Brain constitutes the most essential part of the central nervous system. It is made up of countless number of neurons.
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It sends out number of cranial nerves to different parts of the body and thus controls the various activities of the body.
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It is very simple in higher invertebrates, not developed in lower invertebrates such as protozoans and coelenterates but well developed in higher vertebrates.
The brain of cyclostomes, elasmobranchs and fishes is of primitive type. It appears a more or less linear elaboration of the swollen end of the dorsal tubular nerve cord of the embryo.
At its anterior end the brain is primarily an olfactory sensory centre, and its removal does not bring any serious effects in behaviour, nor does its stimulation elicit motor responses.
In fishes, however, the rhinencephalon does exhibit spontaneous rhythms, and in its absence some instinctive behaviour may be lost.
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The role of diencephalon in the lower invertebrates has been little explored, and this is unfortunate in view of the importance of that region in the higher vertebrates.
The hypothalamus is, however, the site of numerous neurosecretory cell bodies, with axons leading to the neurohypophysis and directly involved in the regulation of osmoregulation and other functions.
The midbrain of the lower vertebrates appears to be the highest integrative centre. It consists of the dorsal tectum (terminus of the optic fibres) which exhibits a close correspondence to the retinal receptor pattern, and the ventral tegmentum, including the oculomotor and other motor centres.
The cerebellum is absent from the cyclostomes and appears merely as a small commissure in elasmobranchs, but in fishes it appears as the archicerebellum, receiving the vestibular input.
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It has coordinating and locomotor functions. The medulla contains the piratory centres, generally two in number, coordinating the alternating movements of inspiration and expiration. The circulatory centre for cardiac and vasomotor reflexes is also found here.
In the lower vertebrates the medulla also contains an important vestibular centre which is the main centre for locomotor integration. In fishes, the nervous centre for regulation of colour change is also found.
It appears that the brain of lower vertebrates contains a set of motor centres in the midbrain, sensory centre in the forebrain and an integrative region between.
In the amphibians some notable differences have been observed. Most notable is a movement of integrative functions from midbrain to diencephalon. The latter appears to have some motor control.
The optic termini have moved from the optic lobes to the diencephalon. The midbrain retains its function in coordination of proprioceptive and exteroceptive information into motor patterns and has been shown to control aspects of reproductive behaviour.
The brain of reptiles lies between the amphibians and the higher vertebrates from functional point of view.
The brain of the bird shows extensive structural differences from brains of the lower vertebrates, especially in the well developed cerebrum and cerebellum.
The cerebrum of the birds appears to be primarily a sensory projection area with limited motor function.
The cerebellum is highly developed, is the palaeocerebellum, and has extremely important functions in ocomotor and postural coordination.
The diencephalon of birds has a well-developed centre for control of body temperature. The red nucleus and an extensor function in the vestibular centre appear first in the reptiles and are well-developed in birds as well.
Finally, in the mammals the cerebrum comes to dominate in size and function, the entire brain.
It contains projection areas for all the sensory functions, hence is the terminus for all sensory information.
The cerebellum is more elaborate in mammals than in birds, with the lateral hemispheres constituting the neocerebellum, where sensory information is coordinated with information from all parts of the brain related to movement.