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Chapter: BIOLOGY (ZOOLOGY) Standard XI first year 11th text book Assignment topics question and answer Explanation Definition

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Human Brain : Prosencephalon (Fore brain), Mesencephalon (mid brain), Rhombencephalon (hind brain), Brain stem, Pachymenix, Leptomeninges

Human Brain : Prosencephalon (Fore brain),   Mesencephalon (mid brain),  Rhombencephalon (hind brain), Brain stem, Pachymenix, Leptomeninges
The brain is safely kept inside the cranial vault. Inside the skull the brain is surrounded by three protective coverings. They may be grouped un-der two divisions.

Human Brain

 

The brain is safely kept inside the cranial vault. Inside the skull the brain is surrounded by three protective coverings. They may be grouped un-der two divisions.

 

1.Pachymenix -It includes the duramater.

 

2.Leptomeninges - In includes the arachnoid mater and pia mater.

 

The duramater is the outermost membrane. It is thick and inelastic in nature. The arachnoid mater is the middle covering over the brain. In between arachnoid and piamater there is a space called the subarachnoid space. It contains cerebro-spinal fluid and blood vessels. The piamater is a delicate membrane closely applied to the brain. This membrane contains blood capil-laries supplying blood to the brain cells.

The human brain weighs about 1.3 Kg. It contains more than a billion neurons. Based on embryological development the brain can be divided as follows.

 

1. Prosencephalon (Fore brain) - It consists of the cerebrum and the dien-cephalon. The cerebrum is the largest part of the brain. It is divided into right and left hemispheres by a longitudinal fissure. However, at the base the two hemispheres are connected by a sheet of nerve fibres called the corpus callossum.

The outer surface of the cerebrum is called the cortex or grey mater. It is 2 to 4 mm thick. The inner content of the cerebrum is the white mater. The surface of the cerebrum has several folds called the gyri. They greatly increase the surface area of the cortex. The shallow grooves in between the gyri are called the sulci. A central sulcus runs in the lateral surface of the cerebrum from superior to inferior region.

 

Each cerebral hemisphere is divided into four lobes. They are the fron-tal at the front, the parietal towards the top of the head, the temporal on theside and the occipital at the rear.

 

The diencephalon contains the thalamus and hypothalamus. This region is found between the cerebrum and the brain stem.

 

The thalamus has a cluster of nuclei which act as the relays for par-ticular sensory pathways. Just beneath the thalamus, the hypothalamus is present. It contains reflex centres linked to the autonomic system. A funnel shaped stalk called the infundibulum extends from its floor. It is connected to theneurohypophysis of the pituitary gland.

 

2.  Mesencephalon (mid brain) - It is the smallest region of the brainstem. On its dorsal surface there are four rounded bodies called the carporaquadrigemina.

 

3. Rhombencephalon (hind brain) - The three main regions of the rhomben-cephalon are the medulla oblongata, the pons varoli and the cerebellum.

 

The cerebellum consists of two hemispheres. Its surface has many ridges called folia. The cerebellum consists of three parts. They are the small anterior flocconodular lobe, a narrow central vermis and two large lateral hemispheres.

 

The pons is just superior to the medulla oblongata. It contains ascending and descending nerve tracts.

 

The medulla oblongata is about 3 cm long. It is continuous with the spinal cord. It remains as a bridge between the brain and the spinal cord.

 

Brain stem - The medulla oblongata, pons and mid brain form the brain stem. It connects the spinal cord to the brain. Ten of the twelve cranial nerves enter or exit the brain through the brain stem.


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