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.