CHAPTER XXIV. 

 THE SENSES. 



THE GENERAL SENSES. TOUCH AND TEMPERATURE 



SENSE. 



Afferent and Efferent Nerve Currents. Up to this point 

 we have been studying efferent, or out-going, nerve cur- 

 rents, such as control muscles and glands. Now let us 

 turn to the in-coming, or afferent, currents ; for it is by 

 means of the afferent currents to the brain that we get all 

 our sensations. In other words, it is through these cur- 

 rents that we get all our knowledge. 



Two Classes of Sensations. There are two classes of 

 sensations, the special and the general. The special senses 

 include sight, hearing, taste, smell, touch, and temperature 

 sense. Among the general sensations are hunger, thirst, 

 fatigue, nausea, satiety, faintness, pain, muscular sense, etc. 



Special Sensation due to External Force. Sensations 

 from the organs of special sense are due to the action of 

 an external force. For instance, sound waves entering the 

 ear affect the nerves of hearing, and we have a sensation 

 of hearing. Light acting on the optic nerve gives sight. 



General Sensations due to Conditions within the Body. 



There are nerves of general sensibility in all parts of 

 the body. The endings of these nerves are acted on by the 

 blood and lymph. Currents are all the time coming through 



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