Structure of forebrain

Forebrain consists of olfactory lobes, cerebrum and diencephalon.

 i.   Olfactory lobes (Rhinencephalon): These highly reduced in human brain and covered by cerebrum from all sides except ventral. Each lobe consists of an olfactory peduncle and olfactory bulb.

ii.  Cerebrum (Telencephalon): 

a. It is a largest part of the brain, constituting about 85% of total brain. 

b.It is divided into right and left cerebral hemispheres by means of a deep median, long fissure. The two hemispheres are internally connected to each other by corpus callosum. 

C. The outer surface of cerebrum is called cerebral cortex, which has outer thin region composed of grey matter and the deep inner part is cerebral medulla, which is composed of white matter. 

d.  The surface of each cerebral hemisphere is greatly folded by many convolutions or gyri and grooves called sulci. These greatly increase total surface area for accommo dation of the vast number of nerve cells. 

e. Each cerebral hemisphere is further divided into four main lobes by three deep sulci: 

1. Parieto-occipital sulcus: It separates the parietal lobe from occipital lobe.

 2. The lateral or Sylvian sulcus: It demarcates the temporal lobe from the frontal and parietal lobes. 

3. Central sulcus: It separates the frontal lobe from parietal lobe. Since, these three sulci are not complete: the lobes are not clearly demarcated from each other. A fifth median lobe called insula or insular cortex is folded deep within the lateral sulcus. 

f.  The grey matter of cerebral cortex mainly consists of cel| bodies of billions of neurons along with no medulated fibres and dendrons. The white matter mainly has axons of myelinated nerves.

iii. Diencephalon: It is the part of the forebrain that contains thalamus and hypothalamus. It lies below the corpus callosum and above the midbrain. It encloses a single cavity called third ventricle / diocoel which communicates with the two lateral ventricles of cerebrum through a narrow opening called foramen of Monro.