THE VERTEBRATE BRAIN AND CRANIAL NERVES

All the nervous systems of vertebrates conform to the same plan for they all develop in the same way. This similarity is most marked in the earlier stages of development, for the nervous system is the first to develop as a definite set of structures before any other important structures like vertebras, body muscles, the heart and the gut can be recognized. In its earliest stages the central nervous system is a simple tube expanded at the front to form the brain.

The Plan of the Brain

The brain consists of three regions, fore-, mid- and hind-brain. The front of the fore-brain is a pair of olfactory lobes, its hind part a band of axons, the thalam-encephalon, connecting the fore-brain with the mid-brain. The rest of the fore-brain is the cerebrum. The roof of the thalamcocephalon bears a stalk connected with the pineal body, while its floor is extended as the infundibulum, in front of which the optic nerves emerge, crossing to form the optic chiasma. Its walls, the optic thalami, are thick. The roof of the mid-brain is the optic lobes ; its floor the crura cerebri. The front of the roof of the hind-brain is the cerebellum, the rest of the hind-brain being the medulla oblongata. The neural canal in the spinal cord continues forwards into the brain, becoming there a series of four spaces called ventricles, lateral ventricles inside the cerebral hemispheres, a third ventricle in the thalamencephalon, and the fourth ventricle behind the cerebellum. Each optic lobe also has a cavity connecting with the ventricles. Although, apart from the last, all these structures are found in all vertebrates, they vary considerably. The relative size of any part and also its surface area bear a distinct relationship to the habits and habitat of each animal—in general, if any particular sense be extensively used by a particular species of animal, then the part of the central nervous system connected with those sense organs is either relatively large or complex, but we do not know how this development has been brought about, for it is hereditary and not due to the use of the sense organ by each particular individual, such as we find in the case of the expert footballer with his leg muscles well developed by his own continual practice.

Functions of Parts of the Brain

If the brains of a fish, a frog and of a mammal are examined, two facts stand out clearly : there is a gradual increase in the relative size and complexity of the cerebral hemispheres as we go from the simpler to the higher forms of vertebrates ; the relative sizes of other parts can vary considerably in each case ; the dogfish has very large olfactory lobes and the frog a very small cerebellum.

Investigation has shown that the size and complexity of the cerebral cortex is connected with intelligence. In the case of man it is known to be the principal region where he experiences sensations and so becomes conscious of what is going on around and in himself. He notices that certain sensations always occur together or in a definite sequence, and as a result he connects them together, I.e. he associates facts together. For example, if we notice that a man ’s entry into a house is always followed by a childish cry of joy, we should conclude that the cry was due to the child seeing the man. The extent to which an animal can connect facts is a measure of that animal ’s intelligence. It may be that the animal has two sets of facts to deal with, requiring it to take two courses of action at the same time. This proves a difficult puzzle to all but the highest mammals, leading us to believe that voluntary action, I.e. that requiring exercise of the will, is almost entirely limited to such animals. Further, such voluntary action may be due to the animal making use of past happenings, I.e. memory is involved. For example, a dog with its master in the country, having smelt a rabbit, will run to and fro between its master and the rabbit ’s burrow when it hears its master ’s familiar whistle, the dog clearly indicating that it cannot ’make up its mind. ’ An intelligent animal rapidly learns by experience.

In man the process goes further, and he decides what his future conduct will be and what follows by pursuing a certain ’line of thought, ’ I.e. he reasons.

It does not follow that an animal ’s response to the stimuli it receives will be a voluntary action ; it may be, or may be a purely instinctive reflex action or may be a combination of the two termed a conditioned reflex action. The instinctive reflexes are those which an animal does not have to learn or be taught, e.g. swimming by ducklings, spinning a web by newly hatched spiders, the sucking action of a very young infant when anything is put into its mouth. Conditioned reflex action is best explained by means of the concrete example of experiments carried out by Pavlov. He found that a very young puppy ’s mouth only ’watered ’when food was put on its tongue, I.e. the salivary glands secreted saliva. Such food normally has a smell, and when slightly older the puppy dribbled when it smelt food. Later the sight of food produced the same result. Now all these stimuli, touch, smell and sight, were directly related to the food itself, so Pavlov tried a sound stimulus. Just before being fed in a quiet cage where the animal could not see the experimenter, a metronome was started. This was repeated for several days, and then at feeding time the metronome was started and although no food was given then the animal dribbled, I.e. the sound now produced the response. The animal continued to respond in this way for several days, but eventually the response was no longer exhibited. In some way the two stimuli became associated together in the dog ’s brain so that either stimulus produced the response.

