2. The Eye External/accessory structures of the eye. Extrinsic eye muscles aim the eyes for following moving objects and for convergence. Lacrimal apparatus includes a series of ducts and the lacrimal glands that produce a saline solution, which washes and lubricates the eyeball. Eyelids protect the eyes. Associated with the eyelashes are the ciliary glands, modified sweat glands, and the meibomian glands, which produce an oily secretion that helps keep the eye lubricated. The conjunctiva is a mucous membrane that covers the anterior eyeball and lines the eyelids. It produces a lubricating mucus
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4. The sclera is the outer, tough, protective tunic. Its anterior portion is the cornea, which is transparent to allow light to enter the eye.
5. The choroid is the middle coat, which provides nutrition to the internal eye structures and prevents light's scattering in the eye. Anterior modifications include two smooth muscle structures, the ciliary body, and the iris (which controls the size of the pupil).
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10. Outer ear structures are the pinna (auricle), external auditory canal, and tympanic membrane (eardrum). Sound entering the external auditory canal sets the eardrum into vibration. These structures are involved with sound transmission only.
11. Middle ear structures are the ossicles and auditory tube within the tympanic cavity. The ossicles (malleus, incus, and stapes)transmit the vibratory motion from the eardrum to the oval window. The auditory tube allows pressure to be equalized on both sides of the eardrum. These structures are also involved with sound transmission only.
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14. Chemical Senses: Smell and Taste Chemical substances must be dissolved in water to excite the receptors for smell and taste. The olfactory (smell) receptors are located in the superior aspect of each nasal cavity (Sniffing helps to bring more air containing odors) over the olfactory mucosa. Olfactory pathways are closely linked to the limbic system; odors recall memories and arouse emotional responses. Gustatory (taste) cells are located in the taste buds, primarily on the tongue. The four major taste sensations are sweet, salt, sour, bitter. Taste and appreciation of foods is influenced by the sense of smell and the temperature and texture of foods.
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17. The muscles of the tongue belong to two groups: intrinsic and extrinsic. Intrinsic muscles lie entirely within the tongue; that is their origin and insertions are inside the tongue. There are four groups of them: Superior Inferior longitudinal Transverse or horizontal Vertical The extrinsic tongue muscles are those that continue beyond the tongue, anchoring it to other structures. There are four pairs of them: Genioglossus – attaches the tongue to the mandible, the jaw bone Hyoglossus – attaches the tongue to the hyoid bone in the neck Styloglossus – attaches the tongue to the styloid process, a protrusion from the temporal bone, one of the skull bones Palatoglossus – attaches the tongue to the palate Actions of Tongue Muscles Genioglossus – helps to protrude the tongue, depress the central part of tongue making it concave, and move the tongue to the opposite side Hyoglossus – helps to depress the tongue Styloglossus – helps to pull the tongue upwards and backwards to aid swallowing Palatoglossus – pulls the soft palate onto the tongue while swallowing Intrinsic muscles – help in widening, flattening, thickening, lengthening and rolling of the tongue.
18. Developmental Aspects of the Special Senses Special sense organs are formed early in embryonic development. Maternal infections during the first five or six weeks of pregnancy may cause visual abnormalities as well as sensorineural deafness in the developing child. An important congenital eye problem is strabismus. The most important congenital ear problem is lack of the external auditory canal. Vision requires the most learning. The infant has poor visual acuity (is farsighted) and lacks color vision and depth perception at birth. The eye continues to grow and mature until the eighth or ninth year of life, Problems of aging associated with vision include presbyopia, glaucoma (the most common cause of blindness in the U.S.), cataracts, and arteriosclerosis of the eye's blood vessels. The newborn infant can hear sounds, but initial responses are reflexive. By the toddler stage, the child is listening critically and beginning to imitate sounds as language development begins. Sensorineural deafness (presbycusis) is a normal consequence of aging. Taste and smell are most acute at birth and decrease in sensitivity after the age of 40 as the number of olfactory and gustatory receptors decreases.
