(a) hemispheres: The outer 2-4 mm of cerebral

(a) Central nervous system (CNS) – It consists of brain and spinal cord. (b) Peripheral nervous system – It carries all the massages between the CNS and the rest of the body. It consists of 12 pairs of nerves originating from brain and 31 pairs of nerves of the spinal cord. (c) Autonomous Nervous system (ANS) – It is a special part of the peripheral nervous system. It regulates automatic body processes like breathing digestion without conscious control of brain. It is divided into sympathetic nervous system, which responds to the bodies needs during increased activity and emergen­cies. Para sympathetic nervous system in general opposes the actions of sympathetic nervous system.

The balance of activity between the two systems is controlled by CNS. The brain: It is the most complex structure of the body and is poorly understood. Brain contains about 100 thousand million nerve cells. A brief description of parts of brain – Hind brain-

a) Cerebellum:

This is located on the back of the brain under the cerebral hemi spheres. It is a much folded structure and has an outer region, the cortex. It appears grayish in color and is called the gyroscope of the body, because it is concerned with the balance of the body.

(b) Pons:

It is a relay station between the cerebellum, spinal cord and the rest of the brain.

(c) Medulla:

Medulla contains cardiovascular center regulating the heart beat, sneezing, coughing salivation, and vomiting are also controlled by this. Mid brain: Controls the reflex movement of the head, neck and trunk. In response to these visual and auditory stimuli originate from here. Fore brain:

a) Hypothalamus:

Relatively small in size but is vital in importance.

It is located just below thalamus. It is the main coordinating and control center for the ANS. Hypothalamus also controls the pituitary gland.

b) Cerebral hemispheres:

The outer 2-4 mm of cerebral hemispheres is called as cerebral cortex which consists of grey matter of brain beneath this are the white matter. Cerebral hemisphere is the site of con­sciousness.

Right hemisphere controls the left side of the body and vice versa.

c) Corpus callasum:

Right and left hemispheres are connected by the thick band of nerve fibres called the corpus callosum. Spinalcord: It is a long, narrow, almost cylindrical structure that extends from the brain to the bottom edge of the first number vertebra. It has H shaped central area of grey matter and an outer layer of white matter.

There are 31 pairs of spinal nerves. Functions of spinal cord include acting as a coordinating center for simple spinal reflexes, such as knee jerk response and autonomic reflexes such as contraction of the bladder and providing a means of communication between the spinal nerves and brain. Whole CNS is protected by 3 layers of membranes called meanings, inside the meanings cerebrospinal fluid is present and contains lymphocytes to protect against infection. Sense organs:

Eye:

Eyes are held in protective bony sockets of thee skull called orbits.

Parts of eye include external covering of the eye known as sclera, cornea, which is the transparent front part of the sclera, and this curved surface acts as the main structure refracting light towards the retina. Cornea is the part which is actually donated during the eye donation. Conjunctiva is the transparent layer of cells protecting the cornea and is continuous with the epithelium of eyelids. Eye has got a lens which is transparent elastic biconvex structure provides fine adjustment for focusing light on to the retina and separates the aqueous and vitreous humors. Iris is the circular, muscular diaphragm containing the pigment which gives the eye its color.

It controls the amount of light entering the eye. Retina contains the photoreceptor cells, rods and cones and cells bodies and axons of neurons supplying the optic nerve. Fovea is the most sensitive part of retina containing the cones only.

Most of the light rays are focused here.

Eye defects:

(a) Myopia:

Short sightedness. A person cannot see the objects away from him. Corrected by wearing concave lenses.

(b) Hypermetropia:

Long sightedness – A person cannot see the short distance objects corrected by wearing convex lenses.

(c) Astigmatism:

The person can see vertical lines clearly but not the horizontal lines. Corrected by using cylindrical glasses. Ear: Ear consist of outer {pinna) strengthened by elastic cartilage, which focuses and collects sound wave into the ear tube. The sound waves cause the tympanic membrane (ear drum) to vibrate. Damage to the tympanic membrane is prevented by a connection between the air filled middle ear and the pharynx. The Eustachian tube. Auditory receptors are found in the deep part cochlea which is normally coiled.

Human ear is sensitive to wavelength between 40-20,000 Hz. Skin: Skin is the outer covering of vertebrates. Skin is composed of 2 layers, epidermis (outermost) and dermis. Epidermis consists of Malphighian layer where the cell replacement occurs. Dermis consists of hair follicles, sebaceous glands and sweat gland.

Dermis is connected with numerous blood capillaries. Skin protects the body from pathogens, regulates the body temperature, acts as the organ of sense or touch, and acts as excretory organ by excreting nitrogenous waste products.

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