THE SENSORY SYSTEMLEARNING OBJECTIVE: Recognize thesenses of the body, and identify their physicalcharacteristics.The sensory system informs areas of the cerebralcortex of changes that are taking place within the bodyor in the external environment. The special sensoryreceptors respond to special individual stimuli such assound waves, light, taste, smell, pressure, heat, cold,pain, or touch. Positional changes, balance, hunger,and thirst sensations are also detected and passed on tothe brain.SMELLOdor is perceived upon stimulation of the receptorcells in the olfactory membrane of the nose. Theolfactory receptors are very sensitive, but they areeasily fatigued. This tendency explains why odors thatare initially very noticeable are not sensed after a shorttime. Smell is not as well developed in man as it is inother mammals.TASTEThe taste buds are located in the tongue. Thesensation of taste is limited to sour, sweet, bitter, andsalty. Many foods and drinks tasted are actuallysmelled, and their taste depends upon their odor. (Thisinterdependence between taste and smell can bedemonstrated by pinching the nose shut when eatingonions.) Sight can also affect taste. Several drops ofgreen food coloring in a glass of milk will make it allbut unpalatable, even though the true taste has not beenaffected.SIGHTThe eye, the organ of sight, is a specializedstructure for the reception of light. It is assisted in itsfunction by accessory structures, such as the eyebrows, eyelashes, eyelids, and lacrimal apparatus.The lacrimal apparatus consists of structures thatproduce tears and drains them from the surface of theeyeball.Structure of the EyeApproximately five-sixths of the eyeball liesrecessed in the orbit, protected by a bony socket. Onlythe small anterior surface of the eyeball is exposed.The eye is not a solid sphere but contains a largeinterior cavity that is divided into two cavities, anteriorand posterior. The anterior cavity is further subdividedinto anterior and posterior chambers (fig. 1-48).The anterior cavity of the eye lies in front of thelens. The anterior chamber of the anterior cavity isthe space anterior to the iris, but posterior to the cornea.The posterior chamber of the anterior cavity consistsof a small space directly posterior to the iris, butanterior to the lens. Both chambers of the anteriorcavity are filled with a clear, watery fluid calledaqueous humor. Aqueous humor helps to give thecornea its curved shape.The posterior cavity of the eye is larger than theanterior cavity, since it occupies all the space posteriorto the lens, suspensory ligaments, and ciliary body. Theposterior cavity contains a substance, with theconsistency similar to soft gelatin, called vitreoushumor. Vitreous humor helps maintain sufficientpressure inside the eye to prevent the eyeball fromcollapsing.The eyeball is composed of three layers. From theoutside in, they are the sclera, choroid, and retina (fig.1-48).OUTER LAYER.—The outer layer of the eye iscalled the sclera. The sclera is the tough, fibrous,protective portion of the globe, commonly called thewhite of the eye. Anteriorly, the outer layer istransparent and is called the cornea, or the window ofthe eye. It permits light to enter the globe. The exposedsclera is covered with a mucous membrane, theconjunctiva, which is a continuation of the inner liningof the eyelids. The lacrimal gland produces tears thatconstantly wash the front part of the eye and theconjunctiva. The tear gland secretions that do not1-45HM3F0148VITREOUSHUMORSCLERAOPTICDISKOPTICNERVEFOVEACENTRALISPOSTERIORCAVITYRETINACHOROIDCOATCILIARYBODYANTERIORCHAMBERPOSTERIORCHAMBERANTERIORCAVITYAQUEOUSHUMORLENSPUPILIRISCORNEASUSPENSORYLIGAMENTSFigure 1-48.—Transverse section of the eye.
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