The special senses of the body include the major senses of sight, smell, hearing, taste, and several others, such as touch and equilibrium. All the senses have highly specialized receptors enabling them to respond to the appropriate stimuli.
The eye is the organ of sight in the body. It gathers light from the environment and forms an image on nerve cells of the retina. The image is then transformed into nerve impulses, which are interpreted by the brain. The eye is a fluid-filled, somewhat movable sphere. Anatomically, the “eye” is synonymous with the eyeball. The wall of the eye is composed of three layers, or coats: an outer, tough, fibrous coat consisting of the cornea and sclera; middle, highly vascular coat, uvea, containing the choroid layer, iris, and ciliary bodies; and an inner coat, the retina, which contains the receptors of sight. The pupil of the eye is an opening in the iris of the eye. Two layers of smooth muscle compose the iris: a sphincter muscle layer, which constricts the pupil and makes it smaller, and a dilator muscle layer, which makes the pupil larger. The iris contains pigments that give colour to the eye. Behind the pupil is the lens, a transparent, biconvex disk of fibrous protein material. The lens is firmly attached to a structure called the ciliary body by a ligament called the suspensory ligament (zonule). The innermost layer of the eyeball is the retina. The inner, nerve layer of the retina consists of three layers of neurons: first, a layer of receptor neurons numbering about 100 million rod cells and 700 million cone cells; second, a layer of bipolar neurons, the nerve cells that receive impulses initiated by the rod and cone cells; and, third, a layer of ganglionic neurons attached directly to the optic nerve. The accessory structures of the eye include the eyebrows, eyelids, eyelashes, conjunctiva, and lachrymal apparatus.
The ear is the organ of hearing and equilibrium. Its purpose is to gather sound waves from the environment and transmit them to nerves in the inner ear. Here the sound waves are transformed into nerve impulses for transmission to the brain. The major region of hearing in the brain is the cortex of the temporal lobe of the cerebrum. The ear consists of three major portions: the external ear, the middle ear, and the internal ear. The external ear (outer ear) consists of cartilage-filled auricle (pinna) and the external auditory meatus, a tortuous canal that ends at the tympanic membrane (eardrum). Earwax (cerumen) keeps the eardrum soft and waterproof. The middle ear connects the external and inner ears. The three small ossicles, which conduct sound waves through the middle ear, occupy most of this part of the ear. These bones in order of their involvement are the malleus (hammer), the incus (anvil), and the stapes (stirrup). The base of the stapes inserts into the oval window, an oval-shaped opening into the inner ear. A specific connection is maintained between the middle ear and the nasopharynx through an auditory or Eustachian tube. The internal ear contains the functional organs for hearing and balance. It is made up of a system of mazelike chambers and tubes called labyrinths. The bony labyrinth can be divided into three areas: the vestibule, the semicircular canals, and the snail-shaped cochlea. One of the components of the cochlea is the spiral organ (organ of Corti). It contains a number of hair cells or cilia that are receptors for sound waves from the auditory fluids.
The sense of taste is called the gustatory sense. The taste buds are located on the upper surface of the tongue within tiny elevations called papillae. The four basic tastes are sweet, sour, bitter, and salty. Molecules enter taste pores of the papillae and stimulate the specialized gustatory (taste) cells of the taste buds. These cells send impulses over sensory nerve fibres to the brain, where the taste stimuli are interpreted.
The sense of smell is referred to as the olfactory sense. During smell, fine particles of substances enter the nose and stimulate special olfactory cells in the mucous membrane of the nasal uppermost portion. When stimulated, the receptors form impulses that leave the nose region over branches of the olfactory nerve. This nerve enters the skull through the cribriform plate of the ethmoid bone and passes through the olfactory bulb in the olfactory tract, which leads to the frontal and temporal lobes of the cerebrum. Interpretation of the stimuli is made here.
The sense of touch is distributed throughout the body. Nerve endings in the skin and other parts of the body transmit sensations to the brain. Some parts of the body have a larger number of nerve endings and, therefore, are more sensitive. Four kinds of touch sensations can be identified: cold, heat, contact, and pain. Hairs on the skin magnify the sensitivity and act as an early warning system for the body.