THE SENSORY SYSTEM LEARNING OBJECTIVE: Recognize the senses of the body, and identify their physical characteristics. The sensory system informs areas of the cerebral cortex of changes that are taking place within the body or in the external environment. The special sensory receptors respond to special individual stimuli such as sound waves, light, taste, smell, pressure, heat, cold, pain, or touch. Positional changes, balance, hunger, and thirst sensations are also detected and passed on to the brain. SMELL Odor is perceived upon stimulation of the receptor cells in the olfactory membrane of the nose. The olfactory receptors are very sensitive, but they are easily fatigued. This tendency explains why odors that are initially very noticeable are not sensed after a short time. Smell is not as well developed in man as it is in other mammals. TASTE The taste buds are located in the tongue. The sensation of taste is limited to sour, sweet, bitter, and salty. Many foods and drinks tasted are actually smelled, and their taste depends upon their odor. (This interdependence between taste and smell can be demonstrated by pinching the nose shut when eating onions.) Sight can also affect taste. Several drops of green food coloring in a glass of milk will make it all but unpalatable, even though the true taste has not been affected. SIGHT The eye, the organ of sight, is a specialized structure for the reception of light. It is assisted in its function by accessory structures, such as the eye brows, eyelashes, eyelids, and lacrimal apparatus. The lacrimal apparatus consists of structures that produce tears and drains them from the surface of the eyeball. Structure of the Eye Approximately five-sixths of the eyeball lies recessed in the orbit, protected by a bony socket. Only the small anterior surface of the eyeball is exposed. The eye is not a solid sphere but contains a large interior cavity that is divided into two cavities, anterior and posterior. The anterior cavity is further subdivided into anterior and posterior chambers (fig. 1-48). The anterior cavity of the eye lies in front of the lens. The anterior chamber of the anterior cavity is the space anterior to the iris, but posterior to the cornea. The posterior chamber of the anterior cavity consists of a small space directly posterior to the iris, but anterior to the lens. Both chambers of the anterior cavity are filled with a clear, watery fluid called aqueous humor. Aqueous humor helps to give the cornea its curved shape. The posterior cavity of the eye is larger than the anterior cavity, since it occupies all the space posterior to the lens, suspensory ligaments, and ciliary body. The posterior cavity contains a substance, with the consistency similar to soft gelatin, called vitreous humor. Vitreous humor helps maintain sufficient pressure inside the eye to prevent the eyeball from collapsing. The eyeball is composed of three layers. From the outside in, they are the sclera, choroid, and retina (fig. 1-48). OUTER LAYER.—The outer layer of the eye is called the sclera. The sclera is the tough, fibrous, protective portion of the globe, commonly called the white of the eye. Anteriorly, the outer layer is transparent and is called the cornea, or the window of the eye. It permits light to enter the globe. The exposed sclera is covered with a mucous membrane, the conjunctiva, which is a continuation of the inner lining of the eyelids. The lacrimal gland produces tears that constantly wash the front part of the eye and the conjunctiva. The tear gland secretions that do not 1-45 HM3F0148 VITREOUS HUMOR SCLERA OPTIC DISK OPTIC NERVE FOVEA CENTRALIS POSTERIOR CAVITY RETINA CHOROID COAT CILIARY BODY ANTERIOR CHAMBER POSTERIOR CHAMBER ANTERIOR CAVITY AQUEOUS HUMOR LENS PUPIL IRIS CORNEA SUSPENSORY LIGAMENTS Figure 1-48.—Transverse section of the eye.