The document discusses the comparative anatomy of brains across vertebrates. It notes that all vertebrate brains share a common underlying form and early development, with three initial swellings that become the forebrain, midbrain, and hindbrain. Across classes like fish, amphibians, and mammals, the relative sizes of these parts change. In mammals, the forebrain grows much larger and dominates. The functions of different brain regions are also described. The development of brain vesicles and flexures is outlined. Finally, features of the brain in different vertebrates like sharks, frogs, lizards, pigeons, and rabbits are compared.

1. GDC Uttersoo
Comparative Anatomy of Brain
The brain is an organ that serves as the Centre of Nervous System in all Vertebrate animals. It is
located in the head, usually close to the sensory organs for senses such as vision.
All vertebrate brains share a common underlying form which appears most clearly during
early stages of development. In its earliest form the brain appears as three swellings at the front
end of the neural tube; these swellings eventually become the forebrain, midbrain and hindbrain
(prosencephalon, mesencephalon and rhombencephalon resp). In the initial stages of brain
development, the three areas are roughly equal in size. In many classes of vertebrates, such as
Fish and Amphibians, the three parts remain similar in size in the adult, but in Mammals the
forebrain becomes much larger than the other parts, and the midbrain becomes very small.
Functions of Brain
1. Olfactory lobes: Sense of smell.
2. Cerebral hemispheres: Seat of intelligence and memory.
3. Diencephalon: Controls the general metabolic functions of the body.
4. Optic lobes: Sense of vision.
5. Cerebellum: Co-ordinates the movements of voluntary muscles.
6. Medulla oblongata: Controls the involuntary functions of the body.
Development and differentiation of brain in Vertebrates
1. The entire nervous system arises from the ectoderm.
2. The ectodermal neural folds are developed which close dorsally to form neural tube.
Development and differentiation of primary brain vesicles and their cavities:

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The vesicles of vertebrate brain:
1. Anterior part of neural tube is slightly swollen which initially differentiated in to three
primary vesicles of brain namely first prosencephalon (forebrain), second
mesencephalon (midbrain) and third rhombencephalon (hindbrain) containing cavities.
2. At later stages two secondary vesicles form in the area of the prosencephalon, 1a and
1b (see fig. below) developing into telencephalon and diencephalon respectively.
3. The diencephalon gives rise to sac like lateral outgrowths called the optic vesicles. The
optic vesicles are pushed downward by two large processes growing forward from the
anterior vesicle (the primitive cerebral hemispheres). These ultimately develop into the
retina, and other nervous parts of the eye.
4. The other secondary vesicles are developed in the rhombencephalon namely 3a and 3b
(see fig. below) developing into metencephalon and myelencephalon respectively.
5. Mesencephalon reduces to form a narrow commissure known as cerebral aqueduct (see
fig. below).
The cavities of vertebrate brain:
1. The cavities of brain are referred as ventricles. There are four ventricles in vertebrate
brain namely 1. Olfactory ventricle. 2. Lateral ventricle. 3. Third ventricle. 4. Fourth
ventricle
2. The olfactory lobes carry cavities known as olfactory ventricles arising from the
anterior part of the cerebral hemispheres. These grow forward, and soon lose olfactory
ventricles.
3. The telencephalic vesicles become the cerebral hemispheres, and their cavities become
the paired lateral ventricles (lateral telocoeles) of the adult brain.

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4. The third ventricle lies in the diencephalon (diocoele). Later in development the lateral
walls of the diencephalon become greatly thickened to form the thalami, thus reducing
the size and changing the shape of the diocoele, which is known in adult anatomy as the
third brain ventricle.
5. The fourth ventricle lies in metencephalon. The anterior part of the fourth ventricle is
known as metacoele and the posterior part as myelocoele.
Flexures of brain:
1. As the result of unequal growth of these different parts of brain, three flexures are formed
and the embryonic brain becomes bent on itself in a somewhat zigzag fashion; the two
earliest flexures are concave ventrally and are associated with corresponding flexures of
the whole head.
2. The first flexure appears in the region of the mid-brain, and is named the ventral
cephalic flexure. By means of it the fore-brain is bent in a ventral direction around the
anterior end of the notochord and fore-gut, with the result that the floor of the fore-brain
comes to lie almost parallel with that of the hind-brain. This flexure causes the mid-brain
to become, for a time, the most prominent part of the brain, since its dorsal surface
corresponds with the convexity of the curve.
3. The second bend appears at the junction of the hind-brain and medulla spinalis. This is
termed the cervical flexure, and increases from the third to the end of the fifth week,
when the hind-brain forms nearly a right angle with the medulla spinalis; after the fifth
week erection of the head takes place and the cervical flexure diminishes and disappears.
4. The third bend is named the pontine flexure, because it is found in the region of the
future pons Varoli.
5. Both the cervical and the pontine flexures eventually straighten out, but the cephalic
flexure remains prominent throughout development.

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Evolutionofcerebral hemispheres & cerebellum with reference to shark,frog,lizard,
pigeon & rabbit:
1. Telencephalon, called lamina terminalis and the roof of the cerebrum is called cortex or
pallium. The telencephalon then appears to be composed of two divisions, the anterior of
which is subsequently developed into the cerebral hemispheres, corpora striata, and the
olfactory lobes. The hemispheres undergo enormous enlargement in their later development
and extend dorsally and posteriorly as well as anteriorly, eventually covering the entire
diencephalon and mesencephalon under their posterior lobes.
2. Posterior part of forebrain, the diencephalon, consisting of the thalamus and
hypothalamus, representing the anterior vesicle. The median evagination in the roof of the
diencephalon develops into epiphysis.
3. The mesencephalon becomes specialized as the optic lobes, visual centers associated with
the optic nerves. The dorsal and lateral walls of the mesencephalon later increase rapidly in
thickness and become the optic lobes (copora quadrigemina) of the adult brain. . It serves as
the main pathway of the fiber tracts which connect the cerebral hemispheres with the
posterior part of the brain and the spinal cord.
4. The hindbrain became divided into anterior metencephalon and posterior myelencephalon.
The metencephalon shows ventrally and laterally an extensive ingrowth of fiber tracts giving
rise to the pons and to the cerebellar peduncles of the adult metencephalon. The roof of the
metencephalon undergoes extensive enlargement and becomes the cerebellum of the adult
brain. The ventro-lateral wall of the cerebrum becomes thick and called corpus striatum.
The ventral and lateral walls of the myelencephalon become the floor and side-walls of the
medulla of the adult brain. The medulla becomes specialized as a control center for some
autonomic and somatic pathways concerned with vital functions (such as breathing, blood
pressure, and heartbeat). The dorsal wall of myelencephalon is thin and receives rich blood
vessels thus forming posterior choroid plexus. The pons is above the medulla and also acts as
a connecting tract. The cerebellum enlarged and became a structure concerned with balance,
equilibrium, and muscular coordination.

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Comparativeanatomyofbraininvertebrates
Brain of all vertebrates, from fish to man, is built in accordance with the same architectural plan.
However, form of brain differs in different vertebrates in accordance with the habits and
behavior of the animals.
1. Elasmobranches:
Olfactory lobes are correspondingly large.
Optic lobes and pallium are relatively moderate in size.
Saccus vasculosus, thin-walled vascular sensory organ, is attached to pituitary and
connected with cerebellum.
Pineal apparatus is well-developed.
Cerebellum is especially large due to active swimming habit.
Ruffle-like restiform bodies are present.
2. Amphibians:
Olfactory lobes are smaller than optic lobes.
Corpus striatum receives greater number of sensory fibres.
Cerebral hemispheres are more developed than in fishes.
Less-developed cerebellum.
Medulla is also small.
Pineal body is small.
3. Reptilians:
Telencephalon becomes the largest region of brain.
Olfactory lobes are larger than in amphibians.
A pair of auditory lobes is found posterior to optic lobes.
The third ventricle is reduced to a narrow cerebral aqueduct.
Cerebellum is somewhat pear-shaped and larger than in amphibians.
Modern reptiles show a great development in the basal parts of the forebrain. There are large
numbers of nervous connections between the thalamus and the hemispheres. The latter are
larger than the optic lobes, showing the increased importance of the former. The walls of the
thalamus are very thick and many of the optic nerve paths end there as well as many in the
midbrain.
4. Birds:
Brain is proportionately larger than that of a reptile.
Olfactory lobes are small.
Two cerebral hemispheres are larger, smooth and projected posteriorly over the

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diencephalon to meet the cerebellum.
Pallium is thin but corpus striatum is greatly enlarged.
Optic lobes are conspicuously developed.
The cerebellum is greatly enlarged with several superficial folds.
5. Mammals:
Brain is proportionately larger than in other vertebrates.
Cerebral hemispheres of Prototheria, Metatheria & Eutheria are smaller & smooth, larger
& smooth, greatly enlarged & divided into lobes respectively.
The two hemispheres are jointed internally by transverse band-like fibres called corpus
callosum.
Olfactory lobes are relatively small but well defined.
Four almost solid optic lobes are present.
The mammalian brain is completely dominated by the cerebral hemispheres. The roof has
developed enormously and spread out forming the cerebral cortex which in humans is thrown
into a number of elaborate folds and almost covers the rest of the brain. The cortex is made up
millions of cells. The more folded the surface, the more cells it can contain. These cells make up
the gray matter. Their axons, which make up the tracts or pathways in the brain, form the white
matter underneath the cortex. The white matter of the spinal cord is also made up of nerve axons,
surrounding the central gray matter.
Conclusion: Discussing the evolution of brain in these vertebrate groups, it is clear that they are
originated from a common ancestral stock.