is the neuron, which is not rigidly fused on to any other cell, but which can make synaptic connexions with a great number of other cells and pass impulses on to them. New connexions can be made, and new kinds of response can be evolved, which become " conditioned " reflexes, or habits.

The places in the central nervous system where these adjustments are made are called centres, and they lie in the grey matter. When the skin of a dog is stimulated by a small irritation, the receptor in the skin sends an impulse through an afferent neuron which runs into the spinal cord by the dorsal root. This neuron makes a synaptic connexion with an association-neuron in the grey matter of the spinal cord. The fibre of this association-neuron runs down the spinal cord in the white matter to the segment of the body where the hind leg is situated. There it makes a synaptic connexion with an efferent neuron (in the grey matter) which passes out through the ventral root to the muscle of the leg. The result of the stimulus is a jerk or " scratch " on the part of the leg. This reflex arc illustrates the fact that the function of the spinal cord is twofold. It contains a number of reflex adjust- ment-centres (in the grey matter), and it conducts impulses up or down the cord to different levels (in the white matter).

In the brain there are the primary centres, connected with the different functional systems of components. These are the " skin brain," " ear-brain," " taste-brain " (in the medulla oblongata), the " eye-brain " (in the midbrain) and the " nose- brain " (in the forebrain). Each of these is a centre where impulses are received of a particular type (from a particular component-system), and where adjustments are made with association and efferent neurons so as to complete the reflex circuit.

Now if these primary centres are marked off in the brain of a dogfish, it is found that except for the cerebellum, they occupy nearly the whole of the brain. Those regions of the brain which conform to the organisation of the spinal cord are called the " segmental apparatus " or " brain-stem," and are to be distinguished from the additions in the shape of the cerebellum, and in higher forms the cerebral cortex, which are " suprasegmental " structures.

DeBeer1928 fig171.jpg

Fig. 171. Transverse sections through the end-brains of, A, dogfish ; B, frog ; C, Chelonian (reptile) ; and D, shrew (mammal).

Showing the development of the cerebral hemispheres and lateral ventricles, and the migration of nerve-cells to the surface forming a cortex.

The various centres of the brain of the fish are mainly concerned with their own functional component-system ; there is not much " team work " between the different centres. The result is that the behaviour of fish largely takes the form of reflex responses to stimuli of certain kinds without much ability for variation or modification by experience. When any particular sensory system is very highly developed, the corresponding centre in the brain is enlarged. So in the carps, which are well supplied with taste-organs, the medulla oblon- gata is enlarged owing to the expansion of the visceral lobe. This expansion is due to the increase in number of neurons in the centre, parallel with the increased number of afferent fibres coming from the numerous receptors. In the catfish, the lateral-line system and the " ear-brain " are well developed.

A certain amount of correlation exists between the primary centres. For example, in the catfish, the " skin-brain " is connected with the " taste-brain," so that food may be recog- nised by touch and by taste, and these two types of sensation co-operate in producing those movements which lead to feeding. In other words, the reflex arc can pass from one functional component system to the other.

But this interrelation and team work between primary centres is best brought about by special correlation- centres, which are not related to any single primary centre but to several. The history of the development and evolution of these correlation-centres really makes up the evolution of the brain in vertebrates.

In the fish, the correlation-centres are not well developed, with the exception of the cerebellum. The cerebellum lies on the dorsal side of the medulla, and from its position its connexions are mostly with the neighbouring centres : " ear- brain " and " eye-brain." The ear-brain is concerned with the balance of the animal as reported from the semicircular canals, and the eyes report its position relatively to external objects. At the same time, fibres of the general somatic system run to the cerebellum and convey impulses of tactile sensations, and of the state of the muscles and joints of the body (proprioceptive). As a result of the commingling of these impulses, the cerebellum comes to be an organ for the regulation of the posture of the body and of bodily movements. It keeps the muscles in " tone," and as a whole regulates the execution of reflexes. It may, in a sense, be compared with the steam steering gear of a ship, which smoothly carries out the directions of the man at the wheel ; and it has been called the head of the proprioceptive system.

