CHAPTER X. 



SENSATION. 



AN organism is brought into relation with its environment 

 through its irritability to external stimuli. This property, of irri- 

 tability, is common to all protoplasm. As the organism increases 

 in complexity from single-celled individuals to individuals con- 

 sisting of groups of cells, this property of irritability or sensa- 

 tion becomes differentiated into a variety of sensations, depending 

 upon the part of the external surface or special end-organ stimu- 

 lated and the nature of the stimulus. 



Conscious sensation first occurs, so far as we know, in those 

 animals provided with a nervous system and brain. The sensory 

 impulse is conducted over nerve pathways to the sensory portions 

 of the cerebral cortex, and there interpreted in terms of sensation 

 and corresponding judgments formed. 



All sensations occur as a result of some form of stimulus applied 

 to the outer body envelope and its connection through afferent 

 nerves with the centres of consciousness in the brain. For the re- 

 ception and transmission of certain stimuli, the outer envelope has 

 become markedly modified, as, for example, the receiving appara- 

 tus for audition and vision. 



The localities for the reception of certain sensory impressions 

 are limited to certain sharply defined areas. These include the 

 end-organs of taste, smell, sight, and hearing. Others have a wide 

 distribution over the entire cutaneous surface and, to a lesser 

 degree, over the mucous surfaces. Such are the tactile sense, the 

 sense of temperature, the pain sense, and the pressure sense. The 

 so-called muscular sense also has a wide distribution. 



All parts of the body are brought into relation with the central 

 nervous system through afferent or centripetal nerves. Only part 



