Central Nervous System

Brain and spinal cord develops from infolding of the neural plate of ectoderm of the embryo immediately dorsal to the notochord.

It is first formed as hollow, tube and later differentiate as anterior wider part, brain and long cylindrical posterior part spinal cord.

(a) Brain (Encephalon) : It is soft, whitish, large sized and slightly flattened structure present inside cranial cavity of cranium of the skull. In man, it is about 1200-1400 gm in weight and has about 10,000 million neurons. Brain is made up of 3 parts

(1) Fore brain (Prosencephalon)

(i) Olfactory lobe – Rhinencephalon

(ii) Cerebrum – Telencephalon

(iii) Diencephalon – Diencephalon

(2) Mid brain (Mesencephalon)

(i) Optic lobes – Mesencephalon

(3) Hind brain (Rhambencephalon)

(i) Cerebellum – Metencephalon

(ii) Medulla oblongata – Myelencephalon

(1) Fore brain or Prosencephalon : It forms anterior two-third of brain and is formed of three parts.

(i) Olfactory lobes : These are one pair, small sized, club-shaped, solid, completely covered by cerebral hemisphere dorsally. Each is differentiated into two parts –

(a) Olfactory bulb : Anterior, swollen part, and

(b) Olfactory tract : Posterior and narrow part which ends in olfactory area of temporal lobe of cerebral hemisphere.

Function : These control the smell.

It is normal in frog, rabbit and man.

It is well developed in dog. So power of smell is more in dog.

These are also well developed in dog fish and name dog fish is on the basis of well developed olfactory lobes.

(ii) Cerebrum : (a) Structure is divided into 5 lobes (i) frontal (ii) parietal, (iii) occipital, (iv) temporal and (v) limbic. A lobe called insula is hidden as it lies deep in the sylvian fissure. The cerebral hemisphere are separated from olfactory lobes by rhinal fissure. The median fissure divides the cerebrum into a right and a left cerebral hemisphere.

A few sulci are well developed and form three deep and wide fissures which divide each cerebral hemisphere into four lobes : anterior frontal lobe, middle parietal lobe, posterior occipital lobe and lateral temporal lobe e.g. Fissure lying between the frontal and parietal lobes is central fissure, that lying between the parietal and occipital lobes is parieto-occipital fissure and that demarcating frontal and parietal lobes from the temporal lobe is lateral or Sylvian fissure. Each cerebral hemisphere is with a fluid-filled cavity called lateral ventricle or paracoel .

Two cerebral hemispheres are interconnected by thick band of transverse nerve fibres called corpus callosum. The peripheral portion of each cerebral hemisphere is formed of grey matter and is called cerebral cortex, while deeper part is formed of white matter and is called cerebral medulla. Cerebral cortex is the highest centre for many sensations and activities and is with a number of sensory areas.

Important areas in the human brain

Parts of Adult Brain (Mammals)

Fore brain (or Prosencephalon) has 3 parts, olfactory lobe, cerebrum and diencephalon.

Olfactory Lobes (or Rhinencephalon)

These are the pair of solid club-shaped structures present along the lower surface of cerebrum; each consists of an olfactory bulb and an olfactory tract to relay sense of smell to temporal lobes. This is exceptionally large in sharks for which scoliodon is called ‘dog fish’. In frog it is hollow (with rhinocoel) and prominent. Cerebrum (or big brain)

The main and largest part of brain; divided by median fissure into two lateral halves called cerebral hemisphere.

The outer layer cerebral cortex of grey matter has large number of gyri and sulci on the surface to increase its area.

The dorsal thick roof is called as pallium which in mammals is derived differently hence called as neopallium.