________________
Preksha Dhyana :
(ii) the mid brain and (iii) the hind brain.
The forebrain and mid-brain together make up the cerebrum. It constitutes the bulk of the brain. It is composed of an outer grey layer beneath which is a mass of white tissues. Its wrinkled surface allows a very large number of brain cells to be packed into a limited space.
Buried deep within the brain are various important structures including the hypothalamus. It contains numerous specialised control centres. It is intimately connected with the pituitary gland, thus interlocking the endocrine system. Medulla oblangata joins the brain to the spinal cord.
The Spinal Cord extends from the foraman magaum down to the level of the second lumbar vertebral. The cord ! itself is suspended rather loosely in the vertebral canal. Its
length is about 45 centimetres. Each pair of nerves is composed of (i) sensory pathways which carry the message of sensation to the brain, and (ii) motor pathways which carry the orders for movement from the brain to the muscles in the trunk and the limbs. The spinal cord also contains 'reflex centres' for immediate response to certain incoming stimuli without involvement of the brain. Sense-organs
The human body is equipped with a versatile assortment of sensory outposts. Our eyes, ears, nose, mouth and the entire body surface are endowed with a vast variety of receptors for collecting useful information from outside. Sensations are the conscious results of the sequence : stimulus+receptor-+conducting pathway+sensory area in the brain. Though hundreds of thousands of sensory messages are received by the brain, only those which are important are perceived while the rest are ignored. An intricate network of nerve-cells, called Reticular Activising System (RAS) acts as a filter and allows only strong or novel signals to pass up to higher brain for conscious perception. The process by which the mind converts raw sensations into perception is complicated. The RAS action
Jain Education International
For Private & Personal Use Only
www.jainelibrary.org