In bony fish, the cerebellum is enlarged to form the so- called valvula which projects forwards beneath the roof of the midbrain. In most amphibia and all higher vertebrates the lateral-line system is lost except for the ear, and the cochlea or organ of hearing is better developed. This affects the cerebellum to some extent. In mammals two new features arise, the superficial cerebellar cortex and the pons Varolii. These develop in connexion with the cerebral cortex.

Apart from the cerebellum, the correlation-centres are mostly concerned with responses to the outside world. In the fish there are correlation-centres of this kind in the forebrain and the midbrain, but the most important are those which become evolved above the evolutionary stage of the fish, and which are situated in the sides of the between-brain (thalamus), the floor of the end-brain (corpus striatum), and the roof of the end-brain (cerebral cortex).

It is characteristic of these higher centres of correlation that they are more or less isolated from the primary sensory centres ; in other words, the correlation-centres are not mono- polised by any single sensory system. In much the same way, if the government of a nation sat in the ordinary town-hall of one of its cities, much of its business would be taken up or influenced by local municipal matters, and it would be less able to deal with business affecting not the city but the nation as a whole.

The thalamus is related by fibres to most of the sensory centres, and it is among other things the centre where impulses are analysed into pleasurable and painful. As such, it is of great importance, for a negative reaction to danger and a positive reaction to food and to a mate go far to ensure the perpetuation of the species. Consequently the thalamus has great survival value in evolution.

The corpus striatum reaches a great development in birds, in which it is responsible for the correlation of the many and varied reactions and movements which form part of the instinctive behaviour. Instinct in birds is highly developed, and its hereditary nature is due to the fact that the reflex arcs and association-neurons in the thalamus and corpus striatum conform to a certain pattern which is the result of development. This also accounts for the fact that instincts are specific, that is, they occur in all members of a species, just as they all have kidneys or livers. But because instinct is determined by the hereditary pattern of the neurons, such behaviour is not easily modified to meet unusual circumstances. A good example of such shortcomings is to be found in the meadow pipit, a bird which is parasitised by the cuckoo. In the pipit's nest the cuckoo lays an egg, which hatches into a young cuckoo. This young parasite proceeds to eject the young pipits from the nest. It was observed on one occasion that the young pipit so ejected remained just outside the nest, under the mother- bird's nose, where it lay helpless and squeaking. It never occurred to the mother-bird to put it back in the nest under her, and so the young one died. The situation was novel and had not presented itself to the bird before, and it could not rise to the occasion. The necessary correlation of neurons could not be made ; and if it could, the bird would probably not have been able to act on the experience of a similar previous occasion. The corpus striatum is not well adapted for such powers of individual adaptability, though it is very suitable for ready-made correlations which make the species as a whole well adapted to a particular routine of life. It is interesting to note that the behaviour of birds resembles that of insects in this respect, and that both the brain of the insect and the corpus striatum of birds are solid compact masses of neurons. For really effective and unusual correlations such an arrange- ment appears to be ill suited. The cerebral cortex which fulfils this very function is shaped not as a solid mass, but as a layer of neurons, the number of which is augmented by increasing the area of the layer. The hollow tubular nerve- cord of vertebrates is very suitable for such an arrangement, and it is probable that its possession enabled vertebrates to evolve as they have done, while its absence from insects prevented them from progressing any further.

DeBeer1928 fig172.jpg

Fig. 172. Dorsal views of the brains of A, Petromyzon ; B, Scyllium ; C, Gadus ; D, Ceratodus ; E, Triton ; F, Lacerta ; G, Columba ; and H, sheep. (Not all drawn to the same scale.) c, cerebellum ; ch, cerebral hemisphere ; /, flocculus ; hb, hindbrain ; mb, midbrain ; ol, olfactory lobe ; on, olfactory nerve ; op, optic lobe ; ot, olfactory tract ; p, pineal ; v, vermis.