Since the reflex action produced by the sound was due to the invariable condition of the environment we say it is a conditioned reflex. Later experiments have shown that instinctive reflexes in man at any rate are extremely few, and that nearly all our actions fall into the category of conditioned reflexes. Walking, riding a bicycle, using a cricket bat, the eye movements in reading and hand movements in writing, are all activities we carry out without thinking. The advantage of being able to form conditioned reflexes is very great, for reflexes can be very rapidly carried out, whereas voluntary action usually follows an appreciable length of time after the stimulus. Walking and riding a bicycle would not be pleasurable activities if they were merely a repetition of our first efforts.

To sum up : the cerebral hemispheres in man are the main seat of consciousness, of various sensations, of memory, of voluntary action, of intelligence and of reason. Each area appears to have one of two functions, e.g. in the auditory region we have one area where we experience the sensation of sound, variations in its pitch, loudness and quality ; in the other area we interpret the sound in terms of our state of knowledge, I.e. where it comes from, what is making it, and, if a musical sound, what kind of instrument is being used. These functions may apply also to a very slight extent to other animals which possess a cerebral cortex, but it is dangerous to make such an application, as there are indications that it is not true, e.g. a man who loses his cerebral hemispheres by an accident becomes paralysed and lacks all feeling, quite unlike the dog quoted above.

The other parts of the brain and their functions in man are :—

The olfactory lobes, ’concerned with ’smell, I.e. they relay impulses from the olfactory organs to the cerebral hemispheres.

The optic thalami, important relay stations for impulses going to or from the cerebral cortex. These may pass via the crura cerebri and the pons to the mid-brain, hind-brain and spinal cord.

The corpora striata, controlling the temperature of the body.

The optic lobes, controlling the contraction of the eyeball muscles, correlating them with impulses received from the eye.

The cerebellum, controlling muscles which maintain the balance, I.e. control equilibrium, and co-ordinating the actions of the antagonistic muscles, e.g. the triceps and biceps muscles of the arm.

The medulla, a great centre for the most important reflexes, e.g. rate of heart-beat and of breathing, and variation of the size of the blood vessels in each part of the body, thereby controlling the blood supply to each part.

In the case of other vertebrates, it would seem that particular sensations reside in places other than the cerebral cortex. These are : the olfactory lobes, seat of the sense of smell ; the optic lobes, scat of the sense of sight; the cerebellum, seat of the sense of hearing and control of many muscles, which in man are controlled by spinal reflexes. The optic thalami would seem to be important centres for correlating the sensory impressions. We cannot tell to what extent an animal is conscious of what it is doing, particularly in view of the fact that so many of its activities can be explained in terms of instinctive and conditioned reflexes. These explanations are supported by recent experimental evidence.

The Cranial Nerves

The nerves which pass through openings in the skull before joining the central nervous system are called cranial nerves. In fish and Amphibia there are ten pairs, and in reptiles, birds and mammals twelve pairs, the eleventh and twelfth corresponding with certain spinal nerves in the lower forms. Their distribution and functions are similar in all cases, although varying in the different animals according to their habits and habitats. Some consist almost entirely of afferent fibres and are therefore called sensory nerves ; others, of efferent fibres, are termed motor nerves, and the rest are mixed.

The Nerve Supply to the Viscera

It will be noted that an important branch of the vagus nerve is concerned with conveying impulses from the brain to the viscera, causing them to slow down their activity. These organs also receive impulses by way of the sympathetic nervous system. The response to these impulses is one of starting or speeding up activity, being a spinal reflex action. Where co-ordination is involved the impulses coming from the higher centres via the vagus will slow down this action until it is appropriate to the immediate needs of the animal,

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