Briefly describe the structure of the following:
(a) Brain (b) Eye (c) Ear
Briefly describe the structure of the following:
(a) Brain (b) Eye (c) Ear
(a) Brain: The human brain is well protected by the skull. Inside the skull, the brain is covered by cranial meninges consisting of an outer layer called dura mater, a very thin middle layer called arachnoid and an inner layer (which is in contact with the brain tissue) called pia mater. The brain can be divided into three major parts :
(i) Forebrain: The forebrain consists of the cerebrum, thalamus and hypothalamus. Functions including temperature, reproductive functions, eating, sleeping and the display of emotion.
(ii) Midbrain: It is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. The midbrain serves important functions in motor movement, particularly movements of the eye, and in auditory and visual processing.
(iii) Hindbrain: It is hindbrain comprises pons, cerebellum and medulla. Controls functions outside conscious control, such as breathing and blood flow
(b) Eye: The adult human eyeball is nearly a spherical structure. The wall of the eyeball is composed of three layers.
- The external layer is composed of a dense connective tissue and is called the sclera.
- The anterior portion of this layer is called the cornea. The middle layer, choroid, contains many blood vessels and looks bluish in colour.
- The choroid layer is thin over the posterior two-thirds of the eyeball, but it becomes thick in the anterior part to form the ciliary body.
- The ciliary body itself continues forward to form a pigmented and opaque structure called the iris which is the visible coloured portion of the eye.
The eyeball contains a transparent crystalline lens which is held in place by ligaments attached to the ciliary body. In front of the lens, the aperture surrounded by the iris is called the pupil whose diameter is regulated by the muscle fibres of iris. The inner layer is the retina and it contains three layers of neural cells from inside to outside-ganglion cells, bipolar cells and photoreceptor cells. There are two types of photoreceptor cells, namely, rods and cones. The daylight (photopic) vision and colour vision are functions of cones and the twilight (scotopic) vision is the function of the rods. The innermost ganglionic cells give rise to optic nerve fibre that forms optic nerve in each eye and is connected with the brain.
(c) Ear: It performs two sensory functions, hearing and maintenance of body balance. It can be divided into three major sections called the outer ear, the middle ear and the inner ear :
(i) Outer ear: It consists of the pinna and external auditory meatus (canal). The pinna collects the vibrations in the air which produce sound. The external auditory meatus leads inwards and extends up to the tympanic membrane (the eardrum). There are very fine hairs and wax-secreting glands in the skin of the pinna and the meatus. The tympanic membrane is composed of connective tissues covered with skin outside and with mucous membrane inside.
(ii) Middle ear: It contains three ossicles called malleus, incus and stapes which are attached to one another in a chain-like fashion. The malleus is attached to the tympanic membrane and the staps is attached to the oval window of the cochlea. The ear ossicles increase the efficiency of transmission of sound waves to the inner ear. A Eustachian tube connects the middle ear cavity with the pharynx. The Eustachian tube helps in equalizing the pressures on either side of the eardrum.
(iii) Inner ear: It is also known as the labyrinth. Labyrinth is divided into the bony labyrinth and a membranous labyrinth. Bony labyrinth is filled with perilymph while membranous labyrinth is filled with endolymph. The membranous labyrinth is divided into two parts-Vestibular apparatus and Cochlea.
The vestibular apparatus is composed of three semi-circular canals and the otolith (macula is the sensory part of saccule and utricle). The crista and macula are the specific receptors of the vestibular apparatus responsible for maintenance of the balance of the body and posture.
The cochlea is a long and coiled outgrowth of sacculus. It is the main hearing organ. Cochlea consists of three membranes. The organ of Corti, a hearing organ, is located on the basilar membrane that has hair cells.
(a) Brain: The human brain is well protected by the skull. Inside the skull, the brain is covered by cranial meninges consisting of an outer layer called dura mater, a very thin middle layer called arachnoid and an inner layer (which is in contact with the brain tissue) called pia mater. The brain can be divided into three major parts :
(i) Forebrain: The forebrain consists of the cerebrum, thalamus and hypothalamus. Functions including temperature, reproductive functions, eating, sleeping and the display of emotion.
(ii) Midbrain: It is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. The midbrain serves important functions in motor movement, particularly movements of the eye, and in auditory and visual processing.
(iii) Hindbrain: It is hindbrain comprises pons, cerebellum and medulla. Controls functions outside conscious control, such as breathing and blood flow
(b) Eye: The adult human eyeball is nearly a spherical structure. The wall of the eyeball is composed of three layers.
- The external layer is composed of a dense connective tissue and is called the sclera.
- The anterior portion of this layer is called the cornea. The middle layer, choroid, contains many blood vessels and looks bluish in colour.
- The choroid layer is thin over the posterior two-thirds of the eyeball, but it becomes thick in the anterior part to form the ciliary body.
- The ciliary body itself continues forward to form a pigmented and opaque structure called the iris which is the visible coloured portion of the eye.
The eyeball contains a transparent crystalline lens which is held in place by ligaments attached to the ciliary body. In front of the lens, the aperture surrounded by the iris is called the pupil whose diameter is regulated by the muscle fibres of iris. The inner layer is the retina and it contains three layers of neural cells from inside to outside-ganglion cells, bipolar cells and photoreceptor cells. There are two types of photoreceptor cells, namely, rods and cones. The daylight (photopic) vision and colour vision are functions of cones and the twilight (scotopic) vision is the function of the rods. The innermost ganglionic cells give rise to optic nerve fibre that forms optic nerve in each eye and is connected with the brain.
(c) Ear: It performs two sensory functions, hearing and maintenance of body balance. It can be divided into three major sections called the outer ear, the middle ear and the inner ear :
(i) Outer ear: It consists of the pinna and external auditory meatus (canal). The pinna collects the vibrations in the air which produce sound. The external auditory meatus leads inwards and extends up to the tympanic membrane (the eardrum). There are very fine hairs and wax-secreting glands in the skin of the pinna and the meatus. The tympanic membrane is composed of connective tissues covered with skin outside and with mucous membrane inside.
(ii) Middle ear: It contains three ossicles called malleus, incus and stapes which are attached to one another in a chain-like fashion. The malleus is attached to the tympanic membrane and the staps is attached to the oval window of the cochlea. The ear ossicles increase the efficiency of transmission of sound waves to the inner ear. A Eustachian tube connects the middle ear cavity with the pharynx. The Eustachian tube helps in equalizing the pressures on either side of the eardrum.
(iii) Inner ear: It is also known as the labyrinth. Labyrinth is divided into the bony labyrinth and a membranous labyrinth. Bony labyrinth is filled with perilymph while membranous labyrinth is filled with endolymph. The membranous labyrinth is divided into two parts-Vestibular apparatus and Cochlea.
The vestibular apparatus is composed of three semi-circular canals and the otolith (macula is the sensory part of saccule and utricle). The crista and macula are the specific receptors of the vestibular apparatus responsible for maintenance of the balance of the body and posture.
The cochlea is a long and coiled outgrowth of sacculus. It is the main hearing organ. Cochlea consists of three membranes. The organ of Corti, a hearing organ, is located on the basilar membrane that has hair cells.