The cerebral cortex is a layer of grey matter near the surface of the end-brain. It is scarcely represented in the fish, and in the amphibia most of the neurons remain in the primitive position for grey matter ; that is, near the central cavity. Some neurons, however, migrate towards the surface. At the same time, the end-brain has been evolving in another direction, in that the cerebral hemispheres are formed as outgrowths containing each a cavity (the lateral ventricles) communicating with that of the between-brain through the foramina of Monro. Cerebral hemispheres first appear in the Dipnoi, and it is possible that they are an adaptation to deficient oxygen-supply : a matter of great importance, for the brain requires the purest arterial blood in the body. The formation of cerebral hemispheres increases the surface of the brain- tissue relatively to its volume, not only on the outside in contact with the vascular pia mater, but also on the inside which is bathed by the cerebro-spinal fluid, itself oxygenated by the choroid plexus. The migration of the neurons to the surface to form a cortex may also be an adaptation to oxygen require- ments, for solid masses of neurons would require large arteries to enter the brain, and there are indications that the pulse of large arteries is injurious to the delicate workings of the neurons.

Another advantage of the cortex type of structure is that it allows of the arrangement of centres on its surface after the fashion of a chequer board. The cortex deals with impulses from the outside world, in animals with sense-organs sufficiently well developed to give them good representations of the relations of different objects and events in space. It is apparently necessary that these representations of objects in space should remain separate in the brain until finally co- ordinated. In the same way it would be impossible to judge which of a number of threads was which, if they were all tangled up together in a ball. This analogy also introduces the fact that the function of the cerebral cortex is to receive the impulses which have already been sorted out in the correlation- centres, and to judge which of many possible is the best response to make. The cortex introduces hesitancy and arbitration into behaviour, which, on the level of the reflex arc, is immediate and determined.

Another factor to be borne in mind is that the cerebral cortex is principally concerned with impulses coming from the exteroceptors, and especially those which, like the eye, ear, and nose, can perceive objects at a distance : the distance- receptors. Responses to stimuli which touch the animal usually (when successful) abolish the stimulus which evoked them. So the flea tickling the dog on its skin evokes the scratch which incapacitates the flea from tickling any more. Such a response is consummatory. If, however, an animal sees some of its food at a distance, the response which it makes to start with does not abolish the stimulus. It sets its limbs in motion towards the food ; this is an anticipatory response, and the consummation is not complete until the food has been reached and eaten. Until this time, the food occupies the attention of the animal.

In the reptiles, there are three sheets of superficial grey matter in each cerebral hemisphere. The median sheet is the hippocampal and the lateral sheet the pyriform cortex. Both these regions are predominantly concerned with impulses coming from the nose ; they are not really " impartial " arbitrators of behaviour. That the cerebral hemispheres should in early stages of evolution be largely under the influence of olfactory sensations follows from the proximity of the olfactory lobes, and from the fact that at these stages the vertebrates had recently emerged from life in water to dry land, for the nose is a more highly developed and efficient organ in air than in water. Being at the most anterior end of the brain, it naturally took time in evolution before fibres from all the correlation-centres farther back in the central nervous system reached them. Part of the middle sheet in the cerebral hemispheres of the reptile appears to be the fore- runner of the true cerebral cortex, which reaches such a high development in the mammals. The hippocampal and pyri- form cortex are called archipallium, to distinguish them from this neopallium in which olfactory impulses do not predominate.

In the birds the cerebral cortex is less well developed than in the reptiles, and the corpus striatum with the attendant highly instinctive type of behaviour is specialised instead.

In the mammals, the cerebral cortex is developed out of proportion to the rest of the brain. In the higher mammals (but not in Monotremes or Marsupials) a special commissure is developed to link together the neopallium of the two hemi- spheres ; this is the corpus callosum. The dorsal commissure of the reptiles, which links together the hippocampal archi- pallia, persists in the mammals as the hippocampal commissure.

The volume of the neopallium is increased in higher mammals without much increasing its thickness by throwing it into folds.

The various regions of the neopallium are connected with the other centres by projection-fibres, and in addition, these regions are interconnected by association-fibres. The number of possible combinations between the neurons is so large that it baffles the power of the mind to grasp it. As an example, one million neurons connected together in all possible ways in groups of two neurons each, gives a number of combinations with nearly three million figures in it. There are not far off ten million neurons in the human cerebral cortex.

The neopallium is therefore well fitted to correlate all the stimuli which the animal receives and to make delicately adjusted responses to them. It also serves as a storehouse for impressions which are collected during experience, and an animal which, in determining the response to be made to a set of stimuli, considers the results of experience, is said to show intelligent behaviour. Such an animal has the power of learning, which is not the same thing as the establishment of a habit. Habits can be formed in the lower simple correlation- centres, by means of neurons between certain afferent and certain efferent neurons. The oftener an impulse passes along a reflex arc the easier does its passage become, with the result that the " habitual " response is given to a stimulus. Some habits so formed may be quite complicated, as when a piece of music is " learned by heart." This learning is, however, not necessarily intelligent, because it often happens that when the musician breaks down he is unable to adapt himself to the immediate circumstances and continue, but has to start again at the beginning.

In a similar way animals can be trained to do tricks, or to thread the " Hampton Court " maze without going down any of the blind alleys. If a rat be so trained as to " know " a maze perfectly, and then be placed in a similar maze but with different lengths of alleys and distances between the turnings, it will try to run the distances which it ran in the original maze, and turn where the turnings were in it, and in so doing it bumps into the walls of the new maze. Its learning was therefore not intelligent.

It is interesting to compare this case with that of a chim- panzee confronted with a novel situation. In order to reach food which was placed out of its reach, it hit suddenly on the idea of piling packing-cases on one another and climbing up on them. There is a good deal of evidence to show that in order to " see " what to do in a set of circumstances, the ape must really see the goal and the object which it may use as an instrument, in the same field of view at the same time. There is little doubt that the eyes have played an important part in the evolution of the brain : in man the number of afferent fibres running in from the retina is greater than that running in from all the spinal nerves of one side put together.

The possession of a cerebral cortex and neopallium does not adapt the species to any particular set of environmental circumstances, but instead, it makes all the members of the species individually adaptable to a large variety of circumstances. This is one of the chief differences between the higher and lower vertebrates. All are well supplied with sense-organs, but the lower vertebrates can only make a small number of kinds of responses to the stimuli which they receive. The higher vertebrates have much the same amount of information given them by their sense-organs, but they use it to much better advantage owing to the integrative and retentive properties of the neopallium. The intelligent being does not waste time on trial and error like Paramecium ; the probable results of possible actions are weighed up in what must be called the mind, with the help of experience stored up as memory, and by means of thought, and the action when taken is intentional. Lastly, it must be noticed that the possession of such a mind and its physical basis the neopallium, confers an enormous advantage on its possessor, and has survival value in evolution.


Elliot Smith, G. Some Problems Relating to the Evolution of the Brain. The Lancet, 1910 (1), pp. 1, 147, and 221.

Herrick, C. Judson. Brains of Rats and Men. University of Chicago Press, 1926. . Neurological Foundations of Animal Behaviour. Henry Holt & Co., New York, 1924.

Kohler, W. The Mentality of Apes. Kegan Paul, London, 1925.

Kuhlenbeck, H. Vorlesungen liber das Zentralnervensystem der Wirbeltiere. Fischer, Jena. 1927. Sherrington, C. S. The Integrative Action of the Nervous System. Yale University Press, 1920.

Smith, E. M. The Investigation of Mind in Animals. Cambridge University Press, 1923.

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology

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