Receptors  are  considered  to  be  sensory  organs. They   provide   the   body   with   the   general   senses   of touch,   temperature,   and   pain.   In   addition,   these receptors initiate reactions or reflexes in the body to maintain  homeostasis.  For  example,  receptors  in  the skin   perceive   cold,   resulting   in   goosebumps.   This reaction   is   the   body's   attempt   to   maintain   internal warmth. Receptors   are   classified   according   to   location, structure,   and   types   of   stimuli   activating   them. Classified  according  to  location,  the  three  types  of receptors   are   as   follows:    superficial  receptors (exteroceptors),  deep receptors  (proprioceptors), and internal receptors (visceroceptors). See table 1-4 for receptor  locations  and  the  senses  resulting  from  the stimulation of these receptors. THE DIGESTIVE SYSTEM LEARNING  OBJECTIVE:   Identify    the location   and   function   of   each   part   of   the digestive system. The   digestive   system   includes   the   organs   that digest and absorb food substances, and eliminate the unused residuals. The digestive system consists of the alimentary canal  and several accessory organs. The accessory   organs   release   secretions   into   the   canal. These   secretions   assist   in   preparing   food   for absorption and use by the tissues of the body. Table 1-5  illustrates  principal  digestive  juices  (secretions) produced by alimentary and accessory organs. Digestion   is   both   mechanical   and   chemical. Mechanical  digestion  occurs  when  food  is  chewed, swallowed,   and   propelled   by   a   wave-like   motion called  peristalsis. When peristalsis occurs, a ring of contraction   appears   in   the   walls   of   the   alimentary canal. At the same time, the muscular wall just ahead of the ring relaxes. This phenomenon is called  receptive relaxation.  As  the  wave  moves  along,  it  pushes  the canal's   contents   ahead   of   it.   Chemical   digestion consists of changing the various food substances, with the aid of digestive enzymes, into solutions and simple compounds.   Carbohydrates   (starches   and   sugars) change into simple sugars (glucose); fats change into fatty acids; and proteins change into amino acids. Once the   food   substances   have   been   broken   down   into simple  compounds,  the  cells  of  the  body  can  absorb and use them. THE ALIMENTARY CANAL The alimentary canal (tract) is 9 meters in length, tubular, and includes the mouth, pharynx, esophagus, stomach,   small   intestine,   and   large   intestine (fig. 1-53). Mouth The   mouth,   which   is   the   first   portion   of   the alimentary   canal,   is   adapted   to   receive   food   and prepare   it   for   digestion   (fig.   1-53).   The   mouth mechanically  reduces  the  size  of  solid  particles  and mixes   them   with   saliva.   This   process   is   called mastication. Saliva, produced by the salivary gland, moistens  food  making  it  easier  to  chew.  Saliva  also lubricates the food mass to aid swallowing. The tongue assists with both mastication and swallowing. Pharynx The pharynx (covered earlier in “The Respiratory System”) is the passageway between the mouth and the esophagus   and   is   shared   with   the   respiratory   tract (fig. 1-53). The  epiglottis  is a cartilaginous flap that 1-49 TYPES LOCATIONS SENSES Superficial receptors At or near surface of body Touch, pressure, heat, cold, and pain Deep receptors In muscles, tendons, and joints Sense of position and movement Internal receptors In the internal organs and blood vessel walls Usually none (except hunger, nausea, pain from stimuli such as chemicals (e.g., aspirin) and distension (e.g., stomach expansion from gas)) Table 1-4.—Types of Receptors, Their Location, and Affected Sense