The human brain weighs about 1.5kg (average 1350 gms or 3 pounds) and is enclosed in the cranial (skull) cavity. It is covered by 3 membranes, called meninges. The outer membrane, called dura mater ("tough mother") is a tough, protective covering formed of white fibrous tissue the middle delicate membrane is called arachnoid (web like), and the inner most transparent membrane is called pia mater ("soft mother"), containing blood vessels (nutritive in function). Between the bony and membranous coverings, there is cerebrospinal fluid, which acts as a protective internal cushion. The bony skull, meninges and the cerebrospinal fluid protect the brain from external injury and shocks.

(a) Dorsal view

Figure 23.1 (A) Superior (dorsal) Views of brain to show cerebral hemisphere, with lobes, furrows and gyri.

1. The Forebrain: It consist of two divisions, the anterior telencephalon with olfactory lobes, cerebral hemispheres and the posterior diencephalon with thalamus and hypothalamus.

a) The cerebrum is the largest area of the brain and is divided by a longitudinal fissure into two cerebral hemispheres. Each hemisphere consists of a frontal lobe, parital lobe, occipital lobe and temporal lobe. It shows prominent fissures (deeper furrows) or sulci (shallow grooves) and gyri or convolutions (thin ridges between sulci). (Figure23.1 A and B). Structurally, the hemispheres are made up of 3 portions (i) Cerebral cortex or grey matter, (ii) Cerebral tracts are bundles of axons located inside the brain and extend in three directions forming projection fibers, association fibers, and transverse fibers forming the corpus callosum (Figure 23.2) (iii) Basal nuclei or ganglia located deep within the cerebral hemispheres, connected to each other and with the thalamus and hypothlamus.

b) The thalamus is a large, oval part located above the midbrain. All sensory impulses (except for olfactory sensations) entering the cerebrum must pass through the thalamus.

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Chapter 24