Comparative anatomy of Brain

General plan of brain: The cranial end of early embryonic neural tube of all craniates from fishes to humans exhibits three primary brain vesicles—

Prosencephalon (future forebrain) Mesencephalon (future midbrain) Rhombencephalon (future hindbrain)

The Rhombencephalon was named for its shape. The primary Prosencephalon & Rhombencephalon soon differentiate to form the five major subdivisions of the adult brain—

Telencephalon Diencephalon Mesencephalon Metencephalon Myelencephalon

Differentiation of the primary vesicles to form an adult brain is accomplished by—

Localized thickenings of the lateral walls & floor of some vesicles. Dorsal, lateral or ventral evaginations, median or paired, in others.

A delicate membrane that runs at the anterior wall of the Telencephalon, called lamina terminalis and the roof of the cerebrum is called cortex or pallium. The ventro-lateral wall of the cerebrum becomes thick and called corpus striatum.

Comparative account of brain in vertebrates:

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.

A. Cephalochordates:

In Amphioxus, the brain is made of an anterior Prosencephalon with a single enlarged ventricle. It is lined with cilia and long filamentous process of epidermal cells.

B. Cyclostomes:

Primitive brain. Subdivisions are not well-marked. Two olfactory lobes are present. Cerebral hemispheres are quite small. Medulla oblongata is very well-developed. Cerebellum is a small transverse dorsal band. Infundibulum bears a hypophysis & pituitary body.

C. Fishes:

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. Osteichthyes:

In Perch, olfactory lobes, cerebral hemispheres and diencephalon are smaller while optic lobes and cerebellum are larger than in a shark.

Some bony fishes have restiform bodies. In bottom-feeders, the antero-lateral sides of medulla show vagal lobes. Parapineal body is absent in modern teleosts.

D. 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.

E. 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.

F. 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 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.

G. 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.

Conclusion: Discussing the evolution of brain in these vertebrate groups, it is clear that they are originated from a common ancestral stock. Their general plan of brain supports this.